EP0710946A1 - Double-walled active noise suppression device - Google Patents

Abstract

The device includes two plates (10,11) which forming a double wall and enclose an interior space (12) of rectangular cross-section. Noise is detected e.g. by electret microphones (13) and cancelled by anti-noise loudspeakers (14) within this space. Four loudspeakers are positioned centrally on the edges of the enclosed space which are typically 1.6 m long, 1.2 m wide and 5 cm thick (d). Two microphones are placed on each longer edge, flanking the central loudspeaker, and one quarter-wavelength away from each corner. In a regulator (18) the sum (22) of the microphone outputs undergoes FIR adaptive least mean squares filtering (23) using a reference microphone (24).

Description

The present invention relates to an attenuation device type acoustic comprising two plates substantially parallels delimiting a rectangular space, noise detection means disposed between the two plates, counter-noise emission means arranged between the two plates, and regulating means for controlling the means for emitting noise abatements so as to to minimize a quantity supplied by the detection means noise.

The invention has applications for example in the field of soundproofing of premises, in the realization hoods for noisy equipment, or in the insulation of passenger compartments of means of transport. An important application is in the realization of double glazing.

A device of the type indicated above, said to be double active wall, based on the operating principle recalled below.

ρ₀ :

masse volumique du milieu situé entre les plaques (1,18 Kg/m³ dans le cas de l'air).

c₀ :

célérité du son dans le milieu situé entre les plaques (340 m/s dans le cas de l'air). ρ0 c 2 0 d : rigidité de la lame d'air

m₁,m₂ :

masse surfacique des plaques (en kg/m²)

The mass-spring-mass resonance frequency of a double wall constituted by two parallel rectangular plates separated by an air gap of thickness d is given by the relation: with:

ρ₀:

density of the medium located between the plates (1.18 Kg / m³ in the case of air).

c₀:

speed of sound in the medium between the plates (340 m / s in the case of air). ρ 0 vs 2 0 d : stiffness of the air gap

m₁, m₂:

surface mass of sheets (in kg / m²)

This resonant frequency is generally between 50 and 250 Hz.

• f < f_mrm : les deux plaques vibrent en phase. La variation de volume entre les plaques reste faible. La double paroi se comporte comme une paroi simple de masse équivalente.
• f ≈ f_mrm : les deux plaques, fortement couplées par la lame d'air, vibrent en opposition de phase. Ceci se traduit par de fortes variations de volume de la lame d'air (phénomène de "respiration" des plaques) et par une faible isolation acoustique par la double paroi.
• f > f_mrm : les mouvements des deux plaques sont découplés par la lame d'air. L'isolation acoustique de la paroi augmente alors rapidement avec la fréquence.

Overall, for a given frequency f, the acoustic behavior of a double wall is considered as follows:

• f <f _mrm : the two plates vibrate in phase. The variation in volume between the plates remains small. The double wall behaves like a single wall of equivalent mass.
• f ≈ f _mrm : the two plates, strongly coupled by the air gap, vibrate in phase opposition. This results in large variations in the volume of the air space (phenomenon of "breathing" of the plates) and in poor acoustic insulation by the double wall.
• f> f _mrm : the movements of the two plates are decoupled by the air _gap . The acoustic insulation of the wall then increases rapidly with frequency.

The attenuation device aims to compensate for the low acoustic insulation provided by the double wall in the vicinity of f _mrm . The principle consists in preventing - via an electro-acoustic system - any variation in volume of the air space.

α_lmn :

amplitude du mode l,m,n

_lmn :

base modale associée à la cavité considérée. Dans le cas d'une lame d'air de forme parallélépipédique : Φ lmn (x,y,z)=cos(lπx/Lx )cos(mπy/Ly )cos(nπz/Lz )

L_x,L_y,L_z (=d) :

dimensions de la lame d'air

ω :

pulsation (= 2πf)

x,y :

coordonnées spatiales parallèlement aux plaques

z :

coordonnée spatiale perpendiculairement aux plaques

t :

temps.

The sound pressure field in the air space can be written in the form of a modal series: with:

α _lmn :

mode amplitude l, m, n

 _lmn :

modal base associated with the considered cavity. In the case of an air knife of parallelepiped shape: Φ L M n (( X Y Z ) = cos ( lπx / L x ) cos ( mπy / L y ) cos ( nπz / L z )

L _x , L _y , L _z (= d):

air gap dimensions

ω:

pulsation (= 2π f )

x, y:

spatial coordinates parallel to the plates

z:

spatial coordinate perpendicular to the plates

t:

time.

The _natural frequency f _lmn of a mode of indices (l, m, n) of the air _gap is given by the relation: f L M n = vs 0 2π l π L x + m π L y + not π L z

The variation in volume of the air _gap is directly proportional to the amplitude of the mode (0,0,0) without the amplitude of the other modes in the vicinity of the resonance frequency of the wall f _mrm being affected. However, it is difficult to measure and excite only this mode by actions which, a priori, involve all of the modes. Indeed, the expression of the acoustic pressure given above (2) shows that the measurement carried out by a microphone will include the responses of other modes than the mode (0,0,0).

It is desirable to obtain attenuation effective, reduce the contribution in size to minimize low frequency modes other than mode (0,0,0), and to ensure that the means of emission of counter-noise predominantly excites the mode (0,0,0) by exciting as little as possible the other modes of the air blade.

It is an object of the invention to thus improve the effectiveness of the attenuation provided by a device to active double wall.

To this end, the invention provides a device acoustic attenuation of the type indicated at the beginning, characterized in that the means for emitting noise abatements include four actuators with respective positions parallel to the plates correspond approximately in the middle of the sides of the rectangular shape of said interior space, in that the noise detection means include four sensors whose respective positions parallel to the plates correspond approximately at the four points on the longest sides of the rectangular shape of said interior space and distant each a quarter of the length of a long side relative at a corner of said rectangular shape, in that the four actuators are controlled in phase, and in that the quantity to be minimized is represented by the sum of output signals from the four sensors.

With this arrangement, the sensors and actuators practically do not interact with modes odd order of the space between the two plates (i.e. modes whose indices are of the type (l, m, n) with odd l or m), nor with the mode (2,0,0) which is the one with the lowest natural frequency among even order modes other than mode (0,0,0). So we can   obtain satisfactory mode control (0,0,0) without significantly affect the effectiveness of mitigation by the excitement of natural low frequency modes.

In another embodiment of the invention based on the same principle, the respective positions of sensors and actuators are inverted, i.e. that the noise detection means comprise four sensors whose respective positions parallel to plates roughly correspond to the midpoints of sides of the rectangular shape of said interior space, and that the means of emission of noise abatement include four actuators with respective positions in parallel the plates correspond approximately to the four points located on the long sides of the rectangular shape of said interior space and each one quarter of the length of a large side with respect to a corner of said rectangular shape.

The two embodiments above have the advantage that the sensors and actuators are located on the edges of the plates. This advantage is important when the plates are transparent or when the interplate space is not easily accessible (double wall prefabricated by example). It is not necessary to provide a structure particular between the plates to support the actuators or the sensors.

It has also been found to be advantageous for a gas lighter than air, for example helium, occupies the interior space between the two plates. This decrease the density of the medium between the plates causes an increase in the speed of sound in this middle and therefore an increase in natural frequencies associated with the different modes (see formula (4)). The result less contribution to acoustic transmission modes other than mode (0,0,0), and therefore better attenuation by selective mode control (0,0,0).

• la figure 1 représente schématiquement un dispositif d'atténuation acoustique selon l'invention, vu en coupe suivant la ligne I indiquée sur la figure 2 ;
• la figure 2 est une vue schématique illustrant la position des capteurs et des actionneurs du dispositif de la figure 1 ;
• la figure 3 est un graphique montrant l'atténuation acoustique que peut procurer un dispositif tel que celui des figures 1 et 2 ;
• la figure 4 est un graphique illustrant une gamme de paramètres préférés dans un dispositif selon l'invention ; et
• les figures 5A à 5F sont des graphiques montrant l'atténuation acoustique qu'on peut obtenir avec différents exemples de constitution des plaques.

Other features and advantages of the invention will appear in the description below of a preferred but non-limiting example of embodiment. In the accompanying drawings:

• Figure 1 schematically shows an acoustic attenuation device according to the invention, seen in section along line I indicated in Figure 2;
• Figure 2 is a schematic view illustrating the position of the sensors and actuators of the device of Figure 1;
• Figure 3 is a graph showing the acoustic attenuation that a device such as that of Figures 1 and 2 can provide;
• Figure 4 is a graph illustrating a range of preferred parameters in a device according to the invention; and
• FIGS. 5A to 5F are graphs showing the acoustic attenuation that can be obtained with different examples of construction of the plates.

The device shown in Figure 1 constitutes a active double wall usable to provide insulation acoustics between the spaces on either side of the wall. The wall includes two parallel rectangular plates 10, 11 delimiting between them an interior space 12 rectangular in shape. The plates are shown flat on the face. It will be understood, however, that they could have a certain curvature, while remaining substantially parallel. Sensors 13 and actuators 14 are arranged between the two plates 10, 11 for respectively detect the noise prevailing in space 12 and emit counter-noises in space 12.

The sensors 13 and the actuators 14 are placed on the edges of the interior space 12. The arrangement of sensors 13 and actuators 14 parallel to the plates is illustrated in Figure 2. The actuators 14 are at number of four and arranged at the four points constituting the midpoints of the sides of the rectangular space 12. The sensors   13 are four in number and arranged each on a long side of the rectangular space 12, at a distance of one quarter the length of a long side with respect to a corner.

The sensors 13 can be microphones with electrets chosen to have sensitivity characteristics and phase not varying more than 1% from one sensor to the other. The actuators 14 can be speakers. An example of a usable speaker is the AUDAX model BMX 400 which represents a good compromise between the flow volume and size (nominal power 15 W, resonance frequency of the order of 150 Hz, diameter outside 77.8 mm, total mass 290 g).

A regulation unit 18 and provided for controlling the actuators 14 so as to minimize an error signal e provided by the sensors 13. The error signal to minimize consists of the amplified sum of the output signals of the four sensors 13, delivered by a summator 22. The control unit 18 includes a processing processor signal 23 programmed in a known manner to apply the gradient algorithm (LMS) with filtered reference. This adaptive filtering mode with finite impulse response is well known in the field of noise cancellation (see for example the works "Digital processing of signal "by M. Bellanger, Editions Masson, Paris 1981; and "Adaptive signal processing" by B. Widrow and S.D. Stearns, Prentice Hall, 1985). A reference microphone 24, located on the side of the source of the noises to be attenuated, provides a reference signal which is applied to a bandpass filter 21, the output of which, addressed to processor 23, is subject to finite impulse response filtering. The coefficients of the filter are updated at each sampling cycle to minimize the error signal e. Processor 23 addresses then the same control signal to the actuators 14, so that the actuators 14 are controlled in phase.

In a typical embodiment, the two plates 10, 11 are made of plexiglass and have the surface mass m₁ = m₂ = 6 kg / m². They delimit an interior space 12 of thickness d = 5 cm whose rectangular shape has sides of length L _x = 1.6 m and L _y = 1.2 m. Since space 12 is filled with air, the mass-spring-mass resonance frequency (formula (1)) is _{equal to} f _mrm = 150 Hz. The critical frequency of the plates is 6,400 Hz. The resonance frequencies of the first modes air gap pairs (formula (2)) are given in Table I. (L M n) (2,0,0) (0.2.0) (2,2,0) (4,0,0) (4,2,0) f _lmn (Hz) 216 290 362 434 522

The sum of the output signals from the four sensors, which represents the signal e to be minimized, reflects the response of the mode (0,0,0) of the space 12 located between the plates 10, 11. In the error signal e , there is practically no contribution of the odd order modes (l, m, n) with odd l or m taking into account the symmetrical arrangement of the sensors, nor of the even order mode of lowest natural frequency (2,0,0). Apart from the mode (0,0,0), the mode contributing to the signal e and having the lowest natural frequency is the mode (4,0,0) if L _x ≥ 2L _y or the mode (0,2,0 ) if L _x ≤ 2L _y . However, the _natural frequency of this mode is relatively far from the resonance frequency f _mrm , so that the influence of this mode and of higher index modes on the acoustic transmission is not decisive.

Due to their positions, the actuators controlled in phase hardly excite order modes odd, nor the modes (2,0,0) and (0,2,0). So the excitement actuators 14 mainly acts to compensate for the mode transmission (0,0,0) without significantly increasing the amplitudes of the other modes of low natural frequency.

FIG. 3 shows the results of simulations of the acoustic attenuation provided by the device in FIG. 1 (without the filter 21) in the example of the parameters indicated above. The dashed line curve corresponds to the values of the attenuation index R as a function of the frequency f of the noise to be attenuated in the case where there is active mode control (0,0,0), and the curve in solid line corresponds to the same values in the absence of active control. It can be seen that the active control according to the invention appreciably increases the weakening index in the range of low frequencies close to the resonance frequency f _mrm .

For frequencies far from f _mrm , there is not always an improvement in the attenuation index and, in some cases, there may even be a slight deterioration. This is why the band-pass filter 21 is provided in the regulation unit 18. This filter 21, to which the reference signal is applied before the filtering with finite impulse response, allows the frequencies for which the mode control ( 0,0,0) has a favorable effect on the attenuation index, i.e. the frequencies between f _mrm / 2 and min (2 f _mrm , f₂₀₀), f₂₀₀ denoting the smallest natural frequency even order modes: f₂₀₀ = c₀ / max (L _x , L _y ), where c₀ denotes the speed of the sound in the medium located between the two plates 10, 11.

It will be understood that various modifications of the example described above with reference to Figures 1 and 2 are possible without departing from the scope of the invention.

Thus, it is possible to swap positions respective sensors and actuators (Figure 2) in obtaining such good selective mode control (0,0,0). he is also possible to fill the inside of the plates with a sound insulator such as glass wool. We can still use a regulation mode other than the adaptive filtering previously described.

In a particularly advantageous embodiment, the space 12 located between the plates 10, 11 is occupied by a gas lighter than air. This increases the speed of the sound in the medium located between the plates, which decreases the density of the modes proper to the low frequencies (formula (4)), while the resonance frequency f _mrm is only slightly modified. The relative contribution of the mode (0,0,0) to the acoustic transmission is then increased so that the efficiency of the active control of this mode is improved. This effect is all the more marked when the gas is light. Helium is therefore a preferred example for this gas. This effect also occurs for configurations of sensors and actuators other than that shown in FIG. 2. Thus, in the case of the double wall indicated above by way of example and with a configuration with four sensors and an actuator central, the applicant has experimentally measured the average attenuation indices R _m , in dB (A), given in Table II when space 12 is filled with air or helium. These measurements were carried out with two types of noise to be attenuated: pink noise and road noise. It can be seen that the improvement in the attenuation provided by helium is much greater when the active mode control (0.0,0) is implemented. pink noise R _m (dB (A)) road noise R _m (dB (A)) air without active control 33 27 with active control 40 35 helium without active control 35 28 with active control 49 43

f_c, en hertz, désigne la fréquence critique d'une plaque ou, si les plaques 10, 11 sont de constitutions différentes, la plus grande des fréquences critiques des deux plaques (dans le cas d'une plaque plane homogène,la fréquence critique vaut fc = c 0 2π m/D avec m = masse surfacique de la plaque, D = Eh³/12(1-υ²) = rigidité en flexion de la plaque , E = module d'Young, υ = coefficient de Poisson, h= épaisseur de la plaque) ;

L_x et L_y sont les longueurs des côtés de l'espace rectangulaire, exprimées en mètres ;

f_mrm est la fréquence de résonance masse-ressort-masse donnée par la formule (1) ; et

f₂₀₀ = c₀/max(L_x,L_y) est la fréquence propre du mode pair de la cavité ayant la plus faible fréquence propre.

The applicant has carried out numerous simulations to determine the parameters of the plates giving rise to good acoustic attenuation by controlling the mode (0,0,0). In FIG. 4, the hatched area of the parameters providing the best attenuation characteristics is shown. The domain corresponds to the constitutions of the plates for which the acoustic transmission around the resonance frequency f _mrm is essentially governed by the mode (0,0,0). It corresponds to the relationships: f vs / (L x L y ) 2 > 800 and f mrm <f 200 or f vs / (L x L y ) 2 > 300 and f mrm <f 200 / 2, in which

f _c , in hertz, denotes the critical frequency of a plate or, if the plates 10, 11 are of different constitutions, the greater of the critical frequencies of the two plates (in the case of a homogeneous plane plate, the critical frequency worth f vs = vs 0 2π m / D with m = surface mass of the plate, D = Eh³ / 12 (1-υ²) = bending stiffness of the plate, E = Young's modulus, υ = Poisson's ratio, h = thickness of the plate);

L _x and L _y are the lengths of the sides of the rectangular space, expressed in meters;

f _mrm is the mass-spring-mass resonance frequency given by formula (1); and

f₂₀₀ = c₀ / max (L _x , L _y ) is the natural frequency of the even mode of the cavity having the lowest natural frequency.

Examples of attenuation curves (index attenuation R as a function of frequency) obtained in simulating various constitutions of the plates are represented in FIGS. 5A to 5F which correspond respectively to the points A to F on the diagram in Figure 4. The curves at solid line illustrate the weakening index in the absence of active control, and the line curves interrupted illustrate the simulated loss index in subtracting the contribution from the mode (0,0,0). The plate configurations are shown in Table III below.

It can be seen in FIGS. 5A to 5F that the cases (C, E and F) for which the relationships (5) or (6) are verified are those leading to the most significant improvement in the attenuation around the frequency of resonance f _mrm . Active control using a configuration of sensors and actuators that provides a satisfactory approximation of the mode response (0,0,0) will lead to a significant improvement in the attenuation when the materials and dimensions of the plates obey the relationships ( 5) or (6).

Claims (6)

Acoustic attenuation device, comprising two substantially parallel plates (10,11) defining a interior space (12) of rectangular shape, means for noise detection (13) arranged between the two plates, noise emitting means (14) arranged between the two plates, and regulating means (18) for controlling the means for emitting noise abatements so as to to minimize a quantity (e) supplied by the means of noise detection, characterized in that the means noise canceling systems include four actuators (14) whose respective positions parallel to plates (10,11) correspond approximately to the midpoints sides of the rectangular shape of said interior space (12), in that the noise detection means comprise four sensors (13) whose respective positions parallel to the plates (10,11) correspond approximately at the four points located on the long sides of the rectangular shape of said interior space (12) and distant each a quarter of the length of a long side relative at a corner of said rectangular shape, in that the four actuators (14) are controlled in phase, and in this that the quantity to be minimized is represented by the sum of output signals from the four sensors (13). Acoustic attenuation device, comprising two substantially parallel plates (10,11) defining a interior space (12) of rectangular shape, means for noise detection (13) arranged between the two plates, noise emitting means (14) arranged between the two plates, and regulating means (18) for controlling the means for emitting noise abatements so as to to minimize a quantity (e) supplied by the means of noise detection, characterized in that the means of noise detection include four sensors whose respective positions parallel to the plates (10,11)   roughly correspond to the midpoints of the sides of the rectangular shape of said interior space (12), in that the means for emitting noise canceling comprise four actuators whose respective positions parallel to plates (10,11) correspond approximately to the four points located on the long sides of the rectangular shape from said interior space (12) and each a quarter apart the length of a large side with respect to a corner of said rectangular shape, in that the four actuators (14) are controlled in phase, and in that the magnitude at minimize is represented by the sum of the output signals of the four sensors (13). Device according to claim 1 or 2, characterized in that the materials and the dimensions of the plates (10,11) are chosen so that the relationships are checked: f vs / (L x L y ) 2 > 800 and f mrm <f 200 or relationships f vs / (L x L y ) 2 > 300 and f mrm <f 200 / 2, in which f _c , expressed in hertz, denotes the critical frequency of a plate or the greater of the two critical frequencies if the plates (10,11) are of different constitutions, L _x and L _y , expressed in meters, are the lengths of the sides of the rectangular shape of the interior space (12) located between the two plates, f _mrm is the resonance frequency of the mass-spring-mass system constituted by the two plates (10,11) and the medium located between them, and f₂₀₀ is a natural frequency given by the formula f₂₀₀ = c₀ / max (L _x , L _y ), where c₀ designates the speed of the sound in the medium located between the two plates (10,11). Device according to any one of the preceding claims, characterized in that it comprises a sensor (24) providing a reference signal, and a bandpass filter (21) to which the reference signal is applied, the output of the pass filter -band (21) being subjected to an adaptive filtering with finite impulse response to control the actuators (14), the bandpass filter (21) allowing the frequencies between f _mrm / 2 and min (2 f _mrm , f₂₀₀) to pass through , or f _mrm is the resonance frequency of the mass-spring-mass system constituted by the two plates (10,11) and the medium located between them, and f₂₀₀ is a natural frequency given by the formula f₂₀₀ = c₀ / max (L _x , L _y ), where c₀ denotes the speed of the sound in the medium located between the two plates, and L _x and L _y denote the lengths of the sides of the rectangular shape of the interior space (12) located between the two plates (10,11). Device according to any one of Claims 1 to 4, characterized in that a lighter gas that the air occupies the interior space (12) located between the two plates (10,11). Device according to claim 5, characterized in that said gas lighter than air is helium.