EP0473095A2 - Hybrid sound attenuator - Google Patents

Abstract

The invention relates to a hybrid sound attenuator, in which a passive absorber lining is combined with an active electro-acoustic system in such a way that the sound attenuation is increased due to the effect of the active subsystem on the passive absorber lining, in order to obtain an optimised channel-side acoustic impedance and thereby an improved sound attenuation within a broad frequency range, particularly at low frequencies. <IMAGE>

Description

1. Field of technology

Mufflers are mostly lined with "passive" sound absorbers. The interaction of the absorber lining with the sound field in the muffler duct is essentially due to the acoustic surface impedance of the absorber lining. There is a so-called optimal surface impedance, which is clearly described by the winding point of the so-called absorber function between the first and the second silencer mode. Realization efforts in the specialist literature have shown that it is fundamentally not possible to maintain this winding point impedance over a larger frequency range with purely passive absorber elements.

In another technique, the so-called "active" sound insulation, the noise to be reduced is superimposed by an electro-acoustic system on a sound wave generated by this system in such a way that there is a canceling interference in the direction of sound propagation (method of anti-noise).

2. Outline

The technical terms of the main claim are first briefly explained below, then the state of the art is discussed, from which the object of the invention is derived. Furthermore, the idea of the invention is formulated and the steps for its implementation are treated as examples. Then the technical progress of the invention is demonstrated and finally the claims of the invention are enumerated.

Where appropriate, mathematical formulas are used as the most concise and precise representation of physical-technical relationships. Furthermore, exemplary schematic drawings serve to explain the inventive concept and its implementation.

3. Explanation of terms

The explanations of terms are intended to help describe the invention and to justify its technical progress. For a more complete description of the acoustics of silencers, reference is made to reference [1].

Silencer:

Silencers are used to reduce the passage of sound through openings. In most applications, these openings are the clear cross sections of channels. Mufflers - and thus also the muffler according to the invention - are also used to reduce the passage of sound through joints (for example door joints) and through openings for material or for people to pass through (for example in soundproof capsules of machines and in soundproof booths). The openings are collectively called "channels" below.

The characteristics of the hybrid sound absorber according to the invention must be adapted to the shape of the clear cross section of the silencer duct. The idea of the invention is developed here as an example for rectangular cross sections, as shown in Figure 1 , but is not limited to this shape. The invention can also be used for other cross-sectional shapes, for example in the case of round or annular channels.

Absorber lining:

Silencers with a sound-absorbing lining basically have a sound-absorbing effect by fitting an absorber lining on the entire circumference or on part of the circumference of the clear duct cross-section, which (mostly only in parts, for example the air enclosed in porous materials) is excited to vibrate by the sound wave and thereby interacts with it, so that the sound wave is dampened as it propagates through the channel. In order to intensify the interaction, the clear cross section of the channel is often subdivided by sound-absorbing backdrops. Such a backdrop silencer is shown in Figure 2 . The idea of the invention is also applicable to such forms of absorber lining. Each gap in the backdrop is then treated like a silencer according to Figure 1 .

Sound absorbers are called "passive" if the resonance of the absorber lining is caused by the sound wave solely through mechanical (acoustic) interactions. The strength and phase of the resonance is then determined by the arrangement, the dimensions and the material properties of the absorber lining. The selection of the components of the passive absorber in the silencer according to the invention is not restricted by the inventive concept. These can be, for example: porous sound absorbers made of fiber materials (mineral fibers, metal fibers, plastic fibers, organic fibers, open-cell foams, porous sintered ones Materials etc.), flexible plates, foils and membranes, perforated sheets, fabrics, fleeces etc. These components of the passive absorber can either be in direct contact with one another or at a mutual distance from one another. For a more complete description of such absorber components and their acoustic properties, reference is made to reference [2].

Active subsystem:

The active subsystem according to the invention consists of one or more sound recorders, one or more signal formers and one or more sound generators (usually: loudspeakers). The sound pickups are mostly microphones, but can also be structure-borne sound pickups. Depending on the task, the signal formers for the output signal of the sound pickups are combinations of amplifiers, frequency filters and other electronic components known per se. The sound generator is set into vibration by the electrical output variable of the signal shaper according to the inventive concept in such a way that its vibrating surface (for example a loudspeaker membrane) assumes an acoustic impedance according to the invention.

The electronic signal formers can in turn consist of passive and active electronic components (according to the language used in electronics); these can be known "analog" or "digital" functional units.

A diagram of the active subsystem with the sound pickup (1), the signal former (2) and the sound generator (3) is shown in Figure 3 .

Action on the passive absorber:

In a preferred embodiment of the inventive concept, the loudspeaker of the active subsystem is arranged on the side of the passive absorber layer facing away from the channel and represents an acoustic terminating impedance Z _b for this. This arrangement, hereinafter referred to as "series connection", serves in the following as Example of the development of the inventive concept. It is shown schematically in Figure 4 . However, it is not the only possible arrangement according to the invention.

In another exemplary embodiment, the channel-side surface of the sound-absorbing lining consists of an arrangement of "passive" and "active" partial areas. The passive areas are formed by the surface of passive sound absorbers, the active areas are formed either by the surface of the loudspeakers or by mouths of loudspeaker chambers, which on their part can again be completely or partially filled with passive absorber components. This arrangement is also referred to as "parallel connection". Figure 5 contains a schematic representation .

Acoustic Impedance:

The acoustic impedance Z _{w of} a surface is the ratio of the sound pressure on this surface to the surface normal sound velocity. Occasionally, the reciprocal of the impedance, namely the (acoustic) admittance G _w = 1 / Z _w , is also used below.

The product U = k _o hZ _o G _w from the wall admittance G _{w of} the absorber lining with the frequency variable k _o h from k _o = 2πf / c _o with the frequency f and the speed of sound c _o and from half the width h des ( here, for the sake of simplicity of the description, assumed a gap-shaped duct cross-section and finally the wave resistance Z _{o of} the air (c _o and Z _o are material constants of the air or the sound-carrying medium in the duct) is the so-called "absorber function" U. It essentially determines the effectiveness of a specific absorber lining in a silencer (for more see below and in [1]).

To simplify the notation, it is assumed below that all impedances Z and all admittances G are normalized with the characteristic impedance Z _{o of} the sound-conducting medium. This is done by replacing Z → Z / Z _o and G → Z _o · G.

Silencer modes:

• · sie "passen" in die Berandungen des Schallfeldes, das heißt, sie erfüllen jede für sich die Randbedingungen,
• · man kann durch Überlagerung von Moden jedes beliebige (auch komplizierte) Schallfeld innerhalb dieser Berandung sythetisieren,
• · die durch ein solches Schallfeld transportierte Wirkleistung ist einfach die Summe der Schalleistungen der Moden (was bei einer Zerlegung des Schallfeldes in andere Komponenten als Moden nicht so einfach wäre).

Modes are basic forms of vibration of a sound field. They have the following characteristics:

• · They "fit" into the boundaries of the sound field, that is, they each fulfill the boundary conditions,
• · One can synthesize any (even complicated) sound field within this boundary by superimposing modes,
• · The active power transported through such a sound field is simply the sum of the sound powers of the modes (which would not be so easy if the sound field was broken down into components other than modes).

Darin ist Γ = Γ'+j·Γ'' die komplexe (Längs-) Ausbreitungskonstante und q(y) ist eine geeignete Querverteilungs-Funktion, welche die Randbedingungen an der Oberfläche der Absorberauskleidung erfüllen muß.Let x be the axial coordinate of a channel in the direction of sound propagation and y a transverse coordinate (in the gap channel perpendicular to the absorber surface), then a suitable representation of the sound pressure field is:

${\text{p (x, y) = e}}^{\text{-Γx}}\text{Q (y) (1)}$

Therein Γ = Γ '+ j · Γ''is the complex (longitudinal) propagation constant and q (y) is a suitable transverse distribution function which has to fulfill the boundary conditions on the surface of the absorber lining.

Die Durchgangsdämpfung D_d eines Schalldämpfers der Länge L ist dann:

$${\text{D}}_{\text{d}}\text{= 8,68 · (Γ'h) ·}\frac{\text{L}}{\text{h}}{\text{= D}}_{\text{h}}\text{·}\frac{\text{L}}{\text{h}}\text{[dB] (3)}$$

The real part Γ 'of Γ is the damping in neper per unit length. It is customary (and used in the following) to specify the damping D _h due to the drop in sound level in deci-bel (dB) per half the gap h. This is:

${\text{D}}_{\text{H}}\text{= 8.68 · Γ'h [dB] (2)}$

The transmission loss D _{d of} a silencer of length L is then:

$${\text{D}}_{\text{d}}\text{= 8.68 · (Γ'h) ·}\frac{\text{L}}{\text{H}}{\text{= D}}_{\text{H}}\text{·}\frac{\text{L}}{\text{H}}\text{[dB] (3)}$$

wobei die Hilfsgröße z eine Lösung der Bestimmungsgleichung

$\text{z · tan z = jU (5)}$

ist mit der oben eingeführten (und aus den Absorberdaten bekannten) Absorberfunktion U.The longitudinal propagation constant Γ is obtained from the frequency variable k _o h and the auxiliary variable z from the formula:

where the auxiliary variable z is a solution of the determination equation

$\text{z · tan z = jU (5)}$

is with the absorber function U introduced above (and known from the absorber data).

Für andere Querschnittsformen wird auf [1] verwiesen. Bei einem runden Kanalquerschnitt mit axialsymmetrischem Feld ist beispielsweise die Bestimmungsgleichung:

mit den Besselfunktionen J₀ und J₁ nullter beziehungsweise erster Ordnung. Die Werte im Windungspunkt zwischen der ersten und der zweiten Mode sind hierbei: U_wp = 2,98+j·1,98 und z_wp = 2,2285+j·2,358.The calculation method outlined here applies to the selected example of a slit-shaped rectangular channel with symmetrical modes (in-phase vibration of the sound field on both sides of the symmetry surface of the slit channel). The case of antisymmetric modes (antiphase oscillation on both sides of the symmetry surface) leads to the equation for z (see [1]):

$\text{z / tan z = -jU (6)}$

For other cross-sectional shapes, reference is made to [1]. In the case of a round channel cross section with an axially symmetrical field, the determination equation is, for example:

with the Bessel functions J₀ and J₁ zero or first order. The values at the winding point between the first and the second mode are: U _wp = 2.98 + j · 1.98 and z _wp = 2.2285 + j · 2.358.

