EP0586831A2 - Sound absorption method for motor vehicles - Google Patents

Abstract

In a sound absorption method for motor vehicles, in which a loudspeaker is used to produce sound pressure values and in which a Helmholtz resonator (AHR) is used to absorb sound pressure at the resonant frequency of the resonator, the loudspeaker (L) mounted in the cavity (H) of the Helmholtz resonator (AHR) is used to generate sound pressure values in the cavity (H) by means of which an arbitrary number of cavity volumes deviating from the real cavity volume are simulated simultaneously or in temporal sequence. The cavity volumes to be simulated are determined on the basis of arbitrary desired resonant frequencies for the Helmholtz resonator (AHR). <IMAGE>

Description

The invention relates to a sound absorption method for motor vehicles according to the preamble of patent claim 1.

From DE 37 29 765 A1, for example, a sound absorption method is known in which several Helmholtz resonators, in particular for damping low-frequency cavity vibrations, are connected in parallel and accommodated in the vehicle body to save space. When sound pressure vibrations are damped by Helmholtz resonators, as the frequencies become lower, larger hollow body volumes of the Helmholtz resonators are generally required. The sound absorption system known from DE 37 29 765 A1 tries to solve the problem of large hollow body volumes only by the type of accommodation in the motor vehicle.

It is an object of the invention to provide a sound absorption system which, when using Helmholtz resonators, does not require large hollow body volumes for the absorption of low-frequency sound pressure vibrations.

This object is achieved by the features of patent claim 1.

The hollow body volume of a Helmholtz resonator affects the sound absorption in the form of the prevailing sound pressure behavior. The invention is based on the idea of generating, in a relatively small hollow body of a Helmholtz resonator, a sound pressure behavior which corresponds to the sound pressure behavior of the usually required hollow body volume for the respectively desired resonance frequency of the Helmholtz resonator, at which the sound pressure is at least largely absorbed, corresponds. For this purpose, a loudspeaker is mounted in the hollow body of a Helmholtz resonator, through which the hollow body is sonicated in such a way that the required hollow body volume for the desired resonance frequency is simulated in the hollow body. In this case, only a single hollow body volume can be simulated as well as successively or simultaneously several hollow body volumes. The simulation of so many hollow body volumes is particularly advantageous that a broadband resonance behavior is generated in the form of an acoustic bandpass filter.

On the one hand, this invention enables sound absorption by Helmholtz resonators with a relatively small volume. On the other hand, a single Helmholtz resonator can be tuned both sequentially in time and simultaneously to several resonance frequencies.

In the following, the Helmholtz resonator that is sonicated with a loudspeaker is also referred to as an active Helmholtz resonator, while conventional Helmholtz resonators with predetermined real volumes are also called passive Helmholtz resonators.

Advantageous developments of the invention are specified in the subclaims.

The method according to subclaim 2 achieves, for example, a control of the loudspeaker which is dependent on the current sound pressure values prevailing in the vicinity of the active Helmholtz resonator. The loudspeaker is controlled by a transmission system which is arranged between the microphone picking up the prevailing sound pressure values and the loudspeaker located in the active Helmholtz resonator. The transmission system is designed such that the sound absorption system consisting of the microphone, the transmission system and the active Helmholtz resonator has the same transmission behavior as a passive Helmholtz resonator with the real volume required for the desired resonance frequency.

The transmission system can be implemented, for example, by the construction of an electrical switching arrangement with digital or analog means, which can be matched to the desired acoustic transmission behavior by means of electrical analogies.

The transmission behavior of the transmission system according to subclaim 3 optimally reproduces, particularly with PD₂T₂ elements, the transmission behavior of a passive Helmholtz resonator with a real, simulated volume.

With the device according to subclaim 4, it is possible to control the loudspeaker to adapt the active Helmholtz resonator to a desired resonance frequency by means of the measured sound pressure values within a small passive comparative Helmholtz resonator.

