EP0601934B1 - 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 notably those intended to be occupied by the head of an individual, especially when seated or lying down: when the desired acoustic protection is obtained, the individual concerned is safe from acoustic aggression outside as long as his head remains at inside such a volume.

To ensure such acoustic protection, it has already been proposed to interpose between the volumes in question and the outside of these soundproof partitions insulating.

The insulation obtained by such partitions is limited and physical barriers materialized by said partitions are often prohibitive.

It has also been proposed to neutralize certain sounds received by such volumes by applying to said volumes volumes of "counter-noises" of identical amplitude and opposite phase to those of said sounds. Document GB-A-2 191 063 discloses such a device.

But to date, this type of neutralization, sometimes referred to as active attenuation, leads to encouraging results only for sounds relatively pure sine waves and transmitted directly from their source to the volume to be protected.

In particular, the random noises could not be treated properly in this way and when volumes considered are inside premises, laterally delimited by partitions, lower by a floor and above by a ceiling, one is hardly managed until today to control the phenomena of reflection or reverberation of the noises to neutralize 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 as defined in the claims is, above all, to remedy all of these disadvantages by protecting a volume arranged inside a room opposite noises of all kinds produced outside this room, and in particular from certain directions privileged corresponding for example to windows.

   formule dans laquelle :

• chaque fonction f_ji(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 g_ik(-t) est élaborée à partir de la fonction g_ik(t) qui est elle-même identique à la fonction réciproque g_ki(t), laquelle est à son tour la réponse impulsionnelle, préalablement déterminée et enregistrée, correspondant au bruit engendré sur un capteur supposé 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, 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 _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 of index j of the battery of sensors above by l 'emission of a short acoustic pulse from a supposed source placed at point i,
• and each function g _ik (-t) is developed from the function g _ik (t) which is itself identical to the reciprocal function g _ki (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 of 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 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 to the highest frequency of the range selected 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 them between these elements 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 g _ki (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 g_ki(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:

• initially, 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 g _ki (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 "times" successive of the processes defined above, one could swap the respective roles and locations of sources and sensors.

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 arrangements main, certain other provisions which preferably used at the same time and which will be more explicitly question below.

In the following, we will describe a mode of preferred embodiment of the invention with reference to drawing attached hereto in a manner which is of course not limiting.

Figure 1 of this drawing shows very schematically a room equipped with a device to protect outside noise a limited volume of this room.

Figure 2 is a diagram of the electronic circuit understood by this device.

We propose to protect against noise random E schematized by the arrow 1 a volume 2 relatively limited located in a delimited room 3 laterally by partitions 4, below by a floor and above by a ceiling.

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

Volume 2 presents for example the form of a sphere or 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 we want to isolate noise E, this person being by example sitting in front of a desk or lying in a bed.

To solve the problem, we use the technique known per se of active attenuation which consists in protecting a given point from annoying noises, creating counter-noises at this point opposed to the said noises and determined in such a way that their addition to these noises at said point produces in this one a null resultant, that is to say removes 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, even deletion, of unwanted noise in volume 2 above defined, and 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.

Interpose between volume 2 to be acoustically protected and the source of the noise E towards which we wishes to ensure said protection two "barriers" or "drums" 6 and 8 each made up of acoustic elements distinct kept separate from each other by a rigid frame (respectively 7.9) openwork opposite 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 distinct "nodes" i-1, i, i + 1 ... occupying at least in part of 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 _i .

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 _ji (t) which would be collected by supposed sensors placed at exactly the same locations i that the sources 10 _i 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 into account in particular all 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 _i of the battery 6 is then replaced by acoustic sensors 12 _i 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 g _ki (t) corresponding to the noise which is generated on the sensor 12 _{i is then determined} by the emission of a short acoustic pulse from the source 15 _k .

