EP0601934A1 - Improvements to methods and devices used to protect from external noises a given volume, preferably located inside a room - Google Patents

Abstract

In order to protect a volume (2) located inside a room (3) from external noises E, recourse is had to a bank of acoustic sensors (11j) receiving the noise E and located a distance A from the volume and to a bank of acoustic sources (15k) located a distance B less than A from the volume and signals S are applied to these sources, these signals being summations of the double convolution products of the function Ej(t) with two functions fij(t) and gik(-t) which are directly derivable from the pulse responses collected, on the one hand, on the sensors (11j) from pulses emitted by sources (10i) carried by a fictitious barrier (6) delimiting the volume and, on the other hand, on sensors (12i) placed at the same locations as these latter sources (10i), from pulses emitted by the above sources (15k). <IMAGE>

Description

We often want to protect certain volumes from noise generated outside these volumes.

The volumes in question are in particular those intended to be occupied by the head of an individual, in particular in a seated or lying position: when the desired acoustic protection is obtained, the individual concerned is protected from external acoustic aggressions both that his head remains placed inside such a volume.

To provide such acoustic protection, it has already been proposed to interpose between the volumes in question and the exterior thereof acoustically insulating partitions.

The insulation obtained by such partitions is limited and the physical obstacles materialized by said partitions are often unacceptable.

It has also been proposed to neutralize certain sounds received by such volumes by applying to said volumes "counter-noises" of identical amplitude and of phase opposite to those of said sounds.

However, to date, this type of neutralization, sometimes referred to as active attenuation, has led to encouraging results only for relatively pure sinusoidal sounds transmitted directly from their source to the volume to be protected.

In particular, the random noises could not be treated correctly in this way and, when the volumes considered are inside rooms, delimited laterally by partitions, lower by a floor and higher by a ceiling, one does not To date, it has hardly managed to control the phenomena of reflection or reverberation of the noises to be neutralized on the various walls delimiting said premises as well as on other obstacles, such as furniture, present in these premises.

The object of the invention is, above all to remedy all of these drawbacks by making it possible to protect a volume placed inside a room from noise of any kind produced outside this room, and by particular from certain privileged directions corresponding for example to windows.

formule dans laquelle :

• - chaque fonction f_jl(t) est identique à la fonction réciproque f_ij(t) qui est la réponse impulsionnelle, préalablement déterminée et enregistrée, correspondant au bruit engendré sur le capteur d'indice j de la batterie de capteurs ci-dessus par l'émission d'une courte impulsion acoustique à partir d'une source supposée placée au point i,
• - et chaque fonction gk(-t) est élaborée à partir de la fonction g _k(t) qui est elle-même identique à la fonction réciproque gk(t), laquelle est à son tour la réponse impulsionnelle, préalablement déterminée et enregistrée, correspondant au bruit engendré sur un capteursupposé placé au point i, à partir de l'émission d'une courte impulsion acoustique par la source d'indice k de la batterie de sources ci-dessus.

To this end, the acoustic protection devices of limited volumes according to the invention are essentially characterized in that they comprise, on the one hand, disposed respectively at two distinct distances A and B of the same fictitious reticulated battery defining points i arranged in the volume to be acoustically protected, a battery of acoustic sensors (microphones) receiving the noises to be neutralized E _j (t) and a battery of acoustic sources (loudspeakers), the distance B being less than the distance A, and on the other hand, an electronic circuit interposed between said sensors and said sources and arranged so as to develop, in times less than AB v being the speed of sound in air, for each noise E _j (t), a plurality of signals S _k (t) which are applied instantaneously, respectively, to the sources, each signal S _k (t) being equal to:

formula in which:

• each function f _jl (t) is identical to the reciprocal function f _ij (t) which is the impulse response, previously determined and recorded, corresponding to the noise generated on the sensor of index j of the battery of sensors above by the emission of a short acoustic pulse from a supposed source placed at point i,
• - and each function gk (-t) is developed from the function g _k (t) which is itself identical to the reciprocal function gk (t), which in turn is the impulse response, previously determined and recorded, corresponding to the noise generated on a supposed sensor placed at point i, from the emission of a short acoustic pulse by the source of index k from the battery of sources above.

