EP2467847B1 - Open-worked acoustic barrier for hybrid active/passive noise treatment - Google Patents

Description

Background of the invention

The present invention relates to the general field of acoustic reduction devices and methods.

There are currently two main families of sound reduction devices: the family of passive acoustic reduction devices and the family of active acoustic reduction devices.

In the first family, we find acoustic barriers or acoustic screens based on inert materials. For example, screens or noise barriers made of concrete have a certain effectiveness in countering road noise. Passive acoustic reduction devices also include window glazing that functions as a sound barrier when the window is closed.

The disadvantages of passive acoustic reduction devices are visual opacity and generally thermal opacity. Indeed, noise barriers do not generally allow heat exchange or limit very strongly and are visually opaque. In addition, it is known that acoustic screens generally create a diffraction sound emission at the top of the screen.

Regarding the windows, they have no sound treatment as soon as they are open.

Overall, the passive control of noise by screen effect is to interpose a wall, a door, a wall or a glazing between the source of the noise and the place in which it is desired to obtain a reduced noise.

The second family consists of active noise control. An example of active control is described in the patent WO 1997/02471 . In this document, an active noise control is used to reduce the air duct noise.

The technology that is described in this document consists in the realization of an active acoustic box including a microphone / loudspeaker pair adapted to measure the primary noise emitted in the ventilation duct and adjust the emission of the loudspeaker according to this primary noise to actively reduce this primary noise as emitted into the ventilation duct.

The figure 1 shows the zone of effectiveness ZEP generally observed with a passive system of reduction of the noise. It can be seen that the effectiveness of passive noise control is mainly concentrated in the audible frequency spectrum, but it is only effective in treating the high frequencies.

The figure 1 also presents the spectrum of road noise SR and it is found that this type of noise is characterized by a high concentration of sound in the low frequencies. Thus the noise barriers generally used are relatively ineffective or not effective at all according to the predominant frequencies of the road spectrum SR.

Despite the improvement of screen noise reduction devices over the last three decades, this technology is now reaching its limits and it is now difficult to envisage an improvement in traditional installations.

The figure 2 represents, in this respect, the improvement of the zone of effectiveness ZEP 'of a passive screen when the thickness thereof is doubled with respect to the zone of efficiency ZEP represented on the figure 1 obtained for a wall 10 cm thick. It is noted that the reduction is improved for the high frequencies but remains practically unchanged for the low frequencies.

It is also known that the effectiveness of road noise barriers is a function of their height. The higher they are, the better the reduction of road noise annoyance.

However whatever the height of the wall, it is known that there is a phenomenon of sound re-emission by the edge of the noise barrier. This well-known phenomenon is related to the sound diffraction of the road noise on the edge of the wall which behaves like a sound re-transmitter of this noise.

The figure 3 schematically illustrates this process of retransmission of a plane wave arriving on the left of a wall 10. This plane wave is attenuated by the presence of the wall 10 but also reemitted in the form of a spherical wave by the edge of the wall according to the diffraction phenomenon.

This phenomenon greatly impairs the effectiveness of the wall by creating non-homogeneous areas of sound attenuation. In order to make it possible to move this diffraction problem as far as possible from the wall, it is possible to raise a wall as high as possible. This solution is not really one in that it greatly increases the costs associated with the construction of the wall but also to the extent that it greatly increases the wind resistance of the wall.

The effectiveness of active acoustic box technology is subject to two conditions. The first condition is related to the wavelength and the second condition to the speed of electronic computers. It turns out that the effectiveness of active control is actually limited for high frequencies by techno-economic reasons.

Active systems are thus more efficient for low frequencies than for high frequencies. In addition, it is not currently possible to perform an active control for high frequencies, which reduces the application field of this noise control.

The document US 2006/285697 discloses a hybrid type acoustic reduction system in which beams comprising loudspeakers are arranged obliquely relative to each other.

The document US 2007/223714 discloses a hybrid type acoustic reduction device in which beams comprising loudspeakers are arranged obliquely with respect to each other. The document US 4,665,549 discloses a hybrid type acoustic reduction device with beams comprising loudspeakers in acoustical absorbent material disposed facing each other.

Object and summary of the invention

• fabriquer une pluralité d'éléments de réduction acoustique, comprenant chacun un microphone et un haut-parleur, en réalisant les étapes suivantes, pour chaque élément :
  • placer le microphone dans un caisson, réalisé en matériau absorbant acoustique passif ou comprenant un matériau absorbant acoustique passif, à proximité de la surface d'un côté dit principal du caisson ;
  • placer dans ce caisson, à côté du microphone, le haut-parleur également à proximité de la surface du côté principal et de manière à ce que la direction d'émission principale du haut-parleur soit substantiellement perpendiculaire au côté principal ;
• disposer côte à côte n éléments de réduction acoustique constituant ainsi une poutre de réduction acoustique dite poutre électro-acoustique ;
• disposer m poutres électro-acoustiques côte à côte et séparée par un intervalle et substantiellement parallèles les unes aux autres en dirigeant le côté principal du caisson vers le côté opposé au côté principal de la poutre voisine, pour que les haut-parleurs émettent dans l'intervalle entre deux poutres, constituant ainsi une barrière acoustique à clairevoies combinant un effet passif et un effet actif de réduction du bruit ;
• introduire du matériau absorbant acoustique sur le côté du caisson opposé au côté principal pour ajuster l'impédance acoustique du caisson et éviter l'apparition d'onde stationnaire entre le côté principal d'une poutre et le côté opposé au côté principal de la poutre voisine ;
• pour chaque élément de réduction acoustique, mesurer la fonction de transfert entre le microphone et le haut-parleur ;
• pour chaque élément de réduction acoustique, calculer un filtre électronique de contrôle à contre-réaction à partir de la fonction de transfert entre le microphone et le haut-parleur, cette fonction de transfert étant linéarisée par la présence de matériau absorbant introduit sur le côté du caisson opposé au côté principal, ce filtre électronique permettant, au sein de chaque élément de réduction acoustique, de boucler le haut-parleur sur le microphone électro-acoustiquement en amplifiant la contre-réaction afin d'obtenir un effet d'absorption acoustique en temps réel pour une gamme de fréquences prédéterminée.