The transmission behavior of this passive comparison Helmholtz resonator is also matched to the absorption of the sound pressure at the desired resonance frequency.
Such an electro-acoustic device is easier to set up and adapts to interference in the form of superimposed external pressure fluctuations better than an electrical switching arrangement based on an electrical analogy to an idealized acoustic arrangement.

Fig. 1

den schematischen Aufbau eines bekannten passiven Helmholtz-Resonators

Fig. 2

zwei mögliche Ausgestaltungen für einen aktiven Helmholtz-Resonator

Fig. 3

eine erfindungsgemäße Ansteuerung des Lautsprechers über ein elektronisch ausgebildetes Übertragungssystem und

Fig. 4

eine erfindungsgemäße Ansteuerung des Lautsprechers über ein elektro-akustisch ausgebildetes Übertragungssystem

Gleiche Bauteile in den Figuren 1 bis 4 sind mit gleichen Bezugszeichen versehen.Exemplary embodiments of the invention are shown in the drawing. Show it

Fig. 1

the schematic structure of a known passive Helmholtz resonator

Fig. 2

two possible configurations for an active Helmholtz resonator

Fig. 3

an inventive control of the speaker via an electronically trained transmission system and

Fig. 4

an inventive control of the speaker via an electro-acoustically designed transmission system

The same components in Figures 1 to 4 are provided with the same reference numerals.

On the left in FIG. 1, the section through a conventional Helmholtz resonator consisting of a hollow body with the volume V and an air passage opening with a neck of length l and diameter d is shown in the side view. On the right side of the 1 shows the top view of the Helmholtz resonator with the air passage opening of the cross-sectional area a.

mit der Schallgeschwindigkeit c, mit der Halslänge l und der Querschnittsfläche a der Luftdurchtrittöffnung sowie mit dem Volumen V des Hohlkörpers.
Im folgenden soll auf die Abhängigkeit der Resonanzfreguenz f allein vom Volumen V eingegangen werden. Die Länge l und die Querschnittsfläche a der Luftdurchtrittsöffnung seien konstant. Wie aus der Formel ersichtlich ist, muß mit tiefer werdenden Frequenzen f das Volumen V größer werden. Dieser Zusammenhang bildet die Problematik des großen Platzbedarfs bei der Anwendung eines Helmholtz-Resonators in Kraftfahrzeugen, da dort insbesondere die Schalldrücke tiefer Frequenzen absorbiert werden müssen.The following applies to the resonance frequency of a Helmholtz resonator, at which the sound pressure is at least predominantly absorbed:

with the speed of sound c, with the neck length l and the cross-sectional area a of the air passage opening and with the volume V of the hollow body.
The dependency of the resonance frequency f solely on the volume V will be discussed below. The length l and the cross-sectional area a of the air passage opening are constant. As can be seen from the formula, the volume V must increase as the frequencies f decrease. This connection creates the problem of the large space requirement when using a Helmholtz resonator in motor vehicles, since in particular the sound pressure at low frequencies must be absorbed there.

2 presents two possible embodiments for an active Helmholtz resonator AHR1 and AHR2, the real volumes of which are each determined by the hollow body H. The air passage openings D are also firmly defined.
In the active Helmholtz resonator AHR1, the wall of the hollow body H opposite the air passage opening D is replaced by a loudspeaker L. The loudspeaker L changes the sound pressure values in the hollow body H, so that other imaginary volumes in the hollow body H can be simulated as the volume that actually exists.

The loudspeaker L, which is arranged within the hollow body H of the active Helmholtz resonator AHR2, also fulfills the same function. With this arrangement possibility, the loudspeaker L is not part of a hollow body wall, but is at least almost completely sealed off from the surroundings in the hollow body H. Such an arrangement is required, for example, if special measures are to be taken to prevent undesired interference from superimposed pressure fluctuations from outside the hollow body H.

3 and 4, for the sake of simplicity, only one active Helmholtz resonator AHR is shown in the form of the Helmholtz resonator AHR1. However, this is not intended to be restrictive, but is only meant as an example.

