GB2049035A - Silencing Gaseous Flow - Google Patents

Abstract

A noise reduction system for pipelines includes a plurality of expansion spaces 3, 5, 7, 9 between an inlet 1 and an outlet 10, the expansion spaces 3, 5, 7, 9 being connected together and to the inlet 1 by pipes 2, 4, 6, 8 at least one of which, 2, is divided into passages extending different distances into an associated expansion space 3. The passages are formed by a pair of concentric pipes separated from each other by longitudinally extending plates of different lengths. The attenuation is made greatest for frequencies having the greatest sound input to the system. An expression is given for the minimum length of pipe 4 between the first and second expansion spaces 3 and 5. The wall 14 of the expansion spaces may be perforated or imperforate and in the former case a sound absorbent material 13 and an airtight cover 15 surround the walls 14. <IMAGE>

Description

SPECIFICATION Apparatus and System for Noise Reduction in Flowing Media The invention concerns reduction of noise in flowing gaseous media or multi-phase media, e.g.

a mixture of gas and dust or pulverulent material, e.g. in the suction and delivery ducts of compressors, in the induction and exhaust ducts of internal combustion engines, in dust removal systems, in pneumatic material conveying systems, and in ventilation and air-conditioning ducts.

Numerous solutions are known for damping noise travelling in flow media. Such solutions generally consist of chambers and transfer ducts between the chambers, closed resonator chambers and the like which are connected in parallel via apertures or pipes to the duct serving to convey the flowing medium.

Devices are known which in addition to the above-mentioned constructions or in place thereof, contain or are filled with sound-absorbing material.

Also known are constructions wherein it is desired to increase the frictional losses in the pipes between the individual chambers and to this end contain elements which increase the overall surface area in the direction of flow. These known construction or principles are utilised partly or wholly among others in Hungarian patent specification Nos. 148470, 164,184 and 157,390; U.S. patent specification No. 2,993,559 and West German patent specification No.

1,238,271.

It is a common characteristic of all known constructions that the acoustic mass representing the mass of gas in the ducts is regarded directly or indirectly as a quantity analogous to inductivity as used in electrical technology; the resilient or spring effect of the gas in the chambers is regarded as acoustic capacity analogous with electrical capacity and the acoustic impedances represented by these elements are connected in series or in parallel to produce acoustic filter circuits analogously with electrical filter circuits in an attempt to reduce the noise in ducts.

However, in most practical cases apparatus or devices constructed on this principle do not fulfil the hopes placed in them and the degree of noise attenuation produced by them is considerably less than the design value. This poor result is then sought to be improved by repeated experimental corrections in subsequent operational steps but because of the large number of the elements utilised and thus because of the very large number of the permutations that can take place the practical results of subsequent corrections result not only in high experimental costs but also in random results.Furthermore, to balance or cancel out the unforeseeable effects already at the design stage the damping of the apparatus is "over-dimensioned", i.e. planned to be deliberately excessive to the theoretical desideratum, but this leads to increased flow losses and thus a reduced efficiency in the associated machinery.

-One cause of the difference between the calculated and actual noise attenuation spectra is that the introduction and utilisation of concentrated acoustic impedance elements is based on an assumption which in'practice is not fulfilled; namely, these elements, i.e. pipes and chambers, should not be dimensioned in the direction of propagation of the noise so as to exceed one-eighth of the wavelength, and further, in its dimensions perpendicular to the direction of progapation of the noise there should not exceed the dimensions derived from the so-called "cut off" frequency which is approximately 1.3 times the wavelength; if this is not the case, then a transverse wave propagation in the duct may also arise and thus the conditions for planar wave propagation are not realised.

For the dimensions of the apparatus and the frequency bands to be attenuated, as utilised in practice, the conditions for preventing transverse dimensions are generally realised but the conditions or prescriptions relating to longitudinal dimensions are not realised at all. Disregarding this factor has the consequence that the equivalent circuit or image substituting the actual elements is not realistic and does not characterise sufficiently closely the acoustic noise transfer characteristics of the system and thus the accumulating errors arising from the connection of the impedances give rise to unanticipated transmitting bands in the attenuation spectrum which in the * and * The noise reduction apparatus for flowing media according to the invention is, in contrast to the prior art constructions, built up from acoustic wave conducting elements in which the parameters of acoustic pressure and particle velocity is not given or characterised by impedance type data but instead is given by a quantity depending on four frequencies, the socalled."acoustic four-pole parameters". The wave conducting elements are open ended and closed spaces of identical cross-section and of given length and do not therefore represent varying "acoustic masses" and "acoustic capacitances" but instead their behaviour as regards frequency is determined exclusively by the mutual states or conditions of the four-pole parameters.The acoustic transmission characteristics of the noise reduction apparatus for flowing media made up of acoustic wave conductors of identical character (in principle) is determined as a function of frequency by the interrelated values of the parameters of these four poles. The acoustic fourpole parameters of the individual elemental acoustic wave conductors are as follows: Oi 1=a22=chyx a12=Zoshyx a21=Zo 'shyx wherein: Zo is the critical acoustic impedance of the elemental acoustic wave conductor; y is the so-called propagation constant in the elemental acoustic wave conductor; and x is the length of the elemental wave conductor.