The function tan z is periodic in the real part z 'of the complex argument z = z' + j · Z ''. Their stock of values is repeated along z 'at intervals of π. This leads to the fact that the equations (5) and (6) have (infinitely) many solutions z. Each of these solutions then represents a silencer mode. It can be seen from Eq. (4) that the different modal solutions for z lead to different modal dampings. The mode damping generally increases with the atomic number m of the modes, that is to say with the Run number of the period strip along z 'from which the solution comes.

Which silencer mode prevails, which mode damping is to be used, depends not only on the design and dimensioning of the silencer, but mainly on the transverse distribution of the sound field impinging on the silencer, accordingly on the sound source (blower, compressor, flame, etc. ) and from the defects (corners, branches, internals, etc.) in the duct in front of the silencer. A rule justified by use is therefore to design the silencer for the lowest damped mode. This was recommended in the early works [3] and [4]. This rule is confirmed because the lowest damped mode is among the first two modes and because higher modes are difficult to excite, so they rarely dominate the sound field. Should they still be present, then they would have a higher damping than the design damping.

Turn point between modes:

For the solutions z = z '+ j · z''of the determination equation, curves with constant real part z' and constant imaginary part z '' are plotted over the complex level of the absorber function U = U '+ j · U (which is assumed to be known) '', then the curve networks of Figure 6 and Figure 7 are created . Each of these figures covers approximately one of the period stripes over z ', that is, a fashion. Namely, Figure 6 the first mode of Eq. (5) and Figure 7 the next higher mode. Other forms of representation can be found in [1], [3], [4].

At the lower end of the area in these figures, in which the curve networks are (almost) rectangular, you can see points around which the curves "wind". These are the "winding points".

What is important in connection with the invention is the fact, demonstrated in [4], that a mode has its highest attenuation at its upper winding point.

Also important in connection with the invention is the fact that neighboring modes have these winding points in common. There the two solutions z of neighboring modes coincide. So if the absorber lining of a silencer has an absorber function with the value U = U _wp in the winding point of the first two modes, then there is no mode of oscillation in this silencer with a lower damping, since the least damped mode is among the first two modes.

Approaching the turn point:

[4] describes how the values of the absorber function U _wp at the winding point and the associated solutions z _{wp are} determined and numerical values are given there. For the winding point between the two lowest modes, the following applies for a symmetrical sound field in the rectangular gap channel: U _wp = 2.05998 + j · 1.65061 and z _wp = 2.1062 + j · 1.12536.

Figure 8 shows the frequency curve of the damping D _{h of} the lowest damped mode in a rectangular gap channel when the absorber function U of the absorber lining is at the winding point U _wp at all frequencies. This curve can be obtained from equations (4) and (2) if one uses z = z _wp in equation (4). The abscissa of this (and all of the following) representation of the damping D _h is the product f · h in [Hz · m], because silencers can be summarized that meet certain "similarity laws". Note the logarithmic plot of the damping.

To make the statement in Figure 8 clear, consider a silencer with a gap width H = 2h = 0.2 m, i.e. h = 0.1 m. This is a frequently used gap width for silencers for technical systems. Then a silencer according to Figure 8 with a length L = 1 m at low frequencies would have an attenuation of D _d = 190 dB, a truly extraordinary value!

This extremely high damping is technically hardly fully usable, because then other sound transmission mechanisms, such as structure-borne sound propagation in the silencer housing, would limit the effective noise level reduction by the silencer. Therefore, one does not need to set the value U _{wp of} the absorber function exactly in a silencer according to this invention; it is sufficient to approximate this value in order to achieve significant improvements in sound attenuation compared to conventional silencers.

Desired frequency range:

With purely passive absorber linings, it is not particularly difficult to approximate the value U _{wp of} the absorber function in individual frequencies. The data for a simple absorber layer made of mineral fibers are given in [1, Figure 19.13] (d / h = 1; Ξd / Z _o = 0.4), the absorber function of which even passes through the winding point at two frequencies. The associated damping curve (however with today's absorber parameters of the mineral fibers instead of the parameters used in [1]; therefore only an approximation to the optimal damping curve) is shown in Figure 9 . It is typical of such purely passive realizations of the optimal wall impedance of the absorber lining that the high damping value is only achieved in narrow resonance peaks and is purchased at other frequencies with deep attenuation dips.

This also applies to the efforts to implement passive absorbers in the literature [6], which was devoted solely to this task. The successes of the implementation efforts in [7] are not significantly different.

As already explained in [4], the reason for the failure of realizing U _wp over a wide frequency range is that no passive component is known which connects the reactance of a spring with the frequency response of an inertial mass. The problems with the purely passive, broadband implementation of the optimal impedance of the absorber lining are therefore of a fundamental nature.

It is the decisive advantage of the invention over mufflers with purely passive absorber linings that the optimal impedance can be maintained over a wide frequency range.

In principle, it would certainly be desirable to set the optimal impedance over the entire frequency range, i.e. the damping curve of Realize Figure 8 . However, this would increase the implementation effort considerably. It is a further advantage of the invention that the frequency range and the level of the improved sound attenuation can be set, as a result of which compromises can be made between effort, technical constraints (such as space requirements, pressure loss, etc.) with the sound attenuation to be aimed for.

Most technical applications of silencers have in common that high damping at low frequencies (approx. 30 Hz to approx. 250 Hz) is required. This is due to the fundamental course of the frequency spectra of the noise of flow generators with an increase to low frequencies. The problems in achieving high, broadband sound attenuation values at low frequencies with purely passive absorber linings are almost as fundamental. This is due to the principle with passive acoustic components that all dimensions are measured in units of the sound wavelength, which increases towards low frequencies. As a result, conventional silencers for low frequencies have a large construction volume, the absorber thicknesses d have to be large, which then leads to an increase in the flow pressure losses. It is shown below using an example that the operating costs of a silencer caused by this can far exceed its purchase value.

For this reason, special attention is paid to the attenuation at low frequencies.

4. State of the art

A description of the prior art must deal with two techniques: the technique of "passive" sound insulation with purely passive components and the technology of "active" sound insulation with only electroacoustic, "active" components.

The problems with the implementation of high sound attenuation with purely passive absorber linings - especially at low frequencies - were mentioned above in part in the explanations of the term. The state of the art in a good design of a passive broadband muffler is described [1, Fig. 19,21], which is repeated here as Figure 10 with the designations used here. A similar designation also contains [8, Fig. 12/10] and [9, Fig. 6.138], whereby in the latter reference the upper limit for technical realizations is not set to D _h = 3 dB, but to D _h = 1.5 dB is. The revision of the VDI directive on silencers [10] also repeats this information as the state of the art for good broadband silencers.

The invention presented here does not actually have to deal with passive silencers, since they are comparable neither in terms of the solution concept nor the damping achieved. A modern development is briefly presented to illustrate the current status in this area. In [11] an absorber element made of so-called "membrane absorbers" for silencers is described, which has been painstakingly tuned by the combination of multiple resonance systems in such a way that over a certain frequency bandwidth (usually approx. 1.5 octaves) the highest possible damping with as much as possible shallow depth. Figure 11 shows damping curves of different types of this absorber element, which are tuned to different frequency ranges. The above-mentioned technically feasible attenuations of D _h = 3 dB are hardly exceeded. The advantage of the smaller overall depth remains. But the price of such elements is three to five times the price of conventional shock absorbers with otherwise comparable performance.

The publication DE 34 25 450 Al [20] also deals with the same aim, namely to reduce the depth of the link elements. This solves a particular problem with link silencers, which arises from the fact that the entire width of the channel in which the links are installed (see Fig. 2 of the present description) is constantly changing because this channel width is in the available space must adapt, but that on the other hand the width of a certain backdrop arrangement can only change by leaps and bounds by the width D + H when adding or omitting another backdrop. The cited document indicates that with a backdrop arrangement of backdrops of the same thickness with backdrop gaps of equal width in the intermediate region of such a jump, the channel width either loses sound absorption if the backdrop column increases proportionally with increasing channel width, or the pressure loss the muffler increased inadmissibly if you add another backdrop with increasing channel width and the backdrop column is reduced so that the added backdrop finds space. According to this document, the problem is to be solved by using scenes with different thicknesses. By suitable A combination of these makes it easier to realize a predetermined overall width of the link arrangement without having to change the ratio D / H, which is decisive for the damping and the pressure loss. Suitable combinations are given in detailed lists in this document.

The above-mentioned laid-open specification only has the general aim in common with the present invention of increasing the damping and reducing the pressure loss in the context of silencers. The type of absorber structure of the backdrops used is left entirely open - depending on the task at hand; consequently there is no reference to the optimum impedance of the link surfaces to be realized, which forms the basis of a silencer according to the invention. The cited document also contains no mention of an electroacoustic system for achieving such an optimal impedance. There is therefore no reference to the basic concept for the muffler according to the invention and also to a crucial basic component of the same, namely the active subsystem.

The comparison with the so-called "active" silencers is more important. This development started with the patent specifications [12]. After a long period of time, lively research on the methods of "active" noise abatement began in the early 1970s. A brief overview is given in [13]. A compilation of literature from 1988, [14], contains 1708 relevant citations from publications in this field. This lively research activity was triggered by the natural restrictions of passive acoustic components, which can only interact with the sound field according to the given acoustic laws for their impedances and transfer functions. Another impetus for the lively efforts to "active" noise abatement was the technical development of the electronic components, which allows practically any mathematically formulated impedance or transfer function to be implemented electronically at a reasonable cost.