In FIG. 3, a microphone M is connected to a transmission system U outside the active Helmholtz resonator AHR via an input line. The transmission system Ü is connected to the loudspeaker L of the active Helmholtz resonator AHR via an output line.

In the vicinity of the air passage opening D of the hollow body H, the microphone M picks up the sound pressure values p _a prevailing outside the hollow body H and converts them into a corresponding electrical signal which is passed on to the transmission system Ü. The transmission system Ü is an electronic switching arrangement with PD₂T₂ transmission behavior. The loudspeaker L is driven by the electrical current I from the transmission system Ü.

ausgedrückt werden, wobei C₀, C₁, C₂, C₃ und C₄ Konstanten sind, die von der Form des Hohlkörpers H, von der gewünschten Resonanzfrequenz f sowie vom Frequenz-Leistungs-Verlauf des Lautsprechers L abhängen, und wobei s der Imaginäranteil der komplexen Frequenz $\text{p=σ+jw}$

mit $\text{s=jw}$

als Laplace-Variablen ist.The PD₂T₂ transmission behavior can, for example, mathematically for a single desired resonance frequency by the following formula according to the Laplace transformation

are expressed, where C₀, C₁, C₂, C₃ and C₄ are constants which depend on the shape of the hollow body H, on the desired resonance frequency f and on the frequency-power curve of the loudspeaker L, and where s is the imaginary component of the complex frequency $\text{p = σ + jw}$

With $\text{s = jw}$

as a Laplace variable.

Danach kann eine frequenzbreitbandige Wirksamkeit der Schalldruckabsorption erreicht werden, da sich der Helmholtz-Resonator gleichzeitig bei vielen Frequenzen in Eigenresonanz befindet.The transmission system U shown in FIG. 3 can be used for any number of resonance frequencies. In a further embodiment of the invention, it can therefore also be used simultaneously for a large number of different desired resonance frequencies, the sound pressure of which is to be absorbed.
For example, for a number n of desired resonance frequencies, the transmission system can be represented mathematically as follows:

Thereafter, a broadband effectiveness of sound pressure absorption can be achieved, since the Helmholtz resonator is simultaneously self-resonant at many frequencies.

4 shows an active Helmholtz resonator AHR and a passive comparative Helmholtz resonator PHR. A microphone M₁ outside of the two Helmholtz resonators and a microphone M₂ within the passive comparison Helmholtz resonator PHR are connected to an arithmetic unit R via input lines. An output line of the computing unit R is connected to the loudspeaker L of the active Helmholtz resonator AHR.

), sondern lediglich eine additive und/oder multiplikative Verstärkungsschaltung unter Berücksichtigung der Konstanten C₀, C₁, C₂, C₃ und/oder C₄, die von der Form des Hohlkörpers H des AHR, von der gewünschten Resonanzfrequenz f sowie vom Frequenz-Leistungs-Verlauf des Lautsprechers L abhängen.
Die beiden Helmholtz-Resonatoren AHR und PHR unterscheiden sich lediglich darin, daß aufgrund der Hohlkörpervolumenunterschiede durch den aktiven Helmholtz-Resonator AHR bei gleicher Freuqunz und gleicher Dämpfung eine um ein Vielfaches größere Schallenergie absorbiert wird als durch den passiven Vergleichs-Helmholtz-Resonator PHR. Diese Absorptionsverstärkung findet in der Recheneinheit R statt und wirkt sich über die Ansteuerung des Lautsprechers L über die Ausgangsleitung der Recheneinheit R auf den aktiven Helmholtz-Resonator AHR aus.
Auch die Ausgestaltung der Erfindung gemäß Fig. 4 ist auf mehrere gewünschte Resonanzfrequenzen gleichzeitig anwendbar, indem mehrere Vergleichs-Helmholtz-Resonatoren, d.h. je ein jeder gewünschten Resonanzfrequenz zugeordneter Vergleichs-Helmholtz-Resonator, in Parallelschaltung zur Ansteuerung des Lautsprechers verwendet werden.The passive comparison Helmholtz resonator PHR is to be designed in miniature design at the same resonance frequency or for absorption of the sound pressure of the same frequency that is also to be absorbed by the active Helmholtz resonator AHR. For this purpose, the passive comparative Helmholtz resonator PHR has the same transmission behavior, which is also the basis of a passive Helmholtz resonator with the real hollow body volume, which corresponds to the hollow body volume required to be simulated for the sound pressure to be absorbed at the desired resonance frequency.
The microphone M₁ records the sound pressure values outside the two Helmholtz resonators AHR and PHR in the vicinity of their air passage openings D1 and D2. The microphone M₂ detects the sound pressure values within the passive Helmholtz resonator PHR. The signals of the microphones M₁ and M₂ are fed to the computing unit R. The computing unit R is not a frequency-dependent transmission system Ü (≠ f (p), with $\text{p = σ + jw}$