The main purpose of the present invention is thus to overcome the disadvantages of the devices known from the prior art by proposing a passive and active acoustic reduction method comprising the steps of:

• fabricating a plurality of acoustic reduction elements, each comprising a microphone and a speaker, performing the following steps for each element:
  • placing the microphone in a box, made of passive acoustic absorbing material or comprising a passive acoustic absorbent material, close to the surface of a so-called main side of the box;
  • place in this box, next to the microphone, the speaker also close to the surface of the main side and so that the main transmission direction of the speaker is substantially perpendicular to the main side;
• arranging side by side n acoustic reduction elements thus constituting an acoustic reduction beam called electro-acoustic beam;
• arranging m electro-acoustic beams side by side and separated by an interval and substantially parallel to each other by directing the main side of the box towards the opposite side to the main side of the adjacent beam, for the loudspeakers to emit in the gap between two beams, thus constituting a clear-voice acoustic barrier combining a passive effect and an active noise reduction effect;
• introducing acoustical absorbent material on the side of the housing opposite the main side to adjust the acoustic impedance of the box and avoid the appearance of standing wave between the main side of a beam and the opposite side to the main side of the adjacent beam ;
• for each acoustic reduction element, measuring the transfer function between the microphone and the loudspeaker;
• for each acoustic reduction element, calculating an electronic feedback control filter from the transfer function between the microphone and the loudspeaker, this transfer function being linearized by the presence of absorbent material introduced on the side of the box opposite the main side, this electronic filter making it possible, within each acoustic reduction element, to loop the loudspeaker on the electro-acoustically microphone by amplifying the feedback to obtain a sound absorption effect in time real for a predetermined frequency range.

The invention therefore makes it possible to obtain perforated acoustic barriers that harmoniously combine the processing of noise with the aid of a passive system and an active system.

Indeed, the active system being carried by structures made from passive acoustic absorbent materials and judiciously arranged, ensures a harmonious combination of both active and passive systems. The active systems are furthermore judiciously placed in relation to the structures constituting the passive system so as to optimize their operation by emitting in the interval between the passive structures.

As the passive acoustic reduction allows a reduction of the noise in the high frequencies and the active device allows an acoustic reduction at low frequencies, a broadband sound processing is obtained according to the invention.

In addition, there are very significant benefits including reducing the weight of structures, the possibility of natural ventilation through the acoustic reduction device, a decrease in the wind resistance of the acoustic barrier.

The invention also makes it possible to treat the problem of the open window for ventilating a dwelling. Indeed, the invention allows to create a perforated screen that can be inserted instead of the glazing to let air and block the noise at low frequency in the air passages.

In particular, the invention makes it possible to deal very effectively with road noise which is generally more effectively handled by an active control rather than a traditional passive system.

The introduction of acoustic absorbing material in front of each element makes it possible to ensure the absence of a standing wave between the beams. This could be harmful because a standing wave is characterized by minima and noise maxima related to the interference of two waves that will propagate in the opposite direction. This is annoying because the active control is the control of the wave that propagates using another wave interposed in phase opposition not in reverse. In the case of active control, therefore, there are only minima and no maxima unless there is reverberation of the wave at a point. In this case, in the presence of the active control, the sound absorbing material prevents the wave from bouncing and interfering in the opposite direction. The advantages of the combination between active noise control elements and the presence of passive sound reduction elements in synergy with the active control are explained in the following.

The presence of acoustic material facing the active control elements improves the transfer function of the space separating the active control elements and the supporting structures of these indispensable elements in the case of a barrier to skylights. This makes it possible to smooth the transfer function from a module and phase point of view and optimize it by linearization. The control by the microphone of the filter then makes it possible to buckle the speaker by the microphone. This overall improves the bandwidth and amplitude of the active noise control. The active / passive combination makes it very effective the barrier to skies thus made according to the principles of the invention.

The use of the two principles of noise reduction combined provides a very good efficiency for the treatment of road noise.

The use of several aligned acoustic reduction elements as well as several beams each comprising an alignment of acoustic reduction elements implies the presence of a noise due to the operation of each device taken separately. The alignment on the same side of the beam allows a uniformity of the treatment of the primary wave. Moreover, the parallelism of the beams allows that the sound attenuation is uniform on the barrier. In the case of beams that would not be substantially parallel, attenuation more or less strong depending on the distance between the beams would be observed which would be harmful. By the term "substantially parallel" is meant here that the beams can be strictly parallel which is the most favorable case but also that the beams can make a slight angle between them resulting in a slightly trapezoidal gap between them.

With the invention, the calculation of the electronic counter-reaction control filter can make it possible to carry out an electronic filtering to control this against noise.

It is therefore possible that the secondary noise coming from each box is treated at the same time as the primary noise that the acoustic reduction device is intended to reduce. This avoids the interference of one box on the other.

The realization of the hybrid active / passive acoustic reduction device according to the invention thus makes it possible to create acoustically opaque sound barriers at low and medium frequencies but optically translucent and / or open to allow the passage of light and / or hot or cold flows and let the heat exchange take place.

The applications of the invention therefore relate to windows that can be opened, road noise barriers, all types of sound screens that are sometimes installed at the top of building roofs with air exchangers or any other type of noisy machines that disturb the neighborhood.

According to an advantageous characteristic of the invention, the electronic filter is further such that the emissions of the loudspeakers aligned with the beam interfere positively and additionally.

These positive and additional interferences bring an additional effect to the simple operation by counteracting the filter.

According to one embodiment of the invention, the box is common for several acoustic reduction elements of the same beam.

The use of such a common box for a plurality of acoustic reduction elements allows easier manufacture of the beams according to the invention.