The principle of active sound attenuation, which is often referred to as "noise abatement through antisound", is shown in Figure 12 . From the left, the sound wave from a noise source hits a microphone (1) in a channel. Its output signal is given to a loudspeaker (3) via a signal processor (2) (with amplification, filtering, delay time shift, etc.), which essentially Radiate sound that is intended to interfere with the sound of the noise source in a way that cancels it. In fact, this interfering extinction is effectively only possible in one direction (forward direction); in the backward direction the sound tends to be amplified in most active systems. In a further development, according to Figure 13, a microphone (4) is also arranged behind the loudspeakers, the output signal of which is used to adaptively improve the parameters of the signal processor (2) via a controller (5) in the direction of minimizing the sound pressure at the location of the microphone ( 4).

Most of the research in the research mentioned deals with the development of control algorithms for signal processing and adaptive reworking. Another part deals with the development of suitable loudspeakers and their arrangement.

As impressive as the principle of active noise abatement initially appears, some fundamental and technical difficulties are clearly evident. First of all, it is easy to see with what accuracy the sound field compensation must work across the entire channel cross-section. In order to achieve a transmission loss D _d of 40 dB (a requirement of the usual order of magnitude in the medium frequency range), the residual error of the sound field compensation may only be 10% on a residual area of 1% of the channel cross section or a residual error of approx. 3% of the compensation on a partial area of 10%. This explains why broadband sound attenuation of 40 dB with active systems was only realized under laboratory conditions (exact channel cross-sections, exactly flat sound waves, no flow), since the loudspeakers for the sound at the edges produce sound fields that are difficult to control. Another basic problem arises from the fact that air flows prevail in the silencer ducts. However, the active system cannot initially distinguish between turbulent pressure fluctuations in the flow, which do not emit sound and are therefore harmless, and the sound pressure fluctuations to be reduced. This is the reason why the originally held hopes of obtaining a silencer for low frequencies (below approx. 100 Hz) with the active systems could not be fulfilled, as the turbulent pressure fluctuations become stronger with falling frequency. A technological problem is that most flows in technical silencers transport hot gases and / or chemically aggressive vapors and / or dirt, since both the control microphones and the loudspeakers are directly exposed to the sound field and thus to the flow. Another loudspeaker technical problem is that the sound velocity of the Loudspeaker membranes must be about the same size as the sound velocity of the noise, but the sound pressure is almost zero due to the desired compensation. The loudspeaker thus works on a sound field with a low field impedance. As a result, the mechanical impedance of the loudspeakers must also be very low; the membranes must be light and soft and must make large strokes (at least at low frequencies). Such loudspeakers are in turn not suitable for being exposed to turbulent flow. A technical problem arises from the fact that the control microphone (1) on the side of the noise source not only picks up the sound field to be suppressed, but also the anti-sound emitted to the rear by the loudspeakers. In fact, a large proportion of research has been used to prepare the interference signal from the superimposed signal using suitable control algorithms. Another fundamental problem in the technical application of active silencers is that both the control microphones (1) and control microphones (4) can only scan the sound field in a few points, and the anti-sound field can only be accurately measured in a few points by the loudspeaker arrangement can be specified. This can work as long as the noise field has a constant transverse distribution. However, this transverse distribution changes continuously with technical sound sources and with the large transverse dimensions of the channels. All of this may be the reason why active silencer systems have barely got beyond the laboratory stage; at least they have not yet gained any technical significance despite the intensive research activity. In the case of a technical application, it would also make it more difficult that if the active system failed, there would be virtually no sound attenuation at all.

A method and a device for active sound attenuation are described in the published patent application DE 27 12 534 A1 [21]. It also defines the principle known as active damping, which consists in "that the energy content of (a) wave can be reduced by combining the primary wave with a specially generated secondary wave in such a way that the dilutions of the secondary wave with the Compression of the primary wave collapse and vice versa ". The aforementioned publication emphasizes that the secondary wave must be generated exactly in relation to the primary wave to be canceled by it. It describes the difficulties in the precise generation of the secondary wave the principle of active damping and deals exclusively with methods and devices for obtaining the control signal for the sound source of the secondary wave. The basic structure used in the aforementioned publication is shown in Figures 12 and 13 of the present description. One of the difficulties mentioned in the cited document is the reaction of the sound from the secondary source ((3) in the figures) to the microphone for the primary wave ((1) in the figures), and the known teaching is accordingly primarily concerned with the problem of avoiding this reaction by subtracting an appropriately formed alternating voltage from the output signal of this microphone.

It is one of the main advantages of the present invention that this problem does not occur with the muffler according to the invention, since the sound wave of the loudspeaker does not have to be subtracted from the control microphone of the active subsystem according to the invention, so that the complicated preliminary methods and described in the cited document Devices for avoiding the reaction of the secondary source on the control microphone are not required and the known teaching cannot suggest the invention. In the cited document, a second microphone ((4) in Figure 13 of the present description) is also used to improve the signal processing from the first control microphone ((1) in Figure 13). This so-called backward regulation is generally the state of the art in control technology and is not in itself the subject of the silencer according to the invention. What is important for all control engineering tasks is how the signal from the reverse control is processed and introduced into the control loop. The implementation of the reverse control is completely different from one another in the above-mentioned laid-open publication and on the other hand in the muffler according to the invention in accordance with the fundamentally different task positions. The use of a backward regulation also in the aforementioned publication does not therefore anticipate the inventive concept.

As this published specification confirms, the principle of active sound damping is based, but the problems of this active sound damping are based on the fact that the canceling secondary wave must "be generated exactly". Exactly this need is avoided in the silencer according to the invention by using a passive subsystem which already produces a certain amount of sound attenuation and which is influenced by an active subsystem according to the invention in such a way that this noise attenuation is improved. This publication does not contain any reference to a passive sound absorber. From this it is already clear that it cannot suggest the present inventive concept.

As set out below, most of these problems are approachally avoided in the present invention.

Finally, the state of the art will be discussed in reference [15], in which a silencer is described, which is called "hybrid" there. In fact, this is a series arrangement in the direction of sound propagation of a purely passive silencer and an active silencer. The frequency curves of both mufflers are only tuned in such a way that a favorable frequency curve should result for the passage of sound through both mufflers. An interaction of a passive subsystem of an absorber liner with an active subsystem of this liner in the same silencer as in the submitted invention does not take place. While the effect of the two If silencer sections are additive in the cited literature reference, the effect of the two subsystems in the silencer according to the invention is multiplicative. The literature reference has only the endeavor to improve sound attenuation and the word "hybrid" in common with the presented invention.

5. Task and inventive concept

The object of the invention is therefore
to combine the absorber lining of a passive subsystem and an electroacoustic active subsystem in such a way that both work together to bring about a significant improvement in sound absorption compared to dampers of a known type.

To solve this problem, a hybrid silencer consists of a passive absorber lining (passive subsystem) and an active electroacoustic system (active subsystem), which is characterized in that the electroacoustic active subsystem acts on the combined passive absorber in such a way that in the hybrid absorbers thus formed, the acoustic impedance of the channel-side surface of the absorber lining results in an absorber function U, which in a desired frequency range reaches or approximates the value of the absorber function U _{wp of} the winding point between the first and the second silencer mode (main claim).

Advantageous embodiments are specified in the subclaims.

According to the invention, the absorber function U should therefore come sufficiently close to the value U _{wp of} the absorber function in the upper winding point of the lowest damped mode, ie between the first and second damper modes, over a wide frequency range, especially at low frequencies. "Sufficient" should be measured by a significant improvement in sound absorption compared to dampers of known design.

The following explanations are intended to explain the implementation steps of this inventive concept by way of example. It will become clear that there are different ways of realizing it. An advantage of the invention is seen in this and in the justified possibility for cost-effective solutions. Since the idea of the invention cannot be fully exploited due to the flexibility of its implementation, even with a larger number of examples, more emphasis should be placed on the explanations of the goal-oriented implementation in individual steps. The examples given here are not intended to limit the scope of the invention.

6. Realization of the idea of the invention 6.1 Passive subsystem

The realization of the inventive concept is demonstrated by way of example on a rectangular gap channel (see Figures 1 and 2 ). It is not difficult for the person skilled in the art to transfer to other channel forms.

6.1.1 Series connection of the passive and active subsystem

Furthermore, the implementation of the hybrid absorber lining according to the invention in the series connection according to Figure 4 is discussed first.

For the dimensioning of a muffler according to the invention, it is initially helpful and important that the acoustic effectiveness of the active subsystem can be described simply by an acoustic impedance Z _b on the surface of the sound generator, with which the passive subsystem (1) in Figure 4 on the rear side is completed.

Dabei ist Z_h die Vorderseiten-Impedanz des passiven Subsystems allein, wenn es auf seiner Rückseite schallhart abgeschlossen ist (Leerlauf-Fall), und Z_w ist die Vorderseiten-Impedanz des passiven Subsystems allein, wenn es auf seiner Rückseite schallweich abgeschlossen ist (Kurzschluß-Fall).The passive subsystem is a linear acoustic four-pole. According to the rules of the four-pole calculation (see also [1, Eq. (4.7)]), the terminating impedance Z _b , with which the absorber function U _{wp is} achieved on the front of the hybrid absorber, results from the formula

Z _{h is} the front impedance of the passive subsystem alone if it is sound-proof on its rear side (idle case), and Z _w is the front impedance of the passive subsystem alone if it is soundproofed on its rear side (short circuit -Case).

