EP0898774B1 - Reactive sound absorber - Google Patents

Abstract

A reactive sound attenuator which includes a sensor for detecting a sound parameter in a space, e.g. a duct, consists of a signal amplifier that is used to amplify a detected signal, an electroacoustic transducer and a cavity with at least one membrane. The membrane, which is capable of moving in a vibratory manner, is part of a wall of a space, e.g. a duct wall. A sensor, which is disposed in the immediate vicinity of, in or on the membrane, detects the vibrations of the membrane. The sensor's signal, which is amplified and inverted by the amplifier, controls the membrane vibration via the electroacoustic transducer.

Description

The invention relates to a reactive absorbing silencer (hereinafter more reactive Called silencer) according to the preamble of claim 1.

State of the art

The most persecuted and refined in active noise abatement so-called anti-noise systems (Nelson, P.A., Elliott, S.J .: Active Control of Sound and Vibration. Academic Press Limited, London: 1992) for sound attenuation in ducts are based on a simple concept, Figure 5. An incoming, primary sound wave is detected by a microphone (8), which is clearly in the direction of the noise source separated from the other components in the channel. The captured microphone signal is arithmetically rotated as precisely as possible by signal processing (11) by 180 ° and serves to control a loudspeaker (9), which is ultimately the secondary Sound wave emits. In the direction of sound propagation, both waves overlap in the Ideally until extinction. This extinction can be monitored with a second microphone (10) in the direction of sound propagation, the signal at the same time to adapt signal processing to any changes in sound propagation can serve in the relevant channel. This is achieved with the help of modern signal processors The procedure is very precise, at least under laboratory conditions. Your practical use is however due to high sensitivity with superimposed air flow or at Temperature fluctuations as well as high expenditure on electronics and Signal processing marked.

Another approach is DE 40 27 511 a hybrid silencer, Figure 6, proposed in the front of a known, passive subsystem (12) an optimal acoustic due to an additional active subsystem on the back Impedance of the duct wall (1) is to be realized, the starting point is the acoustic properties of the passive subsystem, e.g. a layer of porous Absorber material. The other elements of the hybrid silencer are used for Generation of a rear termination impedance of the passive subsystem. For Forcing this termination impedance is the sound pressure behind the passive subsystem to measure with a microphone (13). Then the microphone voltage fed back to a loudspeaker (14) via a signal former (15) Membrane area, the calculated impedance should be set. This procedure continues ahead that the signal former proposed in DE 40 27 511 firstly has its own behavior all electromechanical components (microphone, loudspeaker, box, etc.) compensated and secondly impresses the desired terminating impedance on the system. The properties of the electromechanical components have been thoroughly examined and described. After that, the adjustment is only through complex and only approximately realizable transfer functions of the signal former possible.

Active Helmholtz resonators represent a variant of the basic idea of hybrid silencers according to - DE 42 26 885 and Spannheimer, H., Freymann, R., Fastl, H .: Aktiver Helmholtz resonator for damping cavity vibrations. Progress der Akustik - DAGA 1994, DPG-GmbH, Bad Honnef: 1994, pp. 525-528, -dar, picture 7, preferably with the area of application in motor vehicles. Here represents a conventional Helmholtz resonator the passive described in DE 40 27 511 Subsystem that is actively influenced on its back. In detail it is on known Helmholtz resonator through a hollow body (16) and an opening (17) defined. The one provided outside the Helmholtz resonator next to the opening Microphone (18) provides information about the sound pressure prevailing there, with which a transmission system (20) with special (PDT) frequency and time behavior the required voltage for the speaker (19) generated in the hollow body. This Loudspeaker determines or changes the transmission behavior (resonance frequency) of the original Helmholtz resonator. The loudspeaker in the hollow body thus serves the practical enlargement (general change) of the hollow body volume for improved Sound absorption of the Helmholtz resonator at low frequencies. The goal here there is therefore an active reduction in the resonance frequency and thus the sound absorption of the passive Helmholtz resonator.

The object of the invention is to adjust the efficiency of the reactive silencer to increase the preamble of claim 1 and to reduce the technical outlay. According to the invention, this is achieved by the reactive muffler according to claim 1 solved. Advantageous embodiments of the invention are characterized in the subclaims.

The invention relates to a reactive silencer, in which both the detection as well as the active influencing of the sound field directly and immediately the duct wall (1) takes place, Figure 1. The basic building block is a closed, compact one Cassette (2) in which all components are summarized. Your front is part of the channel wall and is covered by at least one vibratable membrane (3), e.g. a loudspeaker membrane, embodied. This membrane (3) forms by its area Mass with the cavity (4) of the cassette housing behind it an acoustic resonance system. The occurring sound waves in the sewer rain this resonance system at and near its natural frequency to vibrate on. The activation takes place with the help of a sensor (5), which is in the immediate vicinity, is arranged in or on the membrane (3) and detects the membrane vibrations. This sensor function can e.g. Microphones, structure-borne sound sensors or optical Take over motion sensors. The output signal of the sensor is used after a inverting, linear amplification (6) of the control of an electroacoustic Transducer (7), e.g. the voice coil of a speaker.