), but only an additive and / or multiplicative amplification circuit, taking into account the constants C₀, C₁, C₂, C₃ and / or C₄, the shape of the hollow body H of the AHR, the desired resonance frequency f and the frequency-power curve of the Unhook speaker L.
The only difference between the two Helmholtz resonators AHR and PHR is that the active Helmholtz resonator causes differences in the volume of the hollow body AHR, with the same frequency and the same damping, absorbs a sound energy many times greater than that of the passive comparative Helmholtz resonator PHR. This absorption amplification takes place in the computing unit R and has an effect on the active Helmholtz resonator AHR by driving the loudspeaker L via the output line of the computing unit R.
The embodiment of the invention according to FIG. 4 can also be applied to a plurality of desired resonance frequencies at the same time by using a plurality of comparison Helmholtz resonators, that is to say a comparison Helmholtz resonator assigned to each desired resonance frequency, in parallel connection for controlling the loudspeaker.

In addition, it should be pointed out that the invention can be applied to Helmholtz resonators with hollow bodies of any shape and air passage openings of any shape. For example, an air passage opening in the form of a large-area sieve-like cover of the hollow body has proven to be particularly advantageous.
An arrangement of several loudspeakers within the active Helmholtz resonator is also conceivable.

Claims (4)

Sound absorption method for motor vehicles, in which sound pressure values are generated by means of a loudspeaker and in which sound pressure is absorbed at its resonance frequency by means of a Helmholtz resonator,
characterized in that sound pressure values are generated in the hollow body (H) by means of the loudspeaker (L) mounted in the hollow body (H) of the Helmholtz resonator (AHR, AHR1, AHR2), by means of which any number of the real hollow body volume (H ) deviating hollow body volumes are simulated, the hollow body volumes to be simulated being determined on the basis of any desired resonance frequencies for the Helmholtz resonator (AHR, AHR1, AHR2). Sound absorption method according to claim 1,
characterized in that by means of a first microphone (M, M₁) the outside of the Helmholtz resonator (AHR, AHR1, AHR2) prevailing sound pressure values (p _a ) are recorded and fed to a transmission system (Ü) through which the loudspeaker (L) is such is controlled that the transmission behavior of Helmholtz resonators with real, the simulated corresponding volumes is simulated by the entire sound absorption process. Sound absorption method according to claim 2,
characterized in that each desired resonance frequency in the transmission system (Ü) is assigned a PD _i T _k element, with i and k greater than or equal to 1, advantageously 1 or 2, and in that the transmission system (Ü) consists of the sum of all PD _i T _k members is generated. Sound absorption method according to claim 2 or 3,
characterized in that the loudspeaker (L) for generating a desired resonance frequency is controlled by simulating a required hollow volume by means of the sound pressure signal of a further microphone (M₂) within a small-sized comparison Helmholtz resonator (PHR) which has the same transmission behavior in relation to has the desired resonance frequency like a Helmholtz resonator with a real hollow body volume corresponding to the simulated one.

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