• - la détection des bruits E_j(t) nécessaire à l'élaboration des signaux S est effectuée par échantillonnage à une cadence correspondant sensiblement au huitième de la période la plus courte caractérisant les ondes sonores à traiter, c'est-à-dire à la fréquence la plus élevée de la gamme retenue pour la sensibilité des capteurs,
• - le domaine des fréquences auxquelles sont sensibles les capteurs est compris entre 10 et 10 000 Hz,
• - le nombre des éléments acoustiques composant chacune des batteries est égal à plusieurs dizaines, étant notamment de l'ordre de 50 à 100, et les distances qui séparent entre eux ces éléments dans chaque batterie est de l'ordre du décimètre,
• - la différence entre les distances A et B est de l'ordre de 1 mètre ;
• - chaque signal S_k(t) est égal à :
  
  
  formule dans laquelle h_jk(t) est une fonction préalablement déterminée et enregistrée égale à :
  

In preferred embodiments, use is also made of one and / or the other of the following arrangements:

• the detection of the noises E _j (t) necessary for the preparation of the signals S is carried out by sampling at a rate corresponding substantially to the eighth of the shortest period characterizing the sound waves to be processed, that is to say at the highest frequency of the range used for the sensitivity of the sensors,
• - the frequency range to which the sensors are sensitive is between 10 and 10 000 Hz,
• the number of acoustic elements making up each of the batteries is equal to several tens, being in particular of the order of 50 to 100, and the distances which separate these elements from each other in each battery is of the order of a decimeter,
• - the difference between the distances A and B is of the order of 1 meter;
• - each signal S _k (t) is equal to:
  
  
  formula in which h _jk (t) is a previously determined and recorded function equal to:
  

The invention also relates to the batteries of acoustic elements specially designed to equip the above devices as well as the methods for determining the impulse responses f _ij (t) and gk (t) which are used for the preparation of the signals S.

• - dans un premier temps, des sources acoustiques, les réponses f_ij(t) étant alors déterminées au niveau des capteurs permanents ci-dessus lors de l'émission d'impulsions acoustiques courtes par lesdites sources,
• - et dans un deuxième temps, des capteurs acoustiques, les réponses gk(t) étant alors déterminées au niveau de ces capteurs lors de l'émission d'impulsions acoustiques courtes par les sources permanentes ci-dessus.

These methods are essentially characterized according to the invention in that one has close to the volume to be acoustically protected, so as to define at least part of this volume, a crosslinked battery defining a plurality of points i in which one places:

• firstly, acoustic sources, the responses f _ij (t) then being determined at the level of the above permanent sensors during the emission of short acoustic pulses by said sources,
• - And secondly, acoustic sensors, the responses gk (t) then being determined at the level of these sensors during the emission of short acoustic pulses by the above permanent sources.

In at least one of the two source-sensor assemblies used respectively during the two successive "times" of the above-defined methods, the respective roles and locations of the sources and the sensors could be swapped.

In the case where the use of the function h _jk (t) above is planned, a preliminary step is further developed and recorded for this function h _jk (t).

The invention includes, apart from these main provisions, certain other provisions which are preferably used at the same time and which will be more explicitly discussed below.

• La figure 1, de ce dessin, montre très schématiquement un local équipé d'un dispositif propre à protéger des bruits extérieurs un volume limité de ce local.
• La figure 2 est un schéma du circuit électronique compris par ce dispositif.

In what follows, a preferred embodiment of the invention will be described with reference to the attached drawing, of course in a non-limiting manner.