Naturally, in the case of a flowing medium the quantities Zo and y which themselves contain the velocity of sound also contain the so-called Mach number, namely the relation between the flwo velocity of the medium and the velocity of sound, in view of the fact that the co-ordinate system of the observation travels with the speed of the flow of media.

These acoustic wave conductors, regarded as four poles, can be connected in series or in parallel and can be branched.

In the technical literature, according to the rules for connecting four poles of given parameters, in principle the frequency dependent sound transmission of any acoustic four-pole system made up of elemental wave conductors can be determined.

This method of dimensioning a system made up of elemental acoustic wave conductors is completely novel and in accordance with the requirements of practice the conditions that arise can be accurately written down even for systems of geometrical dimensions which are larger by an order of magnitude than acoustic systems dimensioned by current methods.

Preferred embodiments of the invention are described merely by way of example in the accompanying schematic drawings wherein: Figure 1 is a connection diagram of a noisereduction apparatus for flowing media according to the invention, Figure 2 is a schematic view of the apparatus according to the invention, Figure 3 is a first practical embodiment of the noise reduction apparatus according to the invention, Figure 4 is a second embodiment of the noise reduction apparatus according to the invention, Figure 5 is an embodiment of an acoustic duct system with tuned attenuation, shown in elevation, broken away section and transverse section along the plane A-A, and Figure 6 is a perspective view partially broken away of the noise reduction apparatus according to the first embodiment.

The interconnected parts or spaces of the noise reduction apparatus according to the invention are in accordance with what was described above not dimensioned individually but instead matched to the desired noise attenuation band in accordance with the characteristics of the complex acoustic system including the source of noise and the characteristics of the acoustic environment, as a function of the mutual effect on each other of these parts. After noting the geometrical dimensions the sound transmitting characteristics are repeatedly determined by determining the discrete frequencies in the frequency band to be damped and by repeatedly modifying the changeable geometrical dimensions a numerical process is used for determining the final geometrical dimensions which match optimally to each other and to the noise spectrum to be attenuated.

In this way, it has been possible to separate the flow losses of the system from its acoustic characteristics and to reduce flow losses to a minimum even with the smallest geometrical dimensions and sudden changes in dimensions. In order to eliminate the pass bands appearing in the sound transmission characteristics of all known noise reduction apparatus operating on the socalled reactance principle, as well as to increase the sound attenuation without causing increased flow losses, a completely novel and hitherto unknown solution has been employed.In the noise reduction apparatus for flowing media according to the invention in the first widened or expansion space along the direction of flow a socalled "tuned" or "resonantly damped" acoustic pipe supply line has been used while in the further expansion spaces the use of such a pipe system was optional, each of these units consisting essentially of a plurality of parallel connected elemental wave conductors projecting to varying depths to the expanded space. With this completely novel constructional element the pass-bands could be completely eliminated from the desired sound or noise attenuation frequency band and the attenuation spectrum of the complete acoustic system could be rendered suitably uniform while significantly increasing the discrete attenuation values.A further advantage of the acoustic pipe system employing tuned damping is that it does not result in a change or increase in cross-section in the direction of flow and thus the flow losses specifically caused by it can in practice be taken as being zero or negligible.

The length of the elemental wave conductors projecting into the extension spaces of the acoustic pipe supply line system with tuned damping must satisfy the following relations: The length of the shortest elemental wave conductor lmin < li /2, while the length of the longest elemental wave conductor is: lmaxSlifmaV7t/ 112 wherein Imam, and Imin are respectively the length of projection of the longest and the shortest elemental pipe supply line while 11 is the dimension in the direction of flow of the i-th expansion space, and fmax is the cross-section transversely to the flow of the longest elemental pipe line.