As a result, the membrane is forced to vibrate more, the sound pressure on the lined wall surface thus further reduced and the sound wave more subdued.

The shape of the housing (2) can be varied, since only the volume of the cavity (4) affects the frequency characteristic. To suppress cavity resonance, absorbers can be provided inside the housing (2), which is soundproof his. For the spectral adaptation of the resonance system, the area-related can continue Membrane mass, e.g. through different speakers become. The principle-related linear amplifier (6) contains no frequency weighting of the sensor signal in order to use filters, signal formers or other transmission systems to avoid associated unwanted phase shifts. This prevents disturbing acoustic interactions between neighboring ones Cassettes and large-area, reactive silencers made of many individual cassettes, e.g. in reactive silencer backdrops, Figure 2, are possible. The provision of the operating voltages for the sensors (5) and amplifiers (6) is done by conventional Power supplies or batteries. The measured insertion loss of an example reactive silencer, Figure 3, consisting of 4 cassettes, is in Figure 4 shown.

Advantages of reactive silencers compared to the prior art From the basic principle of the reactive silencer, i.e. exploitation or reinforcement the membrane vibrations as a sound field image directly in the duct wall, This gives the following advantages over existing active silencers.

The reactive silencer comes without passive subsystems (porous absorbers, Helmholtz resonators etc.). This fact as well as the spatial concentration of membrane (3) and sensor (5) in the channel wall allow the use of a simple amplifier (6). This allows all components of the reactive silencer to be used can be easily integrated in a compact housing (2).

The spatial cascading of several neighboring reactive silencers in the Duct wall or in silencer backdrops is possible and leads to correspondingly higher ones Sound absorption. The damping effect of cascaded, reactive silencers in the Channel is practically only through secondary sound paths (analogous to passive silencers) limited.

The reactive silencer is connected to any sound field and to any sound field limitation, e.g. Channel redirection, adaptable. The reactive silencer cassettes and thus all electroacoustic components can be acoustic using permeable covers against physical and chemical occurring in the channel Loads are protected.

An embodiment of the reactive silencer sees the use of a Microphones as sensors (5) whose positioning behind the membrane (3), i.e. in the cavity (4) of the cassette (2). The principle of operation of the reactive silencer is not only applicable to flat waves in comparatively narrow channels, but also also effects a damping of modal sound fields in any channels or rooms. In these applications, the vibrating membranes reduce the reactive Cassettes also cover the sound pressure on the lined wall surface and dampen the existing sound field.

Descriptions of the pictures

Image 1: Exemplary version of a reactive silencer cassette in one Channel wall (1), consisting of the housing (2) with at least one membrane (3) in front of a cavity (4), a sensor (5), a linear amplifier (6) and one electroacoustic transducer (7).

Figure 2: Cascaded arrangement of reactive silencer cassettes in a silencer backdrop.

Figure 3: Exemplary embodiment of a reactive silencer consisting of 4 cassettes in a. Channel wall (1) with a channel cross-section of 0.25 m x 0.25 m

Figure 4: Measured insertion loss of the exemplary reactive silencer in picture 3.

Claims (6)

1. Reactively absorbing sound attenuator consisting of a cavity (4) restricted such that it will not transmit sound, including at least one membrane (3), an acoustic sensor (5) in the immediate vicinity of or in or on said membrane (3), as well as an electro-acoustic converter (7) acting upon said membrane, and an inverting signal amplifier (6), wherein the resonance vibrations of said membrane (3), which are detected by means of said sensor (5), are amplified by means of said signal amplifier (6) and said electro-acoustic converter (7),
   characterized in
   that

   for sound absorption said membrane (3) constitutes part of the wall of a space,

   that said cavity (4), seen from said space, is disposed behind said membrane, and the acoustic mass of said membrane (3) with the acoustic resilience of said cavity as resonance absorber is matched with that vibration in said space that needs the strongest attenuation,

   and said signal amplifier (6) operates in a linear mode and provides a plane amplitude frequency characteristic.
2. Reactive sound attenuator according to Claim 1,
   characterized in
   that the mass of said membrane and/or the volume of said cavity (4) are matched with the sound field to be attenuated.
3. Reactive sound attenuator according to Claim 1,
   characterized in
   that said sensor (5) is an acoustic or optical sensor and detects the pressure, the sound particle velocity or the movement of said membrane (3).
4. Reactive sound attenuator according to Claim 1,
   characterized in
   that said electroacoustic transducer (7) is a plain linear loudspeaker.
5. Reactive sound attenuator according to Claim 1,
   characterized in
   that an acoustically pervious cover is provided in front of or on said membrane (3).
6. Reactive sound attenuator according to Claim 1,
   characterized in
   that several sound attenuators are disposed in planar juxtaposition in a duct wall (1), a flow duct or in sound-reducing flats.

Images