• Figure 1, of this drawing, very schematically shows a room equipped with a device capable of protecting external noise a limited volume of this room.
• Figure 2 is a diagram of the electronic circuit included by this device.

It is proposed to protect vis-à-vis random noise E shown schematically by the arrow 1 a relatively limited volume 2 disposed in a room 3 delimited laterally by partitions 4, below by a floor and above by a ceiling.

The noises E are, for example, those which come from outside the premises through an open or closed window 5.

Volume 2 has for example the shape of a sphere or a cylinder of revolution whose diameter is of the order of 1 meter and whose central portion is intended to be occupied by the head of a person whom the we want to isolate noise E, this person being for example sitting in front of a desk or lying in a bed.

To resolve the problem posed, use is made of the technique known per se of active attenuation which consists, in order to protect a given point against annoying noises, in creating at this point counter-noises opposite said noises and determined in such a way that their addition to these noises at said point produces in this latter a zero result, that is to say, eliminates said noises.

• - constitution du bruit par un son sinusoïdal pur tel que celui émis par certains moteurs ou instruments de musique,
• - propagation exclusive et directe dudit son depuis sa source jusqu'au point à protéger, sans réflexion ou réverbération de ce son sur des obstacles tels que les parois d'un local.

The achievements that have been proposed in this area to date have been satisfactory only when the following two conditions are met:

• - constitution of noise by a pure sinusoidal sound such as that emitted by certain motors or musical instruments,
• - exclusive and direct propagation of said sound from its source to the point to be protected, without reflection or reverberation of this sound on obstacles such as the walls of a room.

The present invention proposes to solve the problem of attenuation, or even suppression, of undesirable noise in the volume 2 defined above, even if these noises are random and if they are reflected or reverberated by the walls. 4 of room 3.

To do this, we proceed as follows.

There is interposed between the volume 2 to be acoustically protected and the source of noise E against which it is desired to provide said protection two "barriers" or "batteries" 6 and 8 each composed of distinct acoustic elements kept separate from each other by a rigid frame (respectively 7.9) openwork vis-à-vis sounds.

These two barriers or batteries 6 and 8 are spaced from each other by an average distance A.

The first 6 of these two batteries defines a reticulated network, generally three-dimensional, of points or "nodes" distinct i-1, i, i + 1 ... occupying at least partially the volume 2 to be acoustically protected.

The acoustic elements which it comprises are, initially, acoustic sources (loudspeakers or the like) 10 _i-1 , 10 _i , 10 _{i + 1} ... which are located at said nodes.

As for the acoustic elements included by the second barrier 8, these are sensors (microphones) 11 _d-1 , 11 _d , 11 _{d + 1} ... which are located at different points or "nodes" j- 1, j, j + 1 ... of said barrier.

Each of the impulse response laws is then determined as a function of the time tf _ij (t) corresponding to each of the noises generated on each sensor 11 _j by the emission of a short acoustic pulse from each source 10.

We recall here the reciprocity theorem according to which the impulse response f _ij (t) as defined above is exactly identical to the inverse impulse response f _jl (t) which would be collected by supposed sensors placed at exactly the same locations i that the sources 10 above in response to the emission of short acoustic pulses from sources assumed to be arranged at the different points j in replacement of the sensors 11 _j above.

This reciprocity takes account in particular of all the reflections or reverberations of acoustic waves by the walls of room 3 or by other obstacles contained in this room, such as furniture, reflections which are shown diagrammatically in the drawing by the lines R.

In application of said theorem, the resulting noise which arrives at each of the points i of the battery 6 is calculated for each given global noise E _j (t) received at each of the points j.

This resulting noise is the convolution product E _j (t) ⊗f _ji (t).

The total noise F _i (t) which would reach each of the points i is then determined in response to the noises E _j (t) received by the set of points j, noises which are precisely those symbolized by the arrow 1 above. This total noise F _i (t) is equal to:

Each of the sources 10 of the battery 6 is then replaced by acoustic sensors 12 arranged in exactly the same locations i as these sources.