As an example, we consider a passive subsystem consisting of a layer of thickness d of a porous absorber material, which is covered on its front by a thin cover layer. The porous absorber material is described by the flow resistance R = Ξ · d / Z _o (standardized with Z _o ) with the length-related flow resistance Ξ of the porous absorber material. It is known how to calculate the characteristic propagation constant Γ _an (standardized with k _o ) and the characteristic wave resistance Z (standardized with Z _o ) _of the porous absorber material (see for example [2] or [17]).

mit der Luft-Dichte ρ_o . Wählt man darin den Widerstandsterm R_s groß gegen den Reaktanzterm (2.Term im Nenner), dann beschreibt Z_s die Serienimpedanz einer Deckschicht bestehend aus einer (Massen-)Folie. Wählt man umgekehrt den Reaktanzterm groß gegen R_s, dann hat man einen reinen Reibungswiderstand, wie er beispielsweise durch Gewebe und Vliese dargestellt wird. Ist die Deckschicht dicht auf der porösen Absorberschicht aufgebracht, dann ist dieReaktanz der elastischen Abfederung durch die poröse Schicht noch parallel zu schalten. Da es hier nur um die Demonstration des Verfahrens geht, wird auf diese weitere Komplizierung verzichtet.The cover layer with the series impedance Z _s consists of an acoustic parallel connection of a friction resistance R _s with a mass reactance of a mass m _s related to the area. Then the series impedance Z _{s of} the cover layer follows:

with the air density ρ _o . If one chooses the resistance term R _s large against the reactance term (2nd term in the denominator), then Z _{s describes} the series impedance of a cover layer consisting of a (mass) foil. Conversely, if one chooses the reactance term large against R _s , then one has a pure friction resistance, as is represented, for example, by woven and non-woven fabrics. If the cover layer is applied tightly to the porous absorber layer, the reactance of the elastic cushioning through the porous layer must still be connected in parallel. Since this is only a demonstration of the procedure, this further complication is dispensed with.

The impedances Z _h and Z _w required in Eq. (7) then result from:

$${\text{Z}}_{\text{H}}{\text{= Z}}_{\text{s}}{\text{+ Z}}_{\text{at}}{\text{/ tanh (Γ}}_{\text{at}}\text{· K₀h ·}\frac{\text{d}}{\text{H}}{\text{); Z}}_{\text{w}}{\text{= Z}}_{\text{s}}{\text{+ Z}}_{\text{at}}{\text{Tanh (Γ}}_{\text{at}}\text{· K₀h ·}\frac{\text{d}}{\text{H}}\text{) (9)}$$

The terminating impedance Z _b required according to the invention is thus known for the selected example of the passive subsystem.

Figure 14 shows an example of the so-called "locus" of the terminating impedance Z _b in the complex plane with running frequency variables f · h in Hz · m. The points drawn in the curve lie at thirds intervals of these variables. This illustration applies to a simple porous absorber layer (without top layer) with the parameters d / h = 0.5 and R = 0.5. The curve is composed of arcs, which are known to be simulated by resonance systems (see below under "Active subsystem"). At the circular arc to the right of the imaginary axis, sound energy goes into the active subsystem; here it basically represents an electronically controlled passive absorber. However, since this circular arc is run counterclockwise with increasing frequency, it cannot be realized by passive components, because according to a fundamental theorem of circuit theory, the loci curves of passive circuits are always run clockwise. This should be pointed out, since the word "active" in the active subsystem is not necessarily (albeit usually) to be equated with acoustic energy delivery. That must be on the curve parts to the left of the imaginary axis active subsystem actually feed sound energy into the back of the passive subsystem.

For a different structure of the passive subsystem from a porous absorber layer with d / h = 0.5 and R = 1.0, which is covered with a dense film with the mass parameter m _p = m _s / (ρ _o d) = 1.0, Figure 15 shows the associated locus of the terminating impedance Z _b . The locus here consists of an arc of a single-circuit resonance system at low frequencies and approximately the vertical straight line of a spring reactance at high frequencies. The active subsystem must deliver energy at all frequencies.

In the further example of Figure 16 , the locus of the terminating impedance for a passive subsystem is plotted from a porous absorber layer with the parameters d / h = 0.5 and R = 3.5, which in Figure 10 is a passive absorber lining with a broadband character is shown. The shape corresponds to an attenuated resonance circuit, whose impedance contracts at high frequencies to a convergence point near Z _b = -1.

In general, the examples shown have in common with all differences in shape that they start at low frequencies at the value Z _b = - (R + R _s ) and end near Z _b = -1 at high frequencies.

The use of a porous absorber layer in the passive absorber is not inevitable (although, as will be shown below, it is advantageous) for the realization of the inventive idea. In the calculation, a porous absorber layer can be replaced by an air layer of the same thickness by using for it in the formulas Γ _an → j and Z _an → 1. Figure 17 with the locus of the terminating impedance Z _b according to the invention is an example of an air layer with d / h = 0.5, which is covered on its front by a dense fleece with the series resistance R _s = 2 and the mass parameter m _p = 1 is. As you can see, this locus has a rather complicated shape. However, this is not intended to express the fact that, for technical reasons, it may not also be expedient to construct the passive subsystem of the absorber lining according to the invention without a porous absorber layer, only from foils, membranes and frictional resistances.

The structure of the passive subsystem according to the invention is not limited to the exemplary layer made of a porous absorber with a cover layer. A method for determining the front-side impedance of an M-layered absorber is described in [16, Section 4.4]. whereby individual layers can also be air layers and the layers can also each have outer layers. This very general method can be used for the determination of Z _h and Z _w in Eq. (7), in that the reflection factor r _{M of} the stratification on the rear side is r _M = 1 (reverberant) and r _M = -1 (reverberant) is set. This shows how the task of determining the termination impedance Z _{b is} solved as a subtask in the implementation of the invention for practically every structure of the passive subsystem.

The determination of the termination impedance according to the invention is also not limited to the calculation methods exemplified here of an analytical-numerical description of the passive subsystem using dimensions and material data, although this route is preferable because it provides the best information for the dimensioning of the active subsystem (see below) . One can also build a passive absorber intended for the application of the invention directly and measure its input impedance (once with a sound-proof termination for Z _h and then with a sound-proof termination for Z _w ) as a function of frequency using acoustic impedance measurement methods which are known in the literature have been described several times and are also standardized. To create a formula-dependent frequency dependency, the terminating impedance Z _b formed with the measured values according to Eq. (7) can then be subjected to a numerical regression using known methods. When selecting the functions according to which the regression is carried out, the structure of the active subsystem can expediently be taken into account (see below).

This shows that all passive subsystems whose input impedance can be calculated and / or measured can be included in the determination of the terminating impedance Z _b according to the invention.

In the construction of an absorber lining according to the invention, the determination of the terminating impedance Z _b according to the invention is an intermediate step for the design of the active subsystem. Z _b is the acoustic input impedance of this active subsystem connected in series. The essential part of the active subsystem is the acoustic impedance Z _{m of} the loudspeaker diaphragm, since only it can be influenced by electronic control. In technical applications of the invention, the terminating impedance Z _b generally still contains acoustic components which cannot be influenced electronically. Such a portion is provided, for example, by the "loudspeaker box" with which the loudspeaker is bordered on the back. Such a loudspeaker box is usually necessary, firstly, to avoid the sound of the loudspeaker vibration being released to the outside is radiated, but also to avoid that the pressure of the channel flow presses on the membrane.

Figure 18 shows schematically a hybrid absorber lining according to the invention including a loudspeaker box. In many cases, the loudspeaker box is partially filled with sound absorption material in order to avoid disturbing resonances of the membrane mass with the spring stiffness of the air cushion in the box (however, it can also be advantageous to use this resonance in a targeted manner to create one of the resonance loops of the local curve from Z _b acoustically!). In these cases, the loudspeaker box is again to be described as a layered absorber with a reverberant termination. Now, almost without exception, speaker membranes are incompressible and thin compared to the sound wavelength (at least in the frequency range of interest here). The terminating impedance Z _b therefore arises from a series connection of the sought membrane impedance Z _m with the input impedance Z _{e of} the box, which is measured behind the membrane: Z _m = Z _b - Z _e . The terminating impedance Z _b is still determined according to Eq. (7). The input impedance Z _e is obtained by using the calculation method for the input impedance of stratified absorbers described in [16, Section 4.4]. This subtask can thus be solved in the same generality as the determination of Z _b .

Figure 19 shows the locus curve of the membrane impedance Z _m to be set according to the invention of a structure according to Figure 18 , the air gap in front of the loudspeaker (for the meaning see below) here being 10% of the layer thickness in front of the loudspeaker and the loudspeaker box 80% with absorber material is filled. The entire absorber lining is covered on its front side with a film with mass parameters m _p = 2. In this figure, the variable f · h (points on the curve again at intervals of thirds) runs through the value range from approx. 4 [Hz · m] to 100 [Hz · m].

Läßt man den letzten Bruch weg, entsteht die Abschlußimpedanz Z_b . Dabei ist angenommen, daß der passive Absorber vor dem Lautsprecher geschichtet ist mit M Absorberschichten (einschließlich eventueller Luftschichten) der Dicken d_i mit den längenbezogenen Strömungswiderständen Ξ_i und Deckschichten mit Serienimpedanzen Z_s,i . Ebenso sei in der Lautsprecherbox mit der Gesamttiefe d_box eine Anzahl M_box Absorberschichten (einschließlich eventueller Luftschichten) mit den Dicken d_i . Da bei tiefen Frequenzen die Massenreaktanz von Deckschichten meist vernachlässigbar ist, geht dort Z_s.i→R_s,i gegen die Reibungswiderstände der Deckschichten. Dann wird Z_b ein lineares Polynom in k_oh und in Z_m kommt eine Reaktanz hinzu mit dem Vorzeichen einer Massenreaktanz und der Frequenzabhängigkeit einer Federreaktanz. Die elektronische Synthese dieser Näherung im aktiven Subsystem ist sehr einfach. Diese Näherung wird unten verwendet für Beispiele zu Dämpfungskurven erfindungsgemäßer Schalldämpfer.Both for answering the question with which approximations one can work with in order to "approximate the turn point" according to the invention, and for the design of the active subsystem, it is extremely helpful to use the low frequencies of interest in the [16, Eqs. (4.16a), (4.16b) and Eq. (6.78)] give approximations for the absorber parameters Γ _an , Z _an as well as for their product Γ _an · Z _an and their quotient Γ _an / Z _an analytical approximations for the membrane impedance Z _m according to the invention can be derived:

If the last break is omitted, the terminating impedance Z _b arises. It is assumed that the passive absorber is layered in front of the loudspeaker with M absorber layers (including possible air layers) of thicknesses d _i with length-related flow resistances Ξ _i and cover layers with series impedances Z _{s, i} . Likewise, in the loudspeaker _box with the total depth d _{box there is} a number of M _box absorber layers (including any air layers) with the thicknesses d _i . Since the mass reactance of cover layers is mostly negligible at low frequencies, Z _si → R _{s, i} goes against the frictional resistance of the cover layers. Then Z _b becomes a linear polynomial in k _o h and in Z _m there is a reactance with the sign of a mass reactance and the frequency dependence of a spring reactance. The electronic synthesis of this approximation in the active subsystem is very simple. This approximation is used below for examples of damping curves for silencers according to the invention.

