EP1094444A1 - Active device for the attenuation of sonic intensity - Google Patents

Abstract

The noise cancellation system has sensors (30) which receive noise from distant sources and generate (40) cancellation signals to be emitted from an array of electro acoustic sources (25) in a reflecting or absorbing screen close to the desired cancellation zone.

Description

Technical area

The invention relates to the field of acoustics. More specifically, it targets fixed devices for reducing noise generated by mobile sources, such as especially means of transport in general, and airplanes or land transport in particular.

The invention constitutes an improvement of the device described in the patent EP 0 787 340 to the Applicant.

Previous techniques

The Applicant described in the aforementioned patent a device for attenuation of the sound intensity operating on the principle of the emission of a counter-noise wave generated from information from sensors, and emitted by electro-acoustic sources arranged in such a way that the counter-noise waves combine with the noise waves they admit as an envelope.

The principles described in this patent remain valid for the present improvement, so that said document is cited here for reference, and its content will therefore not be detailed further.

In the embodiments illustrated in this document, the different sources against noise are associated by sub-assemblies mounted on vertical masts, that is to say in a direction substantially perpendicular to the direction of average incidence noise waves.

In this document, the different masts are arranged around the area to be protect, and preferably at the periphery of the area to be protected.

However, it has been observed that the spacing of the masts as described in this document does not not sufficiently attenuate the sound waves having a frequency relatively large, and in particular greater than 500 Hz (Hertz).

A problem which the invention proposes to solve is that of attenuation effective sound waves in a range up to kiloHertz, or even up to 2 kHz (kiloHertz).

Statement of the invention

• un ensemble de capteurs susceptibles de déterminer les signaux et les directions des ondes émises par des sources de bruit éloignées ;
• des moyens pour traiter les signaux issus desdits capteurs, et pour générer des signaux correspondants aux ondes de contre-bruit ;
• un ensemble de sources électro-acoustiques, lesdites sources étant installées dans l'espace proche de la zone à protéger, et connectées auxdits moyens de traitement, et susceptibles d'émettre des ondes de contre-bruit dans la même direction et dans le même sens que les ondes incidentes, les capteurs et les sources électro-acoustiques étant placés de telle façon que les ondes incidentes atteignent préalablement les capteurs.

The invention therefore relates to an active device for attenuating the sound intensity at the level of a determined zone, by emission of counter-noise waves, of the type comprising:

• a set of sensors capable of determining the signals and directions of the waves emitted by distant noise sources;
• means for processing the signals from said sensors, and for generating signals corresponding to the counter-noise waves;
• a set of electro-acoustic sources, said sources being installed in the space close to the area to be protected, and connected to said processing means, and capable of emitting counter-noise waves in the same direction and in the same direction that the incident waves, the sensors and the electro-acoustic sources being placed in such a way that the incident waves reach the sensors beforehand.

This device is characterized in that the electro-acoustic sources are arranged on a continuous surface, and according to a regular mesh.

In other words, the invention consists in associating the different sources of such so that they constitute a sufficiently tight mesh to allow a attenuation of high frequency waves, i.e. in application to treatment sound waves on the order of a kiloHertz.

Thus, according to a characteristic of the invention, the sources are spaced from each other areas with a distance between one and two meters.

It is easily understood that the use of masts as described in the document above would be completely unrealistic to cover a frequency range up to kiloHertz, since it would lead to a much too high mast density on the ground.

• "a" désigne la dimension caractéristique de la maille de réseau de source,
• α est un paramètre légèrement supérieur à 1