There is substantially at a distance B from the median zone of the battery 6, B being a length less than A, a third barrier or battery 13 of the same kind as the previous ones: this battery 13 is constituted by a rigid frame 14 now separated the from each other a plurality of acoustic sources 15 _k-1 , 15 _k , 15 _{k + 1} ... located at distinct points or "nodes" k-1, k, k + 1 ... of said framework.

Each impulse response gk (t) corresponding to the noise which is generated on the sensor 12 by the emission of a short acoustic impulse from the source 15 _{k is then determined} .

By virtue of the reciprocity theorem recalled above, each function gk (t) is strictly identical to the reciprocal function gk (t).

Consequently, we can say that the overall noise G _k (t) which would be created at each of the points k of the battery 13 in response to the noise F _i (t) assumed to be emitted from the points i by sources which would be located at these points, would be equal to:

This formula is precious because it makes it possible to determine in an extremely precise manner the noises which would result, at the level of the battery 13, from the production of the noises F _i (t) at the level of the various points i of the first battery 6.

However, these latter noises F _i (t) are precisely those which are generated at said points i by the application to room 3 of the undesirable noises E _j (t) to be neutralized.

• - de remplacer comme variable dans la loi de réponse g_ik(t) qui intervient dans la formule Il ci-dessus la variable (t) par la variable (-t),
• - et d'appliquer l'opposé S_k(t) de chaque signal résultant sur les sources 15_k correspondantes.

To develop the desired counter-noises, intended to neutralize any nuisance of undesirable incident noise E _j (t) at these points i, that is to say to cancel or at least strongly attenuate the noises F _i (t ) created at the points i from these undesirable noises, it suffices:

• - to replace as variable in the response law g _ik (t) which intervenes in the formula II above the variable (t) by the variable (-t),
• - and to apply the opposite S _k (t) of each resulting signal to the corresponding 15 _k sources.

It turns out that, if we send counter-signals g _ik (-t) at each of the points k, the corresponding wave sent to the point i propagates in exactly the opposite way to that corresponding to l emission of a short acoustic pulse from said point i towards said point k, and this wave therefore comes to focus at point i by exactly reconstructing said short pulse there, despite the various deformations of the wave fronts which may have been caused in both directions by the different acoustic reflections due to the walls and other obstacles of the room.

More precisely, the reverse wavefront corresponding to these counter-signals successively occupies the various positions occupied in the past by the initial "direct" wavefront, the phenomenon observed being comparable to the projection of a cinematographic film upside down.

The signals S _k (t) in question can then be considered to be given by the following formula:

The application of these signals S _k (t) on the sources 15 _k makes it possible to generate at the points i counter-noises C -or C (t) - which are capable of canceling the noises F _j (t) produced at these points by unwanted noise E _j (t).

Volume 2 then remains silent and inaccessible to said noises E _j (t), whatever their natures and intensities and whatever the reflections or reverberations undergone by some of their components before reaching said volume.

Of course, after having determined the impulse response laws gk (t), the battery 6 can be completely eliminated, which completely releases the surroundings of the acoustically isolated volume 2.

This is an important advantage of the present invention.

To obtain the desired neutralization of each noise F _i (t), the counter-noises C must reach the level of the points i at the same time as these noises.

This is where the difference between the two distances A and B between the battery 6 and the batteries 8 and 13 comes in respectively.

Care is taken to ensure that this difference is sufficient for the counter-noises to be able to be produced electronically during the time that the sounds take to travel the length A-B.

It turns out that, if this length is of the order of a meter, the resulting time (3 milliseconds) is more than sufficient for said electronic processing.

This is one of the original observations which led to the conception of the present invention.

The electronic circuits in question have been represented by rectangle 16 in FIG. 1.