According to an advantageous characteristic of the invention, the method comprises a step of optimizing the distance between two beams as a function of the acoustic result in terms of the number of decibels and cutoff frequencies of the active reduction, the visual appearance and of heat exchange.

According to a preferred feature of the invention, the acoustic barrier having a free edge, the method further comprises a step of installing acoustic reduction elements on this free edge to reduce the sound diffraction.

The invention makes it possible to perform a treatment of sound diffraction on the edges of the acoustic screens by means of an active system only.

Indeed, passive systems can do nothing against sound diffraction. On the other hand, the calculation of a specific filtering using the transfer function between each microphone and each loudspeaker of the acoustic reduction elements present on the acoustic beam placed at the top of a passive screen makes it possible to carry out an active control of the diffracted noise. . This makes it possible to significantly reduce the sound re-emission on the other side of the acoustic barrier according to the invention.

In a particular embodiment of the invention, the beam consisting of a plurality of acoustic elements has replaced by an elongated form of speaker associated with at least one microphone disposed near the speaker.

This feature is to use a simplified form of speaker that can be miniaturized rather than a plurality of aligned loudspeakers and microphones.

Advantageously, the predetermined frequency range is the range of low frequencies below 500 Hertz.

This frequency range corresponds to the spectrum accessible by active acoustic reduction systems whose implementation remains technically possible at moderate costs. The open screen thus allows the treatment of lower frequencies. Beyond this frequency, the natural beam barrier does its job as a normal screen for high frequencies.

The invention also relates to a passive and active acoustic reduction device comprising m electro-acoustic beams side by side and separated by an interval, each electro-acoustic beam comprising a plurality of acoustic reduction elements arranged side by side, each element of acoustic reduction comprising a microphone and a loudspeaker placed in a box, made of a passive acoustic absorbing material or comprising a passive acoustic absorbing material, close to the surface of a side said main side of the box and so that the the main transmission direction of the loudspeaker is substantially perpendicular to the main side, the microphone and the loudspeaker being connected to a control electronics capable of receiving a measurement of the transfer function between the microphone and the loudspeaker,
each beam comprising an acoustic absorbing material on the side of the box opposite the main side to adjust the acoustic impedance of the box and avoid the appearance of standing wave between the main side of a beam and the opposite side to the main side of the neighboring beam,
the beams being disposed substantially parallel to one another, side by side so that the main sides of the acoustic elements are directed towards the side opposite the main side of the neighboring beam so that the loudspeakers emit in the interval between two beams, thus constituting a clear-voice acoustic barrier combining a passive effect and an active noise-reducing effect,
the control electronics comprising means for calculating an electronic feedback control filter, for each acoustic reduction element, from the transfer function between the microphone and the loudspeaker, this transfer function being linearized by the presence of absorbent material introduced on the side of the box opposite the main side, this electronic filter allowing, within each acoustic reduction element, loop the speaker on the microphone electro-acoustically by amplifying the feedback to obtain a real-time acoustic absorption effect for a predetermined frequency range.

According to a preferred implementation, the last two steps of the method for measuring the transfer and calculation functions of a filter according to the invention are determined by computer program instructions.

Accordingly, the invention also relates to a computer program on an information medium, this program being capable of being implemented in a computer, this program comprising instructions adapted to the implementation of the last two steps of the program. process according to the invention.

This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other form desirable shape.

The invention also relates to a computer-readable information medium, comprising instructions of a computer program as mentioned above.

The information carrier may be any entity or device capable of storing the program. For example, the medium may comprise a storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a floppy disk, a disk hard, a flash memory, a USB stick etc.

On the other hand, the information medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be downloaded in particular on an Internet type network.

Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

Brief description of the drawings

• la figure 1 montre la zone d'efficacité d'un système passif d'atténuation et le spectre fréquentiel du bruit routier ;
• la figure 2 montre la réduction sonore d'un écran passif dont on double l'épaisseur ;
• la figure 3 montre schématiquement un exemple de réémission sonore par l'arête d'un mur ;
• la figure 4 montre une structure de barrière acoustique selon l'invention ;
• la figure 5 montre la combinaison passif/actif obtenue selon l'invention pour le traitement du bruit routier ;
• la figure 6 montre schématiquement une poutre acoustique selon l'invention ;
• la figure 7 montre un schéma d'un élément de réduction acoustique utilisé dans une poutre selon l'invention ;
• les figures 8a et 8b montrent des exemples de barrière acoustique réalisées selon un mode de réalisation préférentiel de l'invention ;
• la figure 9 montre un exemple de fenêtre munie d'un dispositif de réduction acoustique selon l'invention ;
• la figure 10 montre une expression graphique de la fonction de transfert Hex(ω)dans le plan complexe ;
• les figures 11a et 11b montrent des expressions graphiques du module et de la phase d'un système électroacoustique perturbé par des ondes stationnaires ;
• les figures 12a et 12b représentent des fonctions de transfert Hex(ω) non optimisées et optimisées par la présence selon l'invention d'un matériau passif placé à l'avant du haut-parleur.

Other features and advantages of the present invention will emerge from the description given below, with reference to the accompanying drawings which illustrate an embodiment having no limiting character. In the figures:

• the figure 1 shows the efficiency zone of a passive attenuation system and the frequency spectrum of road noise;
• the figure 2 shows the sound reduction of a passive screen whose thickness is doubled;
• the figure 3 schematically shows an example of sound re-emission by the edge of a wall;
• the figure 4 shows an acoustic barrier structure according to the invention;
• the figure 5 shows the passive / active combination obtained according to the invention for the treatment of road noise;
• the figure 6 shows schematically an acoustic beam according to the invention;
• the figure 7 shows a diagram of an acoustic reduction element used in a beam according to the invention;
• the figures 8a and 8b show examples of acoustic barriers made according to a preferred embodiment of the invention;
• the figure 9 shows an example of a window provided with an acoustic reduction device according to the invention;
• the figure 10 shows a graphical expression of the transfer function Hex (ω) in the complex plane;
• the Figures 11a and 11b show graphical expressions of the module and the phase of an electroacoustic system disturbed by standing waves;
• the Figures 12a and 12b represent non-optimized Hex (ω) transfer functions optimized by the presence according to the invention of a passive material placed in front of the loudspeaker.