By virtue of the reciprocity theorem mentioned above, each function g _ki (t) is strictly identical to the reciprocal function g _ik (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 II 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 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 specifically, the corresponding reverse wavefront to these counter signals successively occupies the various positions that the front occupied in the past initial "direct" wave, the observed phenomenon being comparable to the projection of a cinematographic film at 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 _i (t) - which are capable of canceling the noises F _i (t ) produced at these points by undesirable 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 g _ki (t), the battery 6 can be completely eliminated, which completely clears the surroundings of the acoustically isolated volume 2.

This is an important advantage of this 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 respectively separating the battery 6 of batteries 8 and 13.

We take care that this difference is sufficient so that the noise can be produced electronically during the time that sounds take for travel the length A-B.

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

This is one of the original findings which have enabled the design of the present invention.

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

• d'une part, à chacun des capteurs acoustiques 11₁ par une chaíne comprenant un amplificateur 18_j et un convertisseur analogique-numérique 19_j,
• et d'autre part à chacune des sources 15_k par une chaíne 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 ₁ 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 digital-analog converter 20 _k and an amplifier 21 _k .

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

We proceed by sampling, at a rate which roughly corresponds to the eighth of the most short characterizing the sound waves to be processed, i.e. at the highest frequency of the range selected for the sensitivity of the sensors.

The range of frequencies to which are sensitive the sensors 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 at 80 kHz, which corresponds to a sampling carried out every 12 microseconds.

As for the distances between different acoustic elements of the same drum or barrier, these distances are advantageously given a value equal to half of the longest wavelength small of the affected frequency range.

This is how the distance in question can be of the order of 10 centimeters, which provides protection especially good acoustics compared to the components of low frequency of noises to be neutralized: the length wave is indeed 33 centimeters for a frequency 1000 Hz.

Regarding 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 from 50 to 100.

Convolution products, of these different numbers, which are involved in formula III above are then relatively high, which can imply the use of relatively powerful computing devices.

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 be now described, we can considerably simplify the electronic work required.

This improvement is based on considerations following.

Formula III above can also be written:

la la formule IV devient :

If we call h _jk (t) the second member of this convolution (i.e.

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 elaboration of the function h.

But this preparation can be carried out beforehand during a preparatory stage followed by a storage of the elaborate function h, 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 impulsionnelle considérée, correspondant aussi bien à la trajectoire directe qu'aux réflexions parasites,
• on mesure de même chaque réponse impusionnelle g_ki(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=-∞ au temps t=0 et du temps t=T au temps t=+∞,
• on élabore et on mémorise la fonction "inverse" g_ik(-t),
• on calcule la fonction
  
• 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 g_ik(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 considered, corresponding as well to the direct trajectory as to parasitic reflections,
• we likewise measure each impulse response g _ki (t) over the same period T,
• the two functions thus measured are completed by 0s respectively over the two periods extending from t = -∞ at time t = 0 and from time t = T to time t = + ∞,
• we work out and memorize the "inverse" function g _ik (-t),
• we calculate the function
  
• the functions h thus calculated are memorized by noting that they are symmetrical in jk because the two impulse responses f _ij (t) and g _ik (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 array of 8 × 8 points j, or 64 points j,
• similarly, the battery 13 comprises an 8 × 8 k point network, 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 byte: each reading therefore involves 15,000 bytes.

If we directly use the general formula III given above, it is necessary to store each of the impulse responses f _ij (t) and g _ik (-t), ie a total of 64x512 = 32,768 recorded for each of the two families: if 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 for developing and memorizing 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 only has 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 2,080.

Ultimately, the storage to be carried out for the actual implementation of the invention includes 2,080x15,000 bytes, i.e. 31.20 megabytes, which represents quite a 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 2000 functions in the first case whereas, in the in general, it involves some 16,000x16,000 = 256 million functions.

As a result of what, and whatever the mode of adopted implementation, we finally obtain a device which effectively protects from outside noise a given volume, a device whose constitution and operation result sufficiently from the above.