• - d'une part, à chacun des capteurs acoustiques 11 par une chaîne comprenant un amplificateur 18_j et un convertisseur analogique-numérique 19_j,
• - et d'autre part à chacune des sources 15_k par une chaîne comprenant un convertisseur numérique-analogique 20_k et un amplificateur 21_k.

They have been given a little more detail in FIG. 2 where we see a memorization and calculation unit 17 connected:

• on the one hand, to each of the acoustic sensors 11 by a chain comprising an amplifier 18 _j and an analog-digital converter 19 _j ,
• - and on the other hand to each of the 15 _k sources by a chain comprising a 20 _k digital-analog converter and a 21 _k amplifier.

In practice, the noise E _j (t) which is recorded by the sensors 11 _j is not used continuously.

Sampling is carried out at a rate which corresponds substantially to the eighth of the shortest period characterizing the sound waves to be processed, that is to say at the highest frequency of the range used for the sensitivity of the sensors.

The frequency range to which the sensors are sensitive is advantageously between 10 Hz and 10,000 Hz.

Under these conditions, the highest frequency being 10 kHz, which corresponds to a period of 100 microseconds, the sampling frequency is equal to 80 kHz, which corresponds to a sampling carried out every 12 microseconds.

As regards the distances separating the different acoustic elements of the same battery or barrier, these distances are advantageously given a value equal to half the smallest wavelength of the range of frequencies concerned.

This is how the distance in question can be of the order of 10 centimeters, which provides specially good acoustic protection with respect to the low frequency components of the noises to be neutralized: the wavelength is indeed 33 centimeters. for a frequency of 1000 Hz.

As regards the number of acoustic elements making up each of the barriers or batteries, this number is equal to several tens, being in particular of the order of 50 to 100.

The convolution products, of these different numbers, which occur in formula III above are then relatively high, which can imply the use of relatively powerful calculating organs.

For this purpose, a digital signal processor of the DSP (Digital Signal Processor) type can be assigned to each of the sensors 11 j.

According to an advantageous improvement which will now be described, the necessary electronic work can be considerably simplified.

This improvement is based on the following considerations.

EFormula III above can also be written:

E

If we call h _jk (t) the second member of this convolution (i.e. if h _jk (t) = if _ij (t) ⊗g _ik (-t)), the formula IV becomes:

This formula is relatively simple since it no longer involves any of the points i.

Certainly these points i intervene during the development of the function h.

However, this development can be carried out beforehand during a preparatory stage followed by a storage of the function h developed, which is much more flexible than the previous solution.

• - on commence par mesurer chaque réponse impulsionnelle f_ij(t) sur une période de temps T, à compter du temps t=0 correspondant à l'émission de la courte impulsion acoustique initiale à partir du point i, ladite période s'étendant suffisamment pour contenir la totalité de la réponse i mpulsionnelle considérée, correspondant aussi bien à la trajectoire directe qu'aux réflexions parasites,
• - on mesure de même chaque réponse impusionnelle gk(t) sur la même période T,
• - on complète les deux fonctions ainsi mesurées par des 0 sur respectivement les deux périodes s'étendant de t=-oo au temps t=0 et du temps t=T au temps t=+oo,
• - on élabore et on mémorise la fonction "inverse" gk(-t),
• on calcule la fonction h_jk(t)=^Σ _if_ij(t)⊗j_ik(-t),
• - on mémorise les fonctions h ainsi calculées en remarquant que celles-ci sont symétriques en jk du fait que les deux réponses impulsionnelles f_ij(t) et gk(t) sont elles-mêmes symétriques, respectivement en ij et en ik,
• - c'est enfin avec la fonction h_jk(t) ainsi mémorisée que l'on convolue les bruits E_j(t) à neutraliser, conformément à la formule V ci-dessus, afin de déterminer les signaux opposés S_k(t).