Detailed description of an embodiment

The figure 4 represents an example of acoustic barrier structure according to the invention. Between two fixing posts 40 are arranged a plurality of acoustic beams 41, here 5 (m = 5). These beams 41 are as many linear structures separated by a distance D allowing the flow of air and light.

Acoustic beams 41 constitute passive noise reduction elements. They are thus advantageously made from passive acoustic absorbent materials or comprise acoustic absorbing materials so as to allow partial acoustic insulation in the high frequencies.

According to the invention, each acoustic beam 41 includes several identical and independent active systems physically and mechanically associated to achieve an active acoustic effect in the thickness intervals D between the passive acoustic beams. The plurality of acoustic beams 41 makes it possible to obtain a combination of the active and passive treatments.

The invention thus makes it possible to obtain broadband processing as well as represented on the figure 5 . This figure shows the passive efficiency zone ZEP and the zone of active efficiency ZEA and the spectrum of the road noise SR. It is thus noted that, even if the perforated nature of the noise canceling device inevitably leads to a reduction in the efficiency of the passive acoustic reduction, the acoustic reduction generated by the active means largely makes up for this reduction since the most important intensities of the road noise SR are in low frequencies that active treatment can treat.

The figure 6 shows an acoustic beam 41. According to the invention, this beam advantageously consists of a box 60 on which are placed loudspeakers 61 and microphones 62. Each pair microphone / speaker constituting an acoustic reduction element in the sense of the invention. On the example of the figure 6 , the beam 41 comprises fourteen microphone-speaker pairs, ie n = 14 placed on one side of the box 60 called the main side 63.

Note that the main side is not visible on the beams 41 of the figure 4 since, pointing downwards, the proposed perspective does not allow to see them.

The figure 7 shows such an acoustic reduction element noted 70 comprising a metal enclosure 71 on the bottom of which is placed a passive absorbent 72 for example rock wool. This absorbent material is used to adjust the acoustic impedance into which the loudspeakers of the main side 63 of the neighboring beam will be discharged. The enclosure 71 is for example a metal acoustic enclosure serving as a box within the meaning of the invention.

Within the chamber 71, are placed the speaker 61 and the microphone 62. The microphone 62 serves as a reference for calculating the emission of the speaker 61 as a secondary source.

The presence of passive absorbent 72 on the bottom of the metal enclosure 71 allows an adjustment of the acoustic impedance in which the secondary sources, which are the loudspeakers placed on the adjacent beam facing the bottom of the beam considered, flow. . In the acoustic barrier of the figure 4 , the passive absorbent 72 makes it possible to adjust the impedance of the loudspeakers situated on the beam above the considered beam since the loudspeakers emit downwards. Depending on the structure chosen for the barrier, we note here that the speakers can emit upward or downward, or to the left or right, of the acoustic barrier.

Also the passive absorbent 72 must be enough absorbent to prevent the standing waves from settling between the top of a beam and the bottom of the neighboring beam, which is for example that which emits in the thickness range D located between the two beams.

The principles of acoustic adjustment are explained below. Indeed, when one builds an active control system of type "feedback" or feedback, it is necessary to characterize the acoustic medium to know its frequential characteristics and the sound level for each frequency.

An appreciable tool, as claimed, is the measurement of the transfer function between the transducers, microphone and speaker that will create the desired acoustic effect. This effect is an active reduction of the noise in the context of the invention.

The first criterion to check in an active noise control is the stability of the system in operation. Indeed, the fact of filtering and amplifying the sound signal emitted by the loudspeaker to obtain the active control of the noise naturally creates a positive sound amplification and not a sound reduction which, in case of instability, may prove detrimental.

A phenomenon of this order is well known to sound engineers charged with sounding a theater: the Larsen effect. Sound engineers move the microphones away from the speakers to eliminate feedback. There is also a scientific way of dealing with this problem, which is analyzed by a physical criterion called the Nyquist criterion.

The Nyquist criterion is a measure in the complex plane of the expression of the transfer function of the complete electroacoustic string denoted by Hex (ω): loudspeaker, acoustic medium (wall impedance, distance, microphone, amplifier and filter). correction).

When the looped system is unstable, it amounts to writing that the characteristic equation expressed in module and in phase satisfies, for a null phase, the following relation: $$\left(\left|1-\mathrm{K}.\mathrm{C}\left(\mathrm{\omega i}\right).\mathrm{hex}\left(\mathrm{\omega i}\right)\right|,0\right)\le \left(0,0\right)$$

With: K the gain, C the correction filter and Hex (ω) the complex expression of the electroacoustic open loop.

In this case, the place of Nyquist in open loop satisfies for the affix points with zero imaginary part (| KC (ωi) .Hex (ωi) |, 0) the following inequality: $$\left(\left|\mathrm{K}.\mathrm{C}\left(\mathrm{wi}\right).\mathrm{hex}\left(\mathrm{\omega i}\right)\right|,0\right)\ge \left(1,0\right)$$

The Nyquist graphical stability criterion is expressed by: "For a regular and stable open-loop linear system, the system buckled by a reaction will be stable if the Nyquist locus does not surround or leave it to the right in the direction of the ωi increasing the affix point (1,0).

For each frequency, the place of Nyquist in open loop is calculated. The points of intersection of this place of Nyquist with the axis of the real are calculated. The stability constraint is then defined from the largest abscissa of the intersection points of the Nyquist locus in open loop with the real axis. It is rated Rmax. An example of a graphical determination of the stability constraint Rmax is given on the figure 10 .