6.1.2 Parallel connection of the passive and active subsystem

In the case of the hybrid absorber lining in series connection according to Figure 4 , the active subsystem supports the passive absorber by generating an appropriate terminating impedance Z _b .

The primary task of the invention, however, is to generate an absorber function U = k _{o hG} = U _wp on the front of the absorber _lining . This is proportional to the effective wall admittance G. It is now part of the basic experience of acoustics that, in the case of flat structured absorber surfaces, the acoustically effective admittance is the mean area value of the admittances of the individual partial surfaces, as long as the dimensions of the partial surfaces remain small relative to the sound wavelength (typically less than about a quarter of the sound wavelength). This condition creates no problems at the low frequencies that are of interest here.

Die Admittanz G_a des aktiven Teils wird somit erfindungsgemäß duch

bestimmt.So if F _{p is} the area of the passive part of the absorber lining with a wall admittance G _p on the surface and F _{a is} the area of the active part with an admittance G _a on the surface, then the invention must be:

The admittance G _{a of} the active part is thus according to the invention

certainly.

The admittance G _p for any layered passive absorber can then be determined using the method described above. Furthermore, the terminating impedance Z _b and / or the membrane impedance Z _{m of} the active part, which in turn can contain a layered absorber between the channel-side surface and the loudspeaker membrane or can be terminated with a loudspeaker box, can be determined by the methods there.

6.1.3 Mixed types of series connection and parallel connection of the passive and the active subsystem

In a further embodiment of the invention, the hybrid absorber lining can consist of mixed types of series connection and parallel connection of the passive subsystem with the active subsystem.

The two basic types are spatially arranged next to each other in a simple but complex apparatus as shown in Figure 20 . Of particular interest is the embodiment of the invention as shown in Figure 21 , where an active subsystem works both in series and in parallel to a passive subsystem. This makes it possible to implement favorable frequency curves for the transfer function of the signal former.

A preferred mixed form of the hybrid absorber according to the invention is shown schematically in Figure 22 . With her, the front of the loudspeaker works in series on a passive absorber, and the back creates a parallel admittance G _a in parallel. This is possible because the parallel admittance G _a must have the character of a passive admittance, but the terminating impedance Z _b , as the examples above show, often has the character of a negative passive impedance. This change of sign occurs automatically when you use the front and the back of the speaker diaphragm as an active signal generator. One can deal with this shape of the Invention also save the space required for a loudspeaker box, and the pressure equalization of the flow pressure via the loudspeaker is also given.

In the case of the mixed forms, the control signal for the active subsystem can, depending on the expediency, be recorded either in the part of the series connection or in the part of the parallel connection by the signal pickup (microphone).

7. Active subsystem

The active subsystem with the schematic structure of a possible embodiment of the invention according to Figure 3 has the task of generating a prescribed acoustic impedance. This impedance is the ratio of the sound pressure on the vibrating surface (membrane) of the signal generator to the speed of this surface averaged over the surface.

Here, an important advantage of the invention compared to the task of active sound insulation has already been introduced, namely that the precise vibration distribution of the membrane is not important. Differences in the oscillation amplitude and phase can compensate for each other across the cross-section in the upstream passive absorber (with series connection) or in the upstream chamber (with parallel connection), so that flat wall impedances nevertheless occur on the channel-side surface of the hybrid absorber. An air gap between the membrane and the passive absorber, as is indicated in Figures 4, 18, 20, 21 and 22, is particularly beneficial for such a transverse compensation of the sound field.

Another significant advantage of the invention compared to the task of active sound insulation is that the required control signal detected by the signal sensor (No. (1) in Figure 3 ) is not the sound pressure without the contribution of the active subsystem, but the actual sound pressure , which is a superposition of the sound pressure of the noise to be reduced, which penetrates from the front through the passive absorber, and the sound pressure of the loudspeaker of the active system. The above-described difficulties of control in the case of active noise protection, namely having to first eliminate the contribution of the active system from the control signal, are completely eliminated in the present invention from the approach of the inventive concept.

Figure 23 shows a basic diagram of the active subsystem in a basic form for realizing the invention. A microphone (1) takes the Sound pressure p in front of the loudspeaker (5). Together with a microphone amplifier (2) that may be required, the pressure p is transmitted into an electrical voltage u 1. At the other end of the signal chain, a loudspeaker (5) produces a sound velocity v of its membrane by driving the loudspeaker via a power amplifier (4) with the electrical voltage u₂. The signal former (3) then serves to transform the microphone signal according to the invention in order to achieve the required acoustic termination impedance Z _b = p / v. The subdivision of the passive subsystem shown is a functional subdivision rather than an enumeration of necessary components. The required amplifications can also be easily integrated into the signal former. The function of the signal shaper according to the invention becomes particularly clear if one assumes that both on the microphone side the relationship between p and u 1 is linear and regardless of the frequency and on the speaker side there is such a relationship between v and u 2. Then the transmission factor u 1 / u 2 to be achieved by the signal shaper is, apart from a frequency-independent factor, equal to the terminating impedance Z _b according to the invention or the membrane impedance Z _m .

The assumed frequency-independent linear relationship between p and u ₁ is not a problem in the frequency and amplitude range of interest with conventional microphone technology. In contrast, loudspeakers generally have a frequency response of the acoustic oscillation with constant electrical control. The representation and calculation of this frequency response from the design data of the loudspeaker is possible with standard electroacoustic methods (see for example [18,19]). Methods for reducing this frequency response also belong to the prior art. It is therefore not necessary to go into this here. In connection with the invention, it is sufficient if the remaining frequency dependency of the loudspeaker is included in the frequency dependence of its transfer function to be achieved by the signal former (as multiplication of transfer factors or by addition of transfer measures, which are known to each other); it is then referred to as the "combined" transmission factor of the signal converter.

This inclusion of the loudspeaker frequency response in the combined transmission factor of the signal shaper according to the invention makes the application of the invention largely independent of the chosen type of loudspeaker (electrodynamic, electrostatic or magnetodynamic loudspeaker). The selection can be made according to aspects of the transmission range of the Loudspeaker, by cost, by operational reliability and by simple implementation of this combined transmission factor.

The realization of this combined electrical transmission factor is then the last subtask to apply the invention. The function of the signal shaper in the considered basic form of the implementation of the invention is essentially that of a frequency-dependent electrical filter. The filter characteristic required according to the invention (frequency curve of the transmission factor according to amount and phase) is known for the subtasks by using the described solution methods.

If one assumes that with today's known digital-electronic methods practically every transfer function can be realized with reasonable costs, which can be described mathematically (with somewhat continuous functions), then this last sub-task for the application of the invention is also solved by techniques known per se to watch.

Nevertheless, there is a cost interest in keeping the frequency response of the combined transmission factor as simple as possible by suitable selection and combination of the passive subsystem and the loudspeaker. As the local curves of the terminating impedance Z _b and the membrane impedance Z _m shown above as an example, it is an advantage of the invention that there is scope for design here. Such clever combinations make it possible to also implement the combined transmission factor according to the invention in electronic analog technology. A powerful electronic circuit technology is available for this purpose, so that there is no need to go into this here either.

Also of some interest are realizations of the combined transmission factor according to the invention by means of electronic "hybrid technology", that is to say by combining analog and digital components, in particular in the case of the self-adapting active subsystems with which - as will be explained further below - changes in the Operating temperature and / or the flow rate in the silencer can be "tracked".

As has already been repeatedly noted above, an advantage of the hybrid absorber according to the invention is that, by suitable selection of the components of the passive absorber, the locus of the terminating impedance Z _b can be given shapes which can be implemented by simple electronic circuits in the active subsystem.

Another useful embodiment of the invention is described with reference to Figure 24 . Figures 4, 5, 18, 20, 21, 22 each show a hybrid absorber element according to the invention. To line a silencer, in the basic form of the invention, several of these elements are arranged one behind the other in the direction of the channel axis (direction of sound propagation) until a length of the silencer is produced which provides the required transmission loss. Figure 24 shows schematically that in this embodiment of the invention, neither the control microphone nor the signal former need to be repeated for each element. The spreading constant Γ = Γ '+ jΓ''in the damper channel means both the decrease in the sound pressure level between neighboring hybrid absorber elements of width a (namely 8.68 · Γ'a dB) and the phase shift φ (namely 180 · Γ''a / π degrees) known. In Figure 24, these level and phase changes in the excitation of successive loudspeakers are simulated in an electrical chain circuit.

In a further embodiment of the invention, use is made of the fact that loudspeakers are reciprocal converters; This means that they are not only useful for sound generation, but can also be operated as sound recorders. When this property is applied to the invention, the loudspeaker of the active subsystem is switched in "switching breaks" as a sound pickup, which measures the sound pressure which penetrates from the damper channel through the passive absorber. Periods of one to a few seconds are sufficient for these switching breaks. The control microphone can thus be omitted; it is replaced by a timed switch.