In practice, we do the following:

• - we start by measuring each impulse response f _ij (t) over a period of time T, from the time t = 0 corresponding to the emission of the initial short acoustic pulse from point i, said period extending sufficiently to contain the totality of the impulse response i considered, corresponding as well to the direct trajectory as to parasitic reflections,
• - we likewise measure each impulse response gk (t) over the same period T,
• the two functions thus measured are completed by 0s respectively over the two periods extending from t = -oo at time t = 0 and from time t = T to time t = + oo,
• - we develop and store the "inverse" function gk (-t),
• we calculate the function h _jk (t) = ^Σ _i f _ij (t) ⊗j _ik (-t),
• the functions h thus calculated are memorized by noting that they are symmetrical in jk owing to the fact that the two impulse responses f _ij (t) and gk (t) are themselves symmetrical, respectively in ij and in ik,
• - it is finally with the function h _jk (t) thus memorized that we convolve the noises E _j (t) to neutralize, in accordance with the formula V above, in order to determine the opposite signals S _k (t) .

• - la batterie 8 comprend un réseau de 8x8 points j, soit 64 points j,
• - de même la batterie 13 comprend un réseau de 8x8 points k, soit 64 points k,
• - la batterie 6 comprend un réseau maillé tridimentionnel cubique de 8x8x8=512 points i,
• - le temps T est égal à 100ms, l'échantillonnage est effectué à une cadence de 100kHz, ce qui correspond à un nombre de 10 000 échantillons pour chaque relevé, et la résolution de chaque échantillon est de 12 bits, ce qui correspond à 1,5 octet : chaque relevé fait donc intervenir 15 000 octets.

To highlight the advantages brought by the improvement which has just been described, a numerical example is given below of course purely illustrative and not limiting of the invention:

• the battery 8 comprises an 8 × 8 point j network, or 64 point j,
• similarly, the battery 13 comprises an array of 8 × 8 k points, or 64 k points,
• the battery 6 comprises a cubic three-dimensional mesh network of 8 × 8 × 8 = 512 points i,
• - the time T is equal to 100 ms, the sampling is carried out at a rate of 100 kHz, which corresponds to a number of 10,000 samples for each reading, and the resolution of each sample is 12 bits, which corresponds to 1 , 5 bytes: each reading therefore involves 15,000 bytes.

If the general formula III given above is used directly, each of the impulse responses f _ij (t) and gk (-t) must be stored in memory, ie a total of 64x512 = 32,768 recorded for each of the two families: 'We take into account the symmetry, we can roughly divide this number by two, which again corresponds to a number of statements greater than 16,000 for each family.

The convolution product of these two families of impulse responses and the double convolution product of said product with the representative function of noise E _j (t) imply the use of powerful computers.

• - l'étape préparatoire d'élaboration et de mémorisation de la fonction h fait intervenir la sommation de i=1 à i=512 de 512 produits de convolution f_ij(t)⊗g_ik(-t): le résultat de cette sommation, qui constitue la fonction h, est mémorisé,
• - puis l'étape de création réelle des contre-bruits S n'a plus qu'à faire intervenir la détermination de la fonction h ainsi mémorisée pour chacun des couples de variables jk, c'est-à-dire, compte tenu de la symétrie du système en jk, pour un nombre total de tels couples de l'ordre de 2 080 seulement.

In the case of the improvement described above,

• - the preparatory stage of elaboration and memorization of the function h involves the summation of i = 1 to i = 512 of 512 convolution products f _ij (t) ⊗g _ik (-t): the result of this summation , which constitutes the function h, is memorized,
• - then the step of real creation of the counter-noises S has only to involve the determination of the function h thus memorized for each of the pairs of variables jk, that is to say, taking into account the symmetry of the system in jk, for a total number of such couples of the order of only 2080.

Ultimately, the storage to be carried out for the actual implementation of the invention comprises 2,080 × 15,000 bytes, that is to say 31.20 mega-bytes, which represents a completely reasonable number.