We see in this figure that the real axis is cut several times by the place of Nyquist in open loop. The affix constraint Rmax (0.9, 0) is less than that of the critical point. The open-loop location of the system does not surround the point (1, 0) in the direction of the crescents. The electro-acoustic system is stable in closed loop for the module and phase values considered.

To summarize this criterion it is necessary to avoid having a sound amplification when the phase of the complex expression of the sound goes to 0 °. When a Larsen effect occurs it is that there is a significant phase rotation and a 0 ° transition thereof for a frequency whose energy is greater than 0 dB. When the sound engineers move the microphone away from the speaker, the transfer function, and therefore the amplitude of the signal and its phase, changes.

If it is possible to solve this problem of sound stability by this empirical means, for a system of active control of noise by feedback as well as implemented in the invention, it is necessary to work with the signal given by the transfer function of the electroacoustic system in open loop with respect to its module and its phase to avoid any feedback effect.

It is therefore necessary to obtain a sound reduction in a given frequency band. That is to say to respect the Nyquist criterion even with an amplification of the sound in a given frequency band.

• une bande de fréquence choisie ;
• une amplification du son la plus grande possible dans cette bande de fréquence choisie pour obtenir la meilleure réduction sonore possible;
• un système stable au sens de Nyquist quand le système fonctionne.

The problem of active acoustic noise control by feedback is therefore posed according to the following criteria:

• a chosen frequency band;
• amplification of the largest possible sound in this chosen frequency band to obtain the best possible sound reduction;
• a Nyquist stable system when the system is running.

The possibility of modifying the transfer function is in this case limited to modifying only the phase since the frequency band is chosen and the amplification of the sound is imposed by the very idea of active noise control.

In the prior patent FR 2,595,498 describing a headset with active noise control, the solution chosen is to put a special filter called "clover" which amplifies the sound in a given bandwidth without altering the phase and limiting the phase rotation in the frequency band of treatment, thus to give a stable system according to the Nyquist criterion.

The filtering power given by this "clover" filter is used for the helmet and could also be used in an active box of the type of that of the invention. But the filter "clover" does not solve all the problems encountered when the boxes are positioned in a beam.

The limitations of the active acoustic system using such a filter are related to the complex electro-acoustic structure of the system. This complexity is reflected in the frequency response Hex (ω) by a non-constant modulus, formed of resonances and antiresonances, and a phase comprising singular rotations or phase advances.

The construction of skylights where rows of caissons are arranged whose active face of the loudspeaker delivers on the back of the row of following boxes with a distance between rows of small boxes (typically less than or equal to 20 cm) creates in this confined space, stationary wave phenomena.

These standing waves greatly alter the nature and quality of the transfer function for use in active control. They cause significant phase rotations and significant antiresonances at the signal module. These phenomena are usually called sound nodes. This makes any loop system unstable in the sense of Nyquist and limits the bandwidth associated with active acoustic processing.

The Figures 11a and 11b show curves representing an example of transfer function measurement polluted by these standing waves.

Such a transfer function limits active processing, even using the clover filter, to the frequency band [ω _a , ω _b ] because of the presence of numerous phase rotations at the cancellations of the phase generating resonances. , in particular to ω ₁ , and antiresonances, in particular to ω ₂ .

The Kunt tube, equipped with a perfectly reflective termination, is an approximate example of this type of stationary wave phenomenon for which the phase is zero and the module corresponds to a cosine function when the position of the measuring point moves in the tube. A progressive acoustic wave appears in this tube only when the termination is anechoic type.

An optimized Hex (ω) transfer function whose modulus would be as constant as possible and a phase whose rotation would be as small as possible is conceivable if one works on the acoustic impedance of this anechoic termination in the Kunt tube.

When an acoustic wave is generated in a domain Ω partially enclosed by boundaries Γ _.. the wave regime becomes stationary wave regime. The acoustic energy contained in this space is then determined by the nature of the walls.

Regarding the presence of skylights, the dimensions are less than or at most equivalent to the wavelengths for which the active system is to operate. This generates the presence of standing waves.

The transfer function Hex (ω) of a box facing an absorbent passive wall corresponds to the skylight system according to the invention combining active control and passive control. It can be expressed by the speaker transfer function Hhp (ω) modified by the front and rear acoustic load connected to the walls.

The speaker radiation is then provided with a rear acoustic impedance, due to the cavity of the box and a front acoustic impedance, due to the wall of passive material fixed on the back of the successive box.

The influence of the passive materials on the acoustic emission of the loudspeaker makes it possible to determine the role of the passive material placed opposite the loudspeaker in the expression of the global transfer function Hex (ω).

• où : B.l est le produit du champ magnétique B de l'entrefer et de la longueur l du bobinage;
• Ze est l'impédance électrique;
• Zm est l'impédance mécanique.

We can model the loudspeaker in anti-noise operation facing a wall of a given acoustic impedance. The transfer function Hhp (ω) of the loudspeaker, considered as plane piston, is defined as the ratio of the displacement velocity V (ω) of the diaphragm, by the excitation voltage E (ω) delivered to the terminals the speaker such as: $$\forall \mathrm{\omega },{\mathrm{H}}_{\mathrm{hp}}\left(\mathrm{\omega }\right)=\frac{\mathrm{V}\left(\mathrm{\omega }\right)}{\mathrm{E}\left(\mathrm{\omega }\right)}=\frac{\mathrm{B\; l}}{{\mathrm{Z}}_{\mathrm{e}}{\mathrm{Z}}_{\mathrm{m}}+{\left(\mathrm{B\; l}\right)}^2}$$

• where: B1 is the product of the magnetic field B of the air gap and the length l of the winding;
• Ze is the electrical impedance;
• Zm is the mechanical impedance.