The determination of the accurate dimensions with the above initiai values takes place with the above-described numerical approximation of optimum seeking methods.

To adjust the lower limiting frequency af the frequency band to be damped, in the course of planning or design, the following relation was used:

wherein: K0 is the wave number associated with the lower limiting frequency, 13 is the length of the first expansion space 3, 14 is the length of the elemental wave conductor 4, disposed between the first expansion space 3 and the second expansion space 5, and ZO is the critical acoustic impedance of the i-th space.

Further measures for increasing the noise attenuation without increasing flow losses have been used in the apparatus according to the invention.

The wave conductors 4, 6 and 8 disposed between the individual expansion spaces in the apparatus have been dimensioned as fas as their projection is concerned so that the specific input acoustic impendance af the wave conducting branching off formed by the section of the projections and the expansion spaces 3, 5, 7 and 9 should be zero for the pass-band frequencies in the system dimensloned without the projections.

For specific uses, e.g. for damping souces of noise which display a sudden increase in intensity at low frequencies, e.g. below 2QO Hz, regard was had for a branching-off 11, 12 connected to the wave conductor 4 connecting the first and second expansion spaces, this branching off possibly containing a plurality of parallel or series connected elemental wave conductors closed off with an infinitely large closing impedance, the geometrical dimensions of which have been determined in such a way that its specific input acoustic impedance should be zero in the pass- band to be eliminated, i.e. for the low frequency components representing suddenly increasing intensity.

The noise reduction apparatus according to the invention is always mounted in a ducting or pipe system that can be regarded as a complex acoustic system wherein a decisive role in played by the location of mounting from the point of view of the damping of the source of noise.

The apparatus according to the invention must be so mounted into the system that the specific input acoustic impedance of the acoustic subsystem consisting of the source of noise and the hardware from that source to the noise reduction apparatus should be zero for that frequency which represents the most significant noise intensity of the source of noise as well as to its other harmonics.

Exemplary embodiments of the noise reduction apparatus for flowing media according to the invention are shown in section in Figures 3 and 4 while the broken away view of Figure 6 represents the practical embodiment of the system shown in Figure 1. The apparatus consists of series-connected wave conductors 1, 3, 4, 5, 6, 7, 8, 9, and 10 to which a series-connected wave conductor branch 11, 12 is coupled.An acoustic pipe supply line system of tuned damping or attenuation designated by the reference number 2 projects into the first expansion space 3 and possibly to the second expansion space 5 and illustrated by way of example in Figure 5 so that it can be seen to consist of two concentric pipes between which there are longitudinal plates the lengths of which increase stepwise in a radial direction, and between these plates and the outer surface, the elemental wave conductors differ in their longitudinal dimensions. The configuration and geometrical dimensions of the elemental wave conductors are determined according to the above-described principles after due regard is taken of the source of noise and the acoustic environment. The elemental wave conductors making up the apparatus according to the invention may vary in cross-sectional shape.The bounding surfaces of the individual embodiments may be of the form of smooth and planar plates but for certain purposes of use, e.g. for damping the exhaust ducts of internal combustion engines or for use in the suction and delivery ducts of compressors, etc, the bounding surfaces 14 may expediently be perforated and the latter then may be connected or be lined with a porous fibrous woven sound-absorbing material; in the interests of the ready removal and replacement of wornout sound-absorbing material, and of the access to the apparatus for cleaning, the whole apparatus may be covered with an airtightly closed cover 1 5.

In the embodiment where a sound-absorbing lining 1 3 is used it is necessary to prevent a bypass for sound transmission via the perforated plate 14 and this is done by providing a plate strip passing through the complete periphery of the apparatus in the plane separating the expansion spaces, and passing through the lining 13 up to the outer plate 1 5. To prevent sound transmission along the path of noise via secondary radiation, the plates defining the spaces should be of a double layer or double-walled construction, connected to each other by a random point-like fastening, The first and second exemplary embodiments in Figures 3 and 4 differ from each other in that the construction in Figure 3 displays a change of the direction of flow in the expansion space 5 and by concentrically forming the wave conductors 4 and 8 the length of the construction has been significantly decreased; furthermore, the wave conductor 4 is connected to a supply branch line which is closed and consist of members 11 and 12.In contrast, constructional embodiments in Figure 4 also contain in the expansion space 5 the projecting acoustic pipe supply line of tuned damping designated by the reference number 2; furthermore the conductor branch 11 and 12 is omitted and the wave conductor 4 is longer, optionally flexible, so as to result in a noise reduction apparatus and system for flow media comprising two separately manipulatable constructional units.