• - qu'à l'issue de la phase préparatoire, pour l'exemple numérique adopté, le nombre des fonctions à mémoriser est de l'ordre de 2 000 seulement alors qu'il était de l'ordre de 32 000 selon la formule générale,
• - et que, si l'on considère que le produit de convolution à effectuer dans chaque cas admet deux facteurs dont le premier est E_j(t), le second facteur est défini par quelque 2 000 fonctions dans le premier cas alors que, dans le cas général, il fait intervenir quelque 16 000x16 000=256 millions de fonctions.

In short, we can say:

• - that at the end of the preparatory phase, for the numerical example adopted, the number of functions to be memorized is only around 2,000 while it was around 32,000 according to the general formula ,
• - and that, if we consider that the convolution product to be performed in each case admits two factors, the first of which is E _j (t), the second factor is defined by some 2,000 functions in the first case whereas, in in general, it involves some 16,000x16,000 = 256 million functions.

As a result of which, and whatever the embodiment adopted, a device is finally obtained which makes it possible to effectively protect from external noise a given volume, device whose constitution and operation result sufficiently from the above.

This device has many advantages over those previously known and in particular that of providing acoustic protection even against random noise and even if the volume considered is placed inside a room whose walls are not specially treated to oppose acoustic reflections.

• - celles où les microphones 11j et/ou les haut-parleurs 15k utilisés pour la création des contre-bruits ne seraient pas les mêmes que ceux utilisés préalablement pour la calibration ou mise au point de l'installation en présence de la batterie 6, cas dans lequel les facteurs correctifs appropriés seraient introduits dans les calculs pour tenir compte des différences entre les réponses des appareils utilisés,
• - celles où le phénomène variable créé par les haut-parleurs et/ou celui mesuré par les microphones ne serait pas une pression, mais une vitesse de molécules d'air, cas dans lequel les facteurs correctifs appropriés seraient introduits dans les calculs, le passage de l'une de ces variables à l'autre étant obtenu par dérivation ou intégration temporelle,
• - et celles où, au cours de l'élaboration de l'une au moins des fonctions f et g, les rôles et emplacements des sources et des capteurs seraient permutés par rapport à ceux exploités ci-dessus : en effet, vu le théorème de la réciprocité ci-dessus rappelé, la fonction f_ij(t), étant égale à f_ji(t), peut être élaborée indifféremment en mettant en oeuvre des impulsions acoustiques courtes émises des différents points i et en analysant les réponses impulsionnelles correspondantes aux points j ou en mettant en oeuvre des impulsions acoustiques courtes émises des différents point j et en analysant les réponses impulsionnelles correspondantes aux points i ; en particulier on pourrait envisager de placer uniquement des sources acoustiques aux points i pour déterminer toutes les réponses impulsionnelles f_ij(t) et gk(t), les sources 15_k étant alors remplacées par des capteurs aux points k pour la détermination des réponses 9.

As goes without saying, and as it already follows from the above, the invention is in no way limited to those of its modes of application and embodiments which have been more especially envisaged; on the contrary, it embraces all variants, in particular,

• - those where the microphones 11j and / or the speakers 15k used for the creation of the counter-noises would not be the same as those used previously for the calibration or adjustment of the installation in the presence of the battery 6, case in which the appropriate corrective factors would be introduced into the calculations to take account of the differences between the responses of the devices used,
• - those where the variable phenomenon created by the loudspeakers and / or that measured by the microphones would not be a pressure, but a speed of air molecules, case in which the appropriate corrective factors would be introduced into the calculations, the passage from one of these variables to the other being obtained by derivation or temporal integration,
• - and those where, during the development of at least one of the functions f and g, the roles and locations of the sources and the sensors would be permuted with respect to those exploited above: indeed, having regard to the theorem of the reciprocity mentioned above, the function f _ij (t), being equal to f _ji (t), can be developed indifferently by implementing short acoustic pulses emitted from the different points i and by analyzing the impulse responses corresponding to the points j or by implementing short acoustic pulses emitted from the different points j and by analyzing the corresponding impulse responses at points i; in particular one could consider placing only acoustic sources at points i to determine all the impulse responses f _ij (t) and gk (t), the sources 15 _k then being replaced by sensors at points k for determining the responses 9 .