avec les notations Zar et Zav qui sont respectivement les impédances acoustiques arrière et avant, crées par la présence du matériau passif dans le caisson à l'arrière du haut-parleur et du matériau passif placé face au haut-parleur et collé sur l'arrière du caisson suivant.In this expression of Hhp (ω), the acoustic impedances of the absorbent materials at the front and rear of the loudspeaker that modify its acoustic radiation are introduced. These impedances act acoustically and mechanically on the vibration of the membrane. These acoustic-mechanical impedances can therefore be added to the term Zm which represents the mechanical impedance of the loudspeaker. In this case the transfer function of the loaded speaker becomes: $${\mathrm{H}}_{\mathrm{ch}}\left(\mathrm{\omega }\right)=\frac{\mathrm{B}\times \mathrm{l}}{{\mathrm{Z}}_{\mathrm{e}}\times \left({\mathrm{Z}}_{\mathrm{m}}+{\mathrm{Z}}_{\mathrm{ar}}+{\mathrm{Z}}_{\mathrm{BC}}\right)+{\left(\mathrm{B}\times \mathrm{l}\right)}^2}$$

with the notations Zar and Zav which are respectively the acoustic impedances back and front, created by the presence of the passive material in the box at the back of the loudspeaker and the passive material placed in front of the loudspeaker and stuck on the back the following box.

• avec : s est la surface du piston ;
• Scav est la section du caisson considéré; $$\mathrm{k}=\mathrm{\omega }/\mathrm{C};$$
  
  R coefficient de réflexion des parois lié à la présence ou pas de matériau passif acoustiquement absorbant.

To calculate these impedances Zar and Zav, it is useful to make the hypothesis that the acoustic wave emitted by the loudspeaker propagates according to a plane wave. The rear impedance Zar then checks (in pc unit) the following relation: $${\mathrm{Z}}_{\mathrm{ar}}=\frac{\mathrm{s}}{{\mathrm{S}}_{\mathrm{cav}}}\times \frac{1-\mathrm{R}-\mathrm{j}2\mathrm{R\; sin}\left(2{\mathrm{<figure-callout\; id="1"\; label="kl\; cav"\; filenames="imgf0004.png"\; state="\{\{state\}\}">kl\; </figure-callout>}}_{\mathrm{cav}}\right)}{1+\mathrm{R}-2\mathrm{R\; cos}\left(2{\mathrm{kl}}_{\mathrm{cav}}\right)}$$

• with: s is the surface of the piston;
• Scav is the section of the caisson considered; $$\mathrm{k}=\mathrm{\omega }/\mathrm{C};$$
  
  R reflection coefficient of the walls related to the presence or absence of passive acoustically absorbing material.

The transfer function of the loudspeaker loaded by the front and rear acoustic impedances is in fact the product of the following three transfer functions: $${\mathrm{H}}_{\mathrm{ch}}\left(\mathrm{\omega }\right)={\mathrm{C}}_{\mathrm{AT}}\left(\mathrm{\omega }\right){\mathrm{H}}_{\mathrm{hp}}\left(\mathrm{\omega }\right){\mathrm{C}}_{\mathrm{B}}\left(\mathrm{\omega }\right)$$

C _A (ω) and C _B (ω) are the respective acoustic transfer functions of the chamber cavity and the confined space between the skylights in front of the loudspeaker.

If the excitation E (ω) of the loudspeaker is a white noise, the expression of Hch (ω) is none other than the transfer function of the electroacoustic system of the box coupled to the confined space between the box and the passive material from the back of the following box.

Thus, considering that the frequency response of the measurement microphone is perfect, at least in the frequency zone of study, the transfer function Hch (ω) is equivalent to the experimental transfer function Hex (ω) of the flow-through chamber system. on a passive acoustic acoustic wall.

The variations of the three transfer functions that make up the expression of Hch (ω) thus correspond to the variations of the transfer function Hex (ω).

It is therefore necessary to determine which are the key parameters involved in modifying and optimizing the response in terms of module and phase of the transfer function of the electroacoustic chamber when it delivers either on a perfectly reflecting wall or on a acoustically absorbent passive wall.

It is assumed here that the parameters Ze, B.l, Zm remain constant for a given loudspeaker.

The variation Zar as a function of R which varies in the interval [0,1] makes the impedance Zar vary from [+ ∞, -∞]. Its values change sign and have discontinuities at the boundaries of the interval described.

Thus, when R varies from [0,1], the transfer function Hch (ω) can vary from zero to very large values according to the variations of Zar and Zav. These variations explain the appearance of the fast phase rotations as well as the resonances and anti-resonances of the module observed on the Bode diagrams of the experimental transfer functions.

As regards the variations of s and Scav, they essentially result in a modification of the value of the gain for the Hhp (ω) module without really altering the phase.

Smoothing of the module and phase curves of the Hex (ω) transfer function of the complete electroacoustic system is then obtained by adding a passive material which makes it possible to change the acoustic impedance before the loudspeaker according to the 'invention.

The experimental transfer functions Hex (ω) represented on the Figures 12a and 12b are derived from measurements made on a system with clear-ways whose rear wall of the box vis-a-vis the loudspeaker is sometimes metal, sometimes metal covered with a thickness of 5 cm of passive absorbent material.

The non-optimized transfer function corresponds to skylights whose rear of the box is devoid of passive acoustic absorbing material. The measurement of the optimized transfer function corresponds to skylights equipped with passive material according to the invention.

It is thus verified experimentally that the transfer function is smoother in both module and phase. The phase shift is then less important and the module has no more antiresonances.

We can say that the optimization of the experimental transfer function by the addition of a passive material in the expression of the transfer function of the open-loop feedback system Hex (ω) performs a pseudo-linearization of the expressions phase and module. The combination of active noise reduction elements with passive acoustic absorbing elements makes it possible to produce an effective clear-path barrier according to the principles of the invention.

Thus, for each louvre barrier, Hex (ω) should be optimized by combining passive materials on the faces facing the active noise reduction elements. The active control solution is then improved in bandwidth and efficiency thanks to the addition of a passive material which reduces the phase rotations and consequently removes the critical point in the complex plane.