In the previous embodiments of the active subsystem according to the invention, its implementation is essentially an electronic control task, which incidentally is seen as an advantage of the invention over the methods of active noise protection, since these basically represent a control task. In an embodiment of the invention which may be expedient, the function of the active subsystem according to the invention can also be conceived as a control task. The principle is explained using the schematic figure 25 . In addition to the basic equipment according to Figure 3 , the sound velocity v of the membrane is recorded by a structure-borne noise sensor (4) placed on the loudspeaker membrane. The signal shaper is thus controlled using control technology methods known per se so that the quotient of the pressure p recorded by the microphone and the sound velocity v recorded by the structure-borne sound sensor forms the terminating impedance Z _b required according to the invention. This configuration As a control task, it can be useful "to compensate for changes in the properties of the passive subsystem that are difficult to predict, be it, for example, due to fluctuations in the production of the passive absorber, be it due to difficult to predict temperature profiles in the latter, or be it due to changes during the operation of the muffler, for example by Dirt deposits.

In a further control-technical embodiment of the invention, the primary target variable of the silencer, namely the drop in sound level over a distance Δx of the channel, can also be used as a control variable for optimizing the terminating impedance according to the invention. To do this, measure the sound pressure at two microphone locations located one behind the other at a convenient distance according to the diagram in Figure 26 and regulate the amount and phase of the loudspeaker vibration so that this drop in level is maximized.

8. Side effects and other refinements

For the technical applicability of an invention - and thus for its invention value - it is decisive how application-related deviation from standard situations affects the desired performance of an invention - here the achievable noise reduction.

A first tacitly accepted standard situation is the existence of a predominantly symmetrical sound field, which, coming from the noise source, falls on the silencer (symmetrical means: in-phase vibration of the sound field on both sides of the symmetry plane of the damper channel). In fact, this is usually the case. However, probe cases of the sound source and / or the duct in front of the muffler are also conceivable, where the sound field vibration is predominantly antisymmetric (opposite in phase on both sides of the plane of symmetry). Then the determination equation (6) applies instead of the determination equation (5). This is to be taken into account in the design of the muffler according to the invention with a rectangular cross-section simply by using the value U _wp = 3.71944 + j · 1.89528 for the absorber function U at the winding point according to [5].

A second standard assumption is that the channel-side surface of the absorber lining is homogeneous, that is to say without defects and structures. By constructing the absorber lining according to the invention from hybrid absorber elements, in particular when using the parallel connection or mixed arrangements, surface structures arise. However, it is now known that such imperfections produce higher modes at most. However, since these are damped more than the lowest damped mode for which the silencer according to the invention is designed, a disadvantageous effect can be excluded.

Silencers in technical systems are often used at elevated operating temperatures. These change the acoustic properties of the sound-conducting medium in the damper duct. However, it should now be noted that the design steps presented above have been described in dimensionless form. They therefore apply to any gaseous medium, in particular also to a medium that has been changed from normal air by an increased temperature and / or composition (flue gases!). These operating conditions only manifest themselves when you use numerical values for the material constants that occur. These are the speed of sound c _o in k _o = 2πf / c _o , the density ρ _o in Z _o = ρ _o c _o , furthermore the viscosity and the adiabatic exponent in Ξ and thus in Γ _an and Z _an . The influence of temperature and gas composition on these quantities is known from thermodynamics. It can therefore be included in the determination of the terminating impedance Z _b if one wishes to design a silencer according to the invention for a specific operating temperature and / or gas composition. If operating temperatures change frequently and significantly during operation, it may be necessary to provide a self-adaptive active subsystem rather than operating with a specific operating temperature. As a rule, this will make digital or hybrid electronic control technology necessary for the signal former, in which the consideration of an operating temperature recorded by means of a thermometer does not pose any fundamental problems.

mit dem sogenannten "Strömungsfaktor"

Zwar empfiehlt die theoretisch-akustische Literatur auch noch eine Änderung der Randbedingung (von Schnelle-Anpassung zu Elongations-Anpassung an Randflächen), was zu der weiteren Ersetzung G_w→G_w·w führen würde, jedoch zeigen Vergleiche zwischen Messung und Rechnung eine bessere Übereinstimmung, wenn man diese zusätzliche Ersetzung nicht vornimmt. Damit geht die Gl.(4) über in:

wobei für z nunmehr statt Gl.(5) die Bestimmungsgleichung gilt:

$\text{z · tan (z) = jU · w (16)}$

und entsprechend wird bei den anderen oben angegebenen Bestimmungsgleichungen U→ U·w ersetzt. Durch das Auftreten des Strömungsfaktors w in den Bestimmungsgleichungen verschiebt sich nun auch der Windungspunkt U_wp(M_o) in Abhängigkieit von der Machzahl. Mit dem in [5] beschriebenen Verfahren läßt sich bei einer gegebenen Machzahl M_o der zugehörige Windungspunkt numerisch bestimmen. Dabei wird M_o positiv angesetzt, wenn Schall und Strömung im Schalldämpfer die gleiche Richtung haben, und M_o wird negativ angesetzt wenn sie einander entgegengesetzt gerichtet sind.The consideration of a flow with a flow velocity V _o and a Mach number M _o = V _o / c _o is somewhat more complicated. In terms of flow acoustics, the flow superimposition can be completely recorded by replacing wherever k _o occurs with:

${\text{k}}_{\text{O}}{\text{→ k}}_{\text{O}}\text{· W (13)}$

with the so-called "flow factor"

Although the theoretical-acoustic literature also recommends a change in the boundary condition (from rapid adaptation to elongation adaptation at boundary surfaces), which would lead to the further replacement G _w → G _w · w, comparisons between measurement and calculation show a better one Agreement if you do not make this additional replacement. Eq. (4) thus merges into:

the equation of determination now applies to z instead of Eq. (5):

$\text{z · tan (z) = jU · w (16)}$

and, accordingly, U → U · w is replaced in the other determination equations given above. Due to the occurrence of the flow factor w in the determination equations, the winding point U _wp (M _o ) is now also _shifted depending on the Mach number. With the method described in [5], the associated winding point can be numerically determined for a given Mach number M _o . M _o is set positive if sound and flow in the silencer have the same direction, and M _o is set negative if they are directed in opposite directions.

When applied to the muffler according to the invention, the consideration of a flow superimposition essentially means an exchange of the constant U _wp . If the flow velocity changes frequently and strongly during operation of the silencer, an expedient embodiment of the invention consists in introducing this change in the constant U _wp into a self-adapting active subsystem using known control engineering methods, using the display of a flow velocity Measuring device. With simple frequency responses of the combined transmission factor, this can be accomplished in electrical analog technology, with more complex frequency dependencies, regulation in hybrid or in digital technology can be indicated.

9. Technical progress of the invention

The technical progress of the invention of the hybrid silencer compared to the state of both passive damper technology and (and in particular) compared to active damper technology is to be briefly described. This will also result in further details of a preferred embodiment of the invention.

First of all, the importance of the task of improving the sound absorption of silencers in technical systems is explained using a practical example. A flue gas silencer in a link design according to Figure 2 for a medium-sized thermal power plant with an optimized design according to Figure 10 had a length L = 7m with a total width and height of 7x7 m². The gate thicknesses were D = 0.2 m, the gap widths H = 0.1 m, i.e. d / h = 2. The absorber filling had a flow resistance R = 1.7. Figure 10 then shows an attenuation D _h = 0.1 dB at f = 100 Hz, i.e. at f · h = 5 Hz · m. At this frequency, the silencer has a total attenuation of D _d = 14 dB. At the frequency f = 50 Hz, at which noise protection requirements were also made, the total attenuation was only D _d = 3.5 dB. A target price per square meter of backdrop area results in a silencer price of around DM 0.5 million. The average flow velocity in the backdrop columns was V _o = 50 m / s. This results in a drag coefficient (the ratio of pressure loss to dynamic pressure of the flow) of ζ = 2.5. With this information it can be calculated that the flow loss in the silencer, which must be provided by the blowers to push the air through the silencer, is around 3,000 kW; with an efficiency of 80% of the blowers, an electrical output of around 3 800 kW is applied. With an electricity tariff of 0.10 DM / kWh, the silencer causes annual operating costs of 3.33 million DM / a due to its pressure loss, i.e. many times its purchase value. Here a normal temperature T _o was still calculated for a gas flow. At a higher operating temperature of T Kelvin, the power loss increases by a factor of (T / T _o ) ^2.5 for the same mass throughput, i.e. at an operating temperature of around 300 degrees Celsius by around a factor of 5.8, which results in annual operating costs of around 19.3 million DM / a leads.

Assuming that the overall cross section of the muffler is retained, a muffler according to the invention, which is designed for D _h = 9.5 dB (see damping examples below) with an exhibition ratio d / h = 0.5, could not only dampen at f = 50 Hz to D _d = 15 dB and at f = 100 Hz to D _d = 25 dB, furthermore the flow velocity could be halved to V _o = 25 m / s and the drag coefficient would decrease to ζ = 0.35. This would reduce the operating costs (at normal temperature) to around 62 TDM / a, and to around warm when operating 360 TDM / a. Even a multi-expensive silencer according to the invention would have (with significantly better sound insulation!) Refinanced its higher investment costs after a short time from the savings in operating costs (apart from the fact that the fans would be cheaper because of the lower power required!).

Figure 27 shows an example of an application of the invention as a solid curve, the damping for a hybrid absorber consisting of a mineral fiber layer with the parameters d / h = 0.5 and R = 0.5 and an active subsystem, which is a linear approximation of the Eq. (7) generated in k _o h for the terminating impedance Z _b , the exact locus of which is shown in Figure 14 for this example. For comparison, the damping curve with the passive absorber alone is shown in dashed lines. This example not only shows the high gain in attenuation at low frequencies by the muffler according to the invention, it also illustrates that such gains can already be achieved with simple approximations to the prescribed terminating impedance Z _b , and it suggests a further embodiment of the invention. At frequencies above approximately fh = 70 Hz · m, the approximation used for Z _b obviously no longer applies; here the absorber lining of the passive subsystem alone produces higher damping than the absorber lining with the hybrid absorber in the design used. In such cases, it makes sense to "switch off" the active subsystem at higher frequencies. This means that the termination impedance Z _{b should} then have large amounts. With a hybrid absorber connected in series and with an electrodynamic loudspeaker, this can be achieved simply by the signal former generating an electrical short circuit on the loudspeaker. Then the membrane is known to be "braked" electrodynamically.