This device presents compared to those previously known many advantages and in particular that of ensuring acoustic protection even vis-à-vis random noises and even if the volume considered is arranged inside a room whose walls are not not specially treated to oppose reflections acoustic.

• 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 g_ik(t), les sources 15_k étant alors remplacées par des capteurs aux points k pour la détermination des réponses g.

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 calculations, the passage of 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 compared to those exploited above: indeed, considering the theorem of the reciprocity above recalled, 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 corresponding impulse responses at 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 we could consider placing only acoustic sources at points i to determine all the impulse responses f _ij (t) and g _ik (t), the sources 15 _k then being replaced by sensors at points k for determining the responses g.

Claims (10)

1. Device for protecting a given volume from outside noises, characterized in that it comprises, on the one hand, arranged respectively at two distinct distances A and B from a same reticulate fictitious array (6) defining points i arranged in the volume (2) to be acoustically protected, an array (8) of acoustic sensors (11_j) receiving the noises to be cancelled E_j(t) and an array (13) of acoustic sources (15_k), the distance B being less than the distance A, and on the other hand, an electronic circuit (16) interposed between the said sensors and the said sources and configured so as to calculate, in time spans less than A-B / v, 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:

   

   a formula in which:

   each function f_ji(t) is identical to the reciprocal function f_ij(t) which is the impulse response, determined and recorded beforehand, corresponding to the noise generated at the sensor (11_j) of index j of the above array of sensors through the emission of a short acoustic pulse from a source (10_i) assumed stationed at the point i,

   and each function g_ik(-t) is calculated from the function g_ik(t) which is itself identical to the reciprocal function g_ki(t), which is in turn the impulse response, determined and recorded beforehand, corresponding to the noise generated at a sensor (12_i) assumed stationed at point i, from the emission of a short acoustic pulse by the source (15_k) of index k of the above array of sources.
2. Device according to Claim 1, characterized in that the detection of the noises E_j(t) required for calculation of the signals S is performed by sampling at a rate corresponding substantially to one eighth of the shortest period characterizing the sound waves to be processed, that is to say to the highest frequency of the range selected for the sensitivity of the sensors.
3. Device according to either one of the preceding claims, characterized in that the spread of frequencies to which the sensors (11_j) are sensitive is included 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_i, 11_j, 12_i, 15_k) making up each of the arrays (6, 8, 13) is equal to several tens, being especially of the order of 50 to 100 and in that the distances which mutually separate these elements within each array is of the order of a decimetre.
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 metre.
6. Device according to any one of the preceding claims, characterized in that each signal S_k(t) is equal to

   

   in which formula h_jk(t) is a function determined and recorded beforehand equal to:

   
7. Process for determining the impulse responses f_ij(t) and g_ki(t) which are used for the calculation of the signals S according to any one of the preceding claims, characterized in that in proximity to the volume (2) to be acoustically protected there is arranged, in such a way as to delimit a portion at least of this volume, a reticulate array (6) defining a plurality of points i at which are stationed: in a first time span, acoustic sources (10_i), the responses f_ij(t) then being determined in the vicinity of the above permanent sensors (11_j) during the emission of short acoustic pulses by the said sources, and in a second time span, acoustic sensors (12_i), the responses g_ki(t) then being determined in the vicinity of these sensors during the emission of short acoustic pulses by the above permanent sources (15_k).
8. Process according to Claim 7, characterized in that, within at least one of the two source (10_i; 15_k) - sensor (11_j; 12_i) assemblies used in the course of the two successive "time spans" respectively, the respective roles and locations of the sources and sensors are interchanged.
9. Reticulate array of acoustic elements (10_i; 12_i) for the implementation of the process according to either one of Claims 7 and 8.
10. Process for implementing the device according to Claim 6, characterized in that a prior step of calculation and recording of the function h_jk(t) is undertaken.

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