This combination of active / passive control broadens the attenuation frequency band and allows the gain to be increased without the risk of rapidly creating an unstable closed-loop system as soon as the active noise reduction elements are aligned with each other. beam then placed so as to create a barrier to clear-ways according to the invention.

• l'efficacité de l'atténuation acoustique active ;
• la largeur de la bande de fréquences atténuées ;
• la fiabilité du système qui peut être commandé par une électronique plus simple.

The more the transfer function is linearized thanks to the combination with a passive material in the expression of the electroacoustic string Hex (ω), the more are improved:

• the effectiveness of active acoustic attenuation;
• the width of the attenuated frequency band;
• system reliability that can be controlled by simpler electronics.

The optimization of the transfer function performed by the combination of the active control system and acoustic absorbing material placed opposite can be further supplemented by a judicious choice of transducers whose transfer function offers little rotation phase and deformation of the module.

The microphone 62 and the speaker 61 are connected to a control electronics 73. This control electronics 73 comprises a preamplifier for the microphone 61, an electronic filter, for example an N-order filter and a connected audio power amplifier. to the speaker 61.

The combination of several basic pedestals as shown on the figure 7 allows to obtain an acoustic treatment beam as used in the invention. In reality, as well as presented on the figure 6 , the box is advantageously shared, in long form, for fourteen acoustic reduction elements, each consisting of a microphone and a speaker.

The calculations carried out within the control electronics 73 ensure that the control filtering is such that the active sources interfere together positively and additionally. This ensures a consistent overall treatment.

According to the invention, the coherence of the whole of the perforated hybrid acoustic barrier according to the invention is allowed thanks to the adjustment of the filtering which is done according to the transfer function of each independent box. More exactly the transfer function of the secondary path, ie the path between the microphone and the speaker of each microphone / loudspeaker pair, is used to carry out the filtering adjustment. The transfer function of the secondary path is like its electro-acoustic identity card which makes it possible to control everything in the complex plane for the possible possible processing frequency band.

The measurement of the transfer function between each microphone and the corresponding loudspeaker makes it possible to know the module and the phase of this secondary path for all the considered frequencies of the processing. Thus, it is possible to master, by calculation, the behavior and the stability of the microphone / loudspeaker pair by taking into account all the acoustic characteristics of the system in order to optimally calculate the solution filter allowing the maximum gain for a stability of the insured system.

An embodiment of a feedback control that can be implemented in the invention is explained in the patent application. WO 1997/02471 .

It is noted that the cut-off frequency of the active noise processing and the number of desired decibel reduction conditions the thickness D of the air gap that exists between two beams 41. Thus the bandwidth treated in the low frequencies by the active treatment and the number of reduction decibels obtained in this band are inversely proportional to the thickness of the air gap between two beams. As an indication below is given a table of the reduction results in dB as a function of the distance between two beams. Number of decibels reduced in octave band as a function of the distance between two active / passive beams frequencies E = 2 cm E = 4 cm E = 8 cm E = 16 cm E = 32 cm 31.5 24 24 24 24 24 63 24 24 24 24 12 125 24 24 24 12 6 250 24 24 12 6 3 500 24 12 6 3 0

The figures 8a and 8b show examples of acoustic barriers made according to a preferred embodiment of the invention for which the acoustic barrier is provided with acoustic reduction elements on its upper part for the treatment of said fraction by the upper edge of the acoustic barrier.

On the figure 8a , the barrier is provided with an additional acoustic box 42. This additional beam 42 has no problem as to the adjustment of the acoustic impedance since the speakers present on this beam emit in the free space and therefore have an acoustic impedance infinite.

On the figure 8b , the active treatment of the diffraction noise is carried out using a plurality of acoustic reduction elements consisting of microphone pairs 62 / loudspeaker 61 placed at the end of the beams 41 placed vertically between two cross members 80.

It is noted that a speaker of the type described for installation in double glazing in the patent WO 99/05888 may be used in association with a microphone to produce a system according to the invention. In particular, such an elongated secondary source may be used in an acoustic reduction device intended to be used in the manner of a blind with blades in front of a window.

Indeed, the invention makes it possible to have an active system for processing noise on blade slats of the type of blind slats or on the periphery of the slats. cylinders hanging parallel to the window. After the addition of acoustic absorbing material on the faces opposite to the faces on which the active systems are installed, it is thus possible to achieve a pleasant and effective arrangement for the treatment of noise in the low frequencies while allowing to ventilate and refresh effectively a piece.

The use of a cylinder is advantageous from a point of view of passive noise reduction. Indeed, the mass effect of a cylinder compared to a blade is considerably larger. In addition, the presence of the cylinder makes it possible to adjust, if necessary, the acoustic impedance in which the neighboring loudspeakers flow.

By using a succession of translucent active / passive modules equipped with elongated loudspeakers to the dimensions of the window, a sort of blind comprising blades or, preferably, cylinders approximately 9 cm apart and advantageously integrating the electronics of FIG. control in each blade or cylinder. For a standard window (148x123mm), it is then possible to insert five elements, each element having a width or a diameter of about 16 cm.

Advantageously, such a noise treatment system will be connected to the sector and will be controllable by an electrical switch such as those used for controlling the lighting of a room.

Thus, an acoustic reduction device according to the invention may be installed between an outer curtain and a window. It can be fixed or removable. The elements can be moved on the sides of the window or integrated in a partition.

The figure 9 gives an example of possible embodiment of a sound reduction device on a window 90 with cylinders 91 each provided, on one side, a linear structure according to the invention with pairs speaker / microphone.

It is noted here that the invention allows a result of noise reduction more comfortable to the ear than a simple totally passive or fully active control over the entire surface of the barrier and this, despite a partial active control performed only in the days of the barrier and a partial passive control carried out only by the bars of the barrier.

The invention therefore makes it possible to produce effective acoustic screens throughout the audible spectrum without requiring the use of the law of mass or of materials of great thickness. It also has the advantage of being able to be associated with a treatment of air inlets by active control and, therefore, to be installed in acoustic curtain in front of machines requiring significant ventilation: air conditioning or other machines.