Figure 28 also shows the damping curves of a hybrid absorber (solid) with a structure and an exact locus for Z _b as in Figure 15 , whereby an approximation to the exact curve was used here as well, and (dashed) the damping curve with the passive absorber alone. This example is of interest for practical use because the front of the absorber layer is covered by a film (with m _p = 1), which means that the hybrid absorber can be effectively protected against possible dirt in the sewer flow.

The last figure 29 finally shows corresponding damping curves for a passive absorber lining already dimensioned broadly according to figure 10 , namely that described in figure 16 . In contrast to the two aforementioned exemplary embodiments, the Termination impedance is not obtained from analytically derived approximations, but through interpolation of measured values, as described above. Since the limitations of an analytical approximation are eliminated in such a procedure, the damping achieved comes closer to the optimum value.

In addition to the improvements in sound attenuation demonstrated in the examples, the invention has further technical advantages over the prior art, which are listed below.

The implementation of the active subsystem is an electronic control task instead of - as with the active silencer - a control task. This is seen as an advantage, since experience has shown that tax tasks are easier to solve than standard tasks. Nevertheless, the active subsystem according to the invention can be supplemented with control technology, which allows changes in the operating parameters to be “tracked”.

In the invention, the control variable is the total pressure at the measuring location and not - as in the case of the active silencer - the sound pressure of the incident wave, which has to be recovered from the measured sound pressure using complex algorithms.

In contrast to the active silencer, the use of the silencer according to the invention is independent of the sound field distribution in the silencer duct. This is important because the sound pressure distribution in technical systems can neither be reliably predicted nor kept constant. The silencer according to the invention can therefore be developed and dimensioned independently of the noise source in which it is to be used, while active silencers have to be adapted to the respective noise source.

The acoustic field impedance to which the loudspeaker of the active subsystem works can be set over a wide range by appropriate selection of the parameters of the passive subsystem. This has the advantage that relatively "hard" speakers can be used. These are usually cheaper and more robust in operation. In addition, when the hybrid absorber is connected in series, the sound pressure level at the loudspeaker - and thus the required elongation of the loudspeaker diaphragm - is reduced by the upstream passive absorber.

An important advantage of the invention for technical application is that the components of the active subsystem are not - as in the active one Silencers - are directly exposed to the flow. The upstream passive absorber can be used as a heat insulation layer in hot gas flows, since porous fiber absorbers generally also have good heat insulation. The upstream passive absorber can also take on the function of a dirt filter in the invention.

Also important for the use in flow channels is the property of porous passive absorber materials that they have a rectifier effect on turbulent pressure fluctuations. This means that the fields of the turbulent pressure fluctuations break down faster in an absorber layer than the fields of the sound wave, since at the low frequencies that are mainly of interest, the characteristic diameters of the turbulent pressure fluctuations are much smaller than the sound wavelengths. The problem which arises in the case of the active silencer, of having to separate acoustic and turbulent pressure fluctuations from one another, is largely eliminated in the invention.

Another advantage of the muffler according to the invention compared to the active muffler is seen in the fact that, in the event of a failure of the active subsystem (during maintenance or in the event of a fault), the passive absorber can ensure a certain remaining sound absorption.

Another advantage of the muffler according to the invention over the active muffler is that the transition to other channel cross sections can essentially be taken into account by changing the constant U _wp and that - unlike with the active muffler - new algorithms for regulating and new arrangements of the loudspeakers do not develop Need to become.

Finally, it is an extremely helpful advantage in the planning and dimensioning of a muffler according to the invention that the individual design steps can be broken down into standard tasks (such as impedance measurement, determination of converter constants, design of control circuits etc.), while with the active muffler practically only experiments on the complete system can be.

On the other hand, the fact that the invention does not constitute an obvious further development of known technical teachings can be recognized only from the functions which are assigned to a porous absorber layer in a silencer according to the invention. Such a layer provides thermal protection for the active subsystem; it is a dirt and turbulence filter; it lowers the sound pressure level at the location of the active subsystem due to the internal one Propagation damping in the absorber material and thereby makes loudspeakers with a smaller stroke possible; it acts as a passive damper at high frequencies; it ensures residual damping in the event of a fault in the active subsystem; as a passive damper, it ensures the damping of higher channel modes; As a resistance rectifier, it accomplishes pressure equalization in the transverse direction in the case of non-uniformly vibrating loudspeaker membranes.

• Serienschaltung des passiven und des aktiven Elements (vgl. Abb. 4)
• Parallelschaltung des passiven und des aktiven Elements (vgl. Abb. 5)
• Mischtypen hiervon (vgl . Abb. 20-22)

Bezüglich des aktiven Subsystems bestehend aus Signalaufnehmer (1), Signalformer (9) und Schallgeber (3) als elektroakustische Varianten bei entsprechender Ausbildung des Signalaufnehmers:

• Steuerung über Mikrofon oder Lautsprecher-Mikrofon (vgl. Abb. 3 und 23)
• Steuerung über Körperschall-Aufnehmer mit Mikrofon (vgl. Abb. 25)
• Regelung über Kanal-Mikrofone (vgl. Abb. 26)

Als elektroakustische Variante sei besonders die der Steuerung über Mikrofon nach Abb. 3 hervorgehoben, wobei der Quotient aus dem Flächenmittelwert der Schnelle der Lautsprechermembrane und dem Schalldruck vor derselben eine erfindungsgemäße akustische Abschlußimpedanz Z_b bildet, die im Fall der akustischen Serienschaltung des passiven Absorbers und des aktiven Subsystems gemäß der oben angegebenen Gleichung (7) und im Fall der akustischen Parallelschaltung des passiven Absorbers und des aktiven Subsystems durch eine Kombination der oben angebenen Gleichungen (12) und (7) festgelegt wird.In summary, the following are possible ways of realizing the invention:
Regarding the basic acoustic forms:

• Series connection of the passive and the active element (see Fig. 4)
• Parallel connection of the passive and active elements (see Fig. 5)
• Mixed types of these (see Fig. 20-22)

Regarding the active subsystem consisting of signal pick-up (1), signal former (9) and sound generator (3) as electro-acoustic variants with appropriate design of the signal pick-up:

• Control via microphone or loudspeaker microphone (see Fig. 3 and 23)
• Control via structure-borne sound sensor with microphone (see Fig. 25)
• Control via channel microphones (see Fig. 26)

As an electroacoustic variant, the control via microphone according to Fig. 3 is particularly emphasized, the quotient of the area mean value of the speed of the loudspeaker membrane and the sound pressure in front of the latter forming an acoustic terminating impedance Z _b according to the invention, which in the case of the acoustic series connection of the passive absorber and the active subsystem according to equation (7) given above and in the case of acoustic parallel connection of the passive absorber and the active subsystem by a combination of equations (12) and (7) given above.

10. Description of the pictures

Fig.1:

Rectangular silencer with clear duct width H = 2h and absorber lining of thickness d made of porous absorber material with length-related flow resistance Ξ, with a sound-permeable cover layer on the duct side.

Fig.2:

Baffle silencer with baffles of thickness D = 2d and gap widths H = 2h.

Fig.3:

Schematic drawing of the active subsystem with sound pickup (1), signal former (2), loudspeaker (3)

Fig.4:

Series connection of the passive (1) and the active (2) subsystem.

Fig.5:

Parallel connection of the passive (1) and the active (2) subsystem.

Fig.6:

Solution of Eq. (5); 1. Damper mode.

Fig.7:

Solution of Eq. (5); 2. Damper mode.

Fig.8:

Sound absorption in a rectangular silencer with a clear duct width H = 2h with an absorber lining, whose absorber function U has the value U = U _wp .

Fig.9:

Sound absorption in a rectangular silencer with a clear duct width H = 2h with optimal absorber lining, with hybrid absorber lining with passive absorber of thickness d made of porous absorber layer with flow resistance R = Ξd / Zo and with purely passive lining.

Fig.10:

Approximations for damping curves of rectangular silencers with a clear duct width H = 2h and absorber lining of thickness d made of porous absorber material with length-related flow resistance Ξ, optimized for broadband performance [1].

Fig. 11:

Sound absorption with a passive absorber lining made of so-called "membrane absorbers", a resonator combination tuned to low frequencies.

Fig.12:

Schematic drawing of an active silencer with control microphone (1), signal former (2), anti-noise speakers (3)

Fig.13:

Schematic drawing of an active silencer with adaptive control;
Control microphone (1), signal former (2), anti-noise speaker (3), correction microphone (4), controller (5).

Fig.14:

Locus of the terminating impedance Z _{b of} an absorber layer of thickness d with d / h = 0.5 with normalized flow resistance R = 0.5

Fig.15:

Locus of the terminating impedance Z _{b of} an absorber layer of thickness d with d / h = 0.5 with normalized flow resistance R = 1.0 covered with a foil with m _p = 1

Fig.16:

Locus of the terminating impedance Z _{b of} an absorber layer of thickness d with d / h = 0.5 with standardized flow resistance R = 3.5 of a passive broadband absorber.

Fig.17:

Locus of the terminating impedance Z _{b of} an air layer of thickness d with d / h = 0.5 covered with a normalized flow resistance R _S = 2.0 and a foil with m _p = 1.0.

Fig.18:

Series connection of the passive (1) and the active (2) subsystem, with a loudspeaker box (3).
The terminating impedance Z _b and the input impedance of the box Z _e are indicated

Fig.19:

Locus of the membrane impedance Z _m .