The noise reduction apparatus and system for flowing media operates as follows: the noise radiated from the source of noise to the ducting system is reflected to a significant extent at the inlet opening 1 as a consequence of the dimensioned disposition of the noise reduction apparatus, and then passes through the pipe supply line system of tuned damping designated by the reference number 2; in doing so, because of the wide-band acoustic mis-match, it is strongly attenuated. This process is similarly repeated as the noise passes through the wave conductor 4 and optionally again through the pipe supply line system 2, and then the attenuation and the attenuation gradient of the pass-bands are significantly increased on passing through the expansion spaces 5, 6, 7, 8, and 9.The wave conducting branch 11, 12 adjusted to the pass frequencies and the branch lines cut from the expansion spaces give rise to acoustic reflections which eliminate the pass bands. The noise reduced to a permissible value continues to advance along the duct connected to the wave conductor 10 which can be located at any desired position and thus passes through to the acoustic environment. The direction of the flow of the medium may in this apparatus agree with or be opposite to the direction of propagation of the sound and this is taken into account by the suitable choice of the sign (+ or-) of the Mach number in the four-pole parameters.

An advantage of the apparatus and system according to the invention when compared with the prior art is that it has a particularly wide attenuation frequency range of very nearly two decades without pass-bands, very high noise attenuation values of in excess of 40-50 dB. In spite of the greater acoustic efficiency, the resistance to flow of the system does not exceed that of known devices of similar purpose. The possibilities of changing the constructional form make it equally suitable for fixed installations or mobile use.

Claims (11)

Claims 1. Noise reduction apparatus and system for flowing media, particularly for attenuating noises propagating in ducts and pipes, which consists of a plurality of inter-connected enclosure parts, and which between the source of noise radiating into the ducting and the end which connects to the free atmosphere or to rooms or to further apparatus is disposed at a predetermined spacing from the source of noise, this spacing or distance being determined by the radiation characteristics of the source of noise in such a way that the acoustic part system consisting of the source of noise and of the duct extending from the source of noise to the apparatus itself has a specific acoustic input impedance at the inlet opening of the apparatus which is zero for that frequency from among the basic frequencies of the noise source, which respresents the most significant noise intensity as well as to the upper harmonics of this noise; furthermore, the acoustic fourpole network made up of acoustic wave conductors represented by the enclosures of the system and including the source of noise, the duct from the source of noise to the apparatus and the duct from the apparatus to the point of observation, is formed with predetermined longitudinal and transverse dimensions and interrelated configuration, characterised in that:: - the noises propagating in the flowing media are introduced into the apparatus via an acoustic supply pipe line of tuned damping which consists of wave conductors that project into the expansion space to vary extent, further that such acoustic supply pipe systems of damped or tuned damping may occur within the noise reduction apparatus at several locations or in several constructions and the attenuation maxima of which coincide with the maxima of sound output of the source of noise, -furthermore the length of the acoustic wave conductor between the first expansion space of the noise reduction apparatus for flowing media and the second expansion space is related to the wave-number determined by the first limiting frequency of the frequency band to be damped by the relation: wherein:: k,(m-l) is the wave number determined by the lower limiting frequency; l3(m) is the length of the first expansion space along the direction of flow; l4(m) is the length of the wave conductor between the first expansion space and the second expansion space, Z03 and Z04 (kgm~4. s~1) are respectively the critical acoustic impedances of the expansion spaces 3 and 4, -furthermore, the noise reduction apparatus for flowing media contains at least two additional expansion spaces, furthermore, the noise reduction apparatus for flowing media has acoustic wave conductors between its invididual expansion spaces which project into these expansion spaces at certain places optionally to such an extent that the wave conducting branch lines formed by the intersection of the projections and the expansion spaces have a specific input acoustic impedance that is zero at the pass frequencies of the sound output transmission characteristics of the apparatus, furthermore, to eliminate the further pass bands in the transmission characteristics the acoustic wave conductor between the first and second expansion spaces is connected to one or more series connected wave conductor in the form of a closed supply line, the specific input acoustic impedance of which is zero on the characteristic frequency of the pass band to be eliminated. 2. Noise reduction apparatus for flowing media according to claim 1, characterised in that the acoustic supply pipeline of tuned attenuation passing into the first expansion space and optionally continuing to further expansion spaces consists of two concentric tubes of differing lengths and a plurality of plates of stepwise increasing length disposed between the surfaces of the two concentric tubes so as to divide the annular space between the tubes in a radial direction and an outermost cylindrical surface. 3. Noise reduction apparatus for flowing media according to claim 1 and 2, wherein the individual expansion spaces and branches have perforated walls which for the purpose of increasing the acoustic resistance are provided with woven soung absorbing layers or materials to which is airtightly connected a covering plate for covering the core surface, the fastening between the sound absorbing material and the plate cover being by way of a releasable fastening to enable the sound absorbing material to be readily removable and replaceable. 4. Noise reduction apparatus for flowing media according to any of claim 1 to 3, wherein in order to prevent sound transmission between the individual expansion spaces, through perforated plate and the sound absorbing lining, there is provided in the planes of the partitioning walls between the individual spaces a closed plate strip between the perforated and outermost plate cover along the whole circumference of the apparatus, this plate strip passing through the sound absorbing cover. 5. Noise reduction apparatus for flowing media, according to any preceding claim, characterised in that the walls of the apparatus are formed of a double walled plate the walls of which are connected by randomly disposed point-like fastenings to reduce the sound transmission in the walls by conduction or by radiation. 6. A noise reduction apparatus according to any preceding claim, wherein the first expansion space and further spaces of the apparatus are separated from each other in space but are connected by optionally flexible acoustic wave conductors so that the apparatus consists of two or more separately manipulatable constructional units. 7. Noise reduction apparatus and system substantially as herein described with reference to and as shown in any of the Figures of the accompanying drawings. New Claims or Amendments to Claims filed on 1 Aug. 1 980. Superseded Claims 1 to 7. New or Amended Claims:

1. Noise reducing apparatus for the attenuation of noise propagating in flowing media in pipelines, comprising a plurality of expansion compartments or chambers, which are in acoustic coupling with one another and which communicate with an inlet and an outlet, wherein the inlet is provided by a feeder pipeline with tuned attenuation which protrudes into a first of the expansion compartments or chambers, the feeder pipeline being subdivided longitudinally into subcompartments protruding to varying depths into the first expansion compartment or chamber.

2. Noise reducing apparatus according to claim 1, wherein the effective length of the shortest sub-compartment protruding into the first expansion compartment or chamber is: minSli/2 and the effective length of the longest subcompartment is: Imexlll(fmax i(max1)112 where 1max and Imin are respectively the effective lengths of protrusion of the longest and the shortest sub-compartments, I is the dimension of the first expansion compartment or chamber in the direction of flow and fmax is the crosssectional area of the longest sub-compartment taken perpendicularly to the direction of flow.

3. Noise reducing apparatus according to claim 1 or 2, wherein the cross-section of the subcompartments opening into the first expansion compartment or chamber is a plane perpendicular to the direction of flow.

4. Noise reducing apparatus according to claim -, 1 or 2 wherein the cross-section of the subcompartments opening into the first expansion compartment or chamber has an arcuate or curved shape.

5. Noise reducing apparatus according to any of claims 1 to 4, wherein the acoustic feeder pipeline with tuned attentuation consists of two pipes of different lengths within one another and walls of different lengths radially subdividing the annular space formed between the two pipes into the plurality of sub-compartments.

6. Noise reducing apparatus according to any of claims 1 to 5, wherein at least three of said expansion compartments or chambers are coupled in series.

7. Noise reducing apparatus according to claim 6, wherein the coupling element between at least two expansion compartments of chambers is an acoustic feeder pipeline with tuned attenuation.

8. Noise reducing apparatus according to claim 6, wherein the coupling element between two expansion compartments or chambers n, (n+ 1) is a per se known elementary wave guide the length of which is:

where k0 is the wave number per unit length associated with the lower frequency limit, In is the length of the nth expansion compartment or chamber in the direction of the flow, 13 is the length of the elementary wave guide between the expansion compartments or chambers n and (N+1), Zon and Z0(p+1) are the critical acoustic impedances of the expansion compartments or chambers n and (n+ 1).

9. Noise reducing apparatus according to any of claims 1 to 8, wherein the coupling element between at least two expansion compartments or chambers is a flexible acoustic wave guide.

10. Noise reducing apparatus according to any of claims 1 to 9, wherein the apparatus is coupled to the source of noise by an acoustic wave guide with a certain length at the input or inlet of which the value of the critical acoustic impedance is zero.

11. Noise reducing apparatus substantially as herein described with reference to and as shown in any of the Figures of the accompanying drawings.