By way of example, it has been possible to obtain an overall reduction of 7 additional decibels by the hybrid active / passive processing performed according to the invention compared to a passive screen of the same thickness as part of the reduction of the sound radiation. a heat exchange unit of the heat pump type.

In such an application, it is crucial to allow the circulation of air necessary for the operation of heat exchange and the invention is therefore particularly suitable for this type of application. As such systems are particularly noisy, the invention finds a very interesting application.

On the upper part of the acoustic barrier, elongated high-resistance loudspeakers are advantageously used to carry out the diffraction treatment.

On a highway noise barrier wall, the active trellises according to the invention can see through the wall and allow ventilation. The distances D of separation of the beams used, their number and their vertical or horizontal disposition may be various depending on the specifications. Generally, a compromise will be sought between the quantity of passive material to be used and the cost of the active systems so that the openings and the lightness of the structure are preferred.

Finally, it should be noted that various implementations can be made according to the principles of the invention.

Claims (8)

1. A passive and active acoustic reduction method comprising the following steps:

   • producing a plurality of acoustic reduction elements (70), each comprising a microphone (62) and a loudspeaker (61), by carrying out the following steps for each element:

   • placing the microphone (62) in a box (60) produced in a passive acoustically absorbent material or including a passive acoustically absorbent material in the vicinity of the surface of a main side (63) of the box (60);

   • placing the loudspeaker (61) in this box (60), beside the microphone (62), also in the vicinity of the surface of the main side (63) and in such a manner that the main emission direction of the loudspeaker (61) is substantially perpendicular to the main side (63);

   • disposing n acoustic reduction elements (70) side by side to constitute an acoustic reduction bar or electro-acoustic bar (41);

   • disposing m electro-acoustic bars (41) side by side, substantially parallel to one another and separated by gaps (D) and directing the main side (63) of the box towards the side opposite the main side of the adjacent bar, so that the loudspeakers (61) emit into the gap between two bars (41), thus constituting an open-work acoustic barrier combining a passive noise-reduction effect and an active noise-reduction effect;

   the method being characterized in that it comprises the steps of:

   • introducing an acoustically absorbent material (72) on the side of the box (60) opposite the main side (63) to adjust the acoustic impedance of the box (60) and prevent the appearance of standing waves between the main side (63) of one bar (41) and the side opposite the main side of the adjacent bar;

   • for each acoustic reduction element (70), measuring the transfer function between the microphone (62) and the loudspeaker (61);

   • for each acoustic reduction element (70), computing a feedback control electronic filter from the transfer function between the microphone (62) and the loudspeaker (61), the transfer function being linearized by the presence of acoustically absorbent material (72) introduced onto the side of the box (60) opposite the main side (63), the electronic filter acting, within each acoustic reduction element (70), to enable electro-acoustic looping of the loudspeaker (61) to the microphone (62) by amplifying the feedback in order to obtain a real-time acoustic absorption effect for a predetermined range of frequencies.
2. A method according to claim 1, characterized in that the electronic filter is further such that emissions from the loudspeakers (61) aligned on the bar (41) interfere positively and additively.
3. A method according to claim 1 or claim 2, characterized in that the box (60) is common to a plurality of acoustic reduction elements (70) of the same bar (41).
4. A method according to any one of the preceding claims, characterized in that it includes a step of optimizing the distance (D) between two bars (41) as a function of the acoustic result in terms of the number of decibels and the cut-off frequency of the active reduction, of its visual appearance, and of heat exchange.
5. A method according to any one of the preceding claims, characterized in that, for an acoustic barrier having a free edge, it further includes a step of installing acoustic reduction elements (42, 81, 82) on that free edge to reduce sound diffraction.
6. A method according to any one of the preceding claims, characterized in that the bar (41) constituted of a plurality of acoustic elements (70) is replaced by a linear loudspeaker associated with at least one microphone disposed in the vicinity of the loudspeaker.
7. A method according to any one of the preceding claims, characterized in that the predetermined frequency range treated by the open-work screen is the range of low frequencies below 500 Hz.
8. A passive and active acoustic reduction device comprising m electro-acoustic bars (41) disposed side by side, substantially parallel to one another, and separated by gaps (D), each electro-acoustic bar (41) including a plurality of acoustic reduction elements (70) disposed side by side, each acoustic reduction element (70) comprising a microphone (62) and a loudspeaker (61) placed in a box (60) produced in a passive acoustically absorbent material or including a passive acoustically absorbent material, in the vicinity of the surface of a main side (63) of the box (60) in such a manner that the main emission direction of the loudspeaker (61) is substantially perpendicular to the main side (63), the microphone (62) and the loudspeaker (61) being connected to control electronics (73) adapted to receive a measurement of the transfer function between the microphone (62) and the loudspeaker (61);

   each bar further including an acoustically absorbent material on the side of the box opposite the main side for adjusting the acoustic impedance of the box and preventing the appearance of standing waves between the main side of one bar and the side opposite the main side of the adjacent bar;

   the device being characterized in that :

   the bars (41) being disposed side by side in such a manner that the main sides (63) of the acoustic elements (70) are directed towards the side opposite the main side of the adjacent bar so that the loudspeakers (61) fire into the gap (D) between two bars (41), thus constituting an open-work acoustic barrier combining a passive noise-reduction effect and an active noise-reduction effect;

   the control electronics (73) including means for computing a feedback control electronic filter for each acoustic reduction element (70) from the transfer function between the microphone (62) and the loudspeaker (61), the transfer function being linearized by the presence of acoustically absorbent material introduced onto the side of the box (60) opposite the main side (63), the electronic filter acting, within each acoustic reduction element (70), to enable electro-acoustic looping of the loudspeaker (61) to the microphone (62) by amplifying the feedback in order to obtain a real-time acoustic absorption effect for a predetermined range of frequencies.

Images