Fig.20:

Combination of a series connection (1) with a parallel connection (2) of the passive and the active subsystems.

Fig.21:

Combination of a series connection (1) with a parallel connection (2) with a common active subsystem.

Fig.22:

Combination of a series connection (1) with a parallel connection (2) with a common active subsystem using both diaphragm sides of the loudspeaker.

Fig.23:

Schematic drawing of the functional components of the active subsystem with sound pickup (1), microphone amplifier (2), signal former (3), power amplifier (4), loudspeaker (5)

Fig.24:

Hybrid absorbers (1) arranged one behind the other with an active subsystem consisting of a guide element (2) and a circuit of attenuators and phase rotators to simulate the change in level and phase of the sound wave in the damper channel (4).

Fig.25:

Series connection of a passive (1) and an active (2) subsystem, which regulates the ratio of the sound pressure from the microphone (3) and the speed from the structure-borne sound sensor (4) to the required ratio of the terminating impedance.

Fig.26:

Series connection of a passive (1) and an active (2) subsystem, which maximizes the difference in sound levels at two microphones (3) arranged in series in the damper channel (4). Value regulates.

Fig.27:

Attenuation curves of a silencer with a passive absorber and a termination impedance according to Fig.14;
solid: hybrid absorber;
dashed: purely passive absorber.

Fig.28:

Attenuation curves of a silencer with a passive absorber and a termination impedance according to Fig.15;
solid: hybrid absorber;
dashed: purely passive absorber.

Fig.29:

Attenuation curves of a silencer with a passive absorber and a termination impedance according to Fig.16;
solid: hybrid absorber;
dashed: purely passive absorber.

11. Literature used

• [1] MECHEL, FP
  "Silencer"
  in "Taschenbuch der Technischen Akustik", Edit.M. Heckl, HAMüller Springer-Verlag, Berlin, 1975
• [2] MECHEL, FP
  "Sound absorption"
  in "Taschenbuch der Technischen Akustik", Edit.M. Heckl, HAMüller Springer-Verlag, Berlin, 1975
• [3] MORSE, PMJAcoust.Soc.Amer. 11 (1939) 205-210
  "Transmission of sound inside pipes"
• [4] CREMER, L. Acustica, Supplements 3 (1953) 249-263
  "Theory of airborne sound insulation in a rectangular duct with a swallowing wall and the resulting highest degree of damping"
• [6] GERBER, O. Acustica, Supplements 2 (1953) 264-270
  "Experimental investigations to realize the theoretically possible maximum attenuation of sound propagation in a rectangular air duct with swallowing walls"
• [7] SHORT, U. Acustica 21 (1969) 74-85
  "Sound propagation in the canal with periodic wall structure"
• [8] KÖLTZSCH, P.
  "Sound absorbing channels"
  in "Noise Control", Edit. W. Schirmer Verlag Tribüne, Berlin, 1971 [9] ESCHE, V.
  "Ventilation systems and silencers"
  in "Taschenbuch Akustik", W.Fasold, W.Kraak, W. Schirmer Edit. VEB Verlag Technik, Berlin, 1984
• [10] VDI guideline 2567
  "Sound insulation through silencers"
• [11] FUCHS, H., et al
  "Sound absorbing component", DE-Pat. 3412432 (1984/1989)
  "Silencer box", DE-Pat. 3504208 (1985/1989)
• [12] LUEG, P. US Pat.No. 2,043,416; (1934/1936)
  "Process of silencing sound oscillations"
  LUEG, P. DRP No. 655 508; (1933/1937)
  "Method for damping sound vibrations"
• [13] GUICKING, D. Progr. Akustik, DAGA '89 (1989) 23-36
  "Active noise protection - successes, problems and perspectives"
• [14] GUICKING, D.
  "Active Noise and Vibration Control. Annotated Reference Bibliography"
  3rd ed. 1988; III. Physik.Inst.Univ.Göttingen, 3400 Göttingen
• [15] MUNJAL, ML, ERIKSSON, LJ
  J.Acoust.Soc.Amer. 86: 832-834 (1989)
  "Analysis of a hybrid noise control system for a duct"
• [16] MECHEL, FP
  "Sound absorber", Vol.I
  Hirzel Verl., Stuttgart, 1989
• [17] MECHEL, FP progress acoustics, DAGA '84 (1984) 31-52
  "Sound absorber"
• [18] LENK, A.
  "Electromechanical systems. Vol. 1: Systems with concentrated parameters"
  3rd edition, VEB Verl.Technik, Berlin, 1975
• [19] REICHARDT, W.
  "Electroacoustics"
  Teubner Verlagsges., Leipzig. 1971
• [20] DE 34 25 450 Al
• [21] DE 27 12 534 Al

Claims (20)

Hybrid silencer consisting of a passive absorber lining and an active electroacoustic subsystem, characterized in that the electroacoustic active subsystem having a control microphone and a loudspeaker acts on the passive absorber combined with it in such a way that the acoustic impedance of the channel sides in the hybrid absorber thus formed Surface of the absorber lining gives an absorber function U, which in a desired frequency range reaches or approaches the value of the absorber function U _{wp of} the winding point between the first and the second silencer mode. Hybrid muffler according to Claim 1, characterized in that, in the case of predominantly symmetrical sound fields in the muffler duct, the absorber function U _wp at the winding point lies between the first and the second symmetrical muffler mode, and that in the case of predominantly antisymmetrical sound fields in the muffler duct, the absorber function U _wp at the winding point lies between the first and the second antisymmetric silencer mode. Hybrid silencer according to Claim 1 or 2, characterized in that the passive absorber is made from components known per se, such as porous absorber materials, air layers, membranes, foils, flexible plates, cover layers with flow resistances such as nonwovens, felts, fabrics, perforated sheets etc., and also from acoustic ones Resonators known per se, such as Helmholtz resonators, plate and film resonators or combinations of such components. Hybrid muffler according to at least one of claims 1 to 3, characterized in that the hybrid absorber represents the lining of muffler ducts with a rectangular, round or ring-shaped cross section or that it forms so-called backdrops in such ducts. Hybrid silencer according to at least one of Claims 1 to 4, characterized in that the active subsystem consists of a control microphone, possibly with a microphone amplifier, for measuring the sound pressure in front of the loudspeaker, from this loudspeaker, optionally with a power amplifier, and from an intermediate circuit There is an electronic signal former which controls the loudspeaker in such a way that the quotient of the area mean value of the speed of the loudspeaker membrane and the sound pressure in front of it forms an acoustic termination impedance Z _b according to the invention. Hybrid silencer according to one of claims 1 to 5, characterized in that the passive absorber and the active subsystem are arranged in acoustic series connection. Hybrid silencer according to at least one of claims 1 to 6, characterized in that the passive absorber and the active subsystem are arranged in acoustic parallel connection. Hybrid silencer according to at least one of claims 1 to 7, characterized in that combinations of series connection and parallel connection of the passive absorber and the active subsystem are used. Hybrid silencer according to at least one of claims 1 to 8, characterized in that the rapid on one side of the loudspeaker membrane is used for the terminating impedance in series connection and the rapid on the other side of the loudspeaker membrane is used for the terminating impedance in parallel connection. Hybrid silencer according to at least one of claims 1 to 9, characterized in that reciprocal loudspeakers are used in the active subsystem and that the loudspeaker is used to measure the sound pressure on its membrane at timed intervals. Hybrid silencer according to at least one of claims 1 to 10, characterized in that, in addition to the control microphone for measuring the sound pressure in front of the loudspeaker membrane, a structure-borne sound sensor is placed on the loudspeaker membrane, which measures the speed of the membrane, and with known electronic control circuits the quotient of sound pressure and fast is regulated to the terminating impedance according to the invention. Hybrid silencer according to at least one of Claims 1 to 11, characterized in that in the case of hybrid absorber elements which are arranged one behind the other in the direction of sound propagation, the control of the loudspeakers of the subsequent elements is derived by electronic attenuators and phase rotators in such a way that they reduce the level drop and Phase speed corresponds to the lowest damped damper mode in the damper channel. Hybrid silencer according to at least one of claims 1 to 12, characterized in that the drop in the sound level is measured at consecutive points in the damper channel with microphones and that the terminating impedance of the loudspeakers of the active subsystems used to maximize the drop in level by known electronic controllers is regulated. Hybrid silencer according to at least one of Claims 1 to 13, characterized in that the operating temperature of the silencer is measured and that the parameters of the signal former of the active subsystem are controlled using control techniques known per se, using this temperature as an input variable, so that the surface impedance described is maintained at different operating temperatures. Hybrid silencer according to one of claims 1 to 14, characterized in that the flow velocity is measured in the silencer duct and that the parameters of the signal former of the active subsystem are known using control techniques using this flow velocity as an input variable and the absorber function U _wp valid at the respective flow velocity in the winding point are regulated as a system variable such that the surface impedance described in claim 1 is maintained at different flow velocities. Hybrid silencer according to at least one of claims 1 to 15, characterized in that the signal shaper of the active subsystem is constructed in electronic analog technology or in electronic digital technology or in hybrid technology from analog and digital components. Hybrid silencer according to at least one of claims 1 to 16, characterized in that the passive absorber is designed as a heat-insulating layer. Hybrid silencer according to at least one of claims 1 to 17, characterized in that the passive absorber is protected by cover layers on its front against the ingress of dirt and / or moisture and that these cover layers in the acoustic characteristics of the passive absorber and in the dimensioning of the Signal formers of the active subsystem are included. Hybrid silencer according to at least one of claims 1 to 18, characterized in that the length-specific flow resistances of porous absorber layers and / or the flow resistances of cover layers of the passive absorber are selected so that in an air gap arranged in front of the loudspeaker membrane there is a transverse compensation of the sound pressure with an uneven vibration amplitude of the speaker over its surface. Hybrid silencer according to at least one of claims 1 to 19, characterized in that the passive absorber is dimensioned according to known rules in such a way that if the active subsystem fails, the silencer has a certain residual damping.

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