Claims (10)

1. Device for protecting a given volume from external noise, characterized in that it comprises, on the one hand, disposed respectively at two distinct distances A and B of the same fictitious reticulated battery (6) defining points i arranged in the volume (2) to be acoustically protected, a battery (8) of acoustic sensors (11 _d ) receiving the noises to neutralize E _j (t) and a battery (13) of acoustic sources (15 _k ), the distance B being less at distance A, and on the other hand, an electronic circuit (16) interposed between said sensors and said sources and arranged so as to develop, in times less than AB v being the speed of sound in air, for each noise E _j (t), a plurality of signals S _k (t) which are applied instantaneously, respectively, to the sources (15 _k ), each signal S _k (t) being equal to:

formula in which: each function f _ji (t) is identical to the reciprocal function f _ij (t) which is the impulse response, previously determined and recorded, corresponding to the noise generated on the sensor (11 _j ) of index j of the battery of sensors above by the emission of a short acoustic pulse from a source (10i) supposedly placed at point i, - and each function gk (-t) is developed from the function g _k (t) which is itself identical to the reciprocal function gk (t), which in turn is the impulse response, previously determined and recorded, corresponding to the noise generated on a sensor (12 _i ) assumed to be placed at point i, from the emission of a short acoustic pulse by the source (15 _k ) of index k from the battery of sources above. 2. Device according to claim 1, characterized in that the detection of the noise E _j (t) necessary for the preparation of the signals S is carried out by sampling at a rate corresponding substantially to the eighth of the shortest period characterizing the sound waves to be processed, that is to say at the highest frequency of the range used for the sensitivity of the sensors. 3. Device according to any one of the preceding claims, characterized in that the frequency range to which the sensors are sensitive (11 _d ) is between 10 and 10 000 Hz. 4. Device according to any one of the preceding claims, characterized in that the number of acoustic elements (10 ,, 11 _d , 12, 15 _k ) making up each of the batteries (6, 8, 13) is equal to several tens, being in particular of the order of 50 to 100, and in that the distances which separate these elements from each other in each battery is of the order of a decimeter. 5. Device according to any one of the preceding claims, characterized in that the difference between the distances A and B is of the order of 1 meter. 6. Device according to any one of the preceding claims, characterized in that each signal S _k (t) is equal to S _k (t) = - Σ ^E _j j (t) ⊗h _jk (t), formula in which h _jk (t) is a previously determined and recorded function equal to:

7. Method for determining the impulse responses f _ij (t) and g _ki (t) which are used for the preparation of the signals S according to any one of the preceding claims, characterized in that one has close to the volume (2) to acoustically protect, so as to delimit at least part of this volume, a crosslinked battery (6) defining a plurality of points i in which we place: initially, acoustic sources (10), the responses f _ij (t) then being determined at the level of the above permanent sensors (11 _j ) during the emission of short acoustic pulses by said sources; and secondly, acoustic sensors (12), the responses g _ki (t) then being determined at the level of these sensors during the emission of short acoustic pulses by the above permanent sources (15 _k ). 8. Method according to claim 7, characterized in that, in at least one of the two source assemblies (10 _i ; 15 _k ) -sensors (11 _j ; 12 _i ) used respectively during the two successive "times", the respective roles and locations of the sources and sensors are swapped. 9. Crosslinked battery of acoustic elements (10 _i ; 12 _i ) for implementing the method according to any one of claims 7 and 8. 10. Method for implementing the device according to claim 6, characterized in that one proceeds to a prior step of developing and recording the function h _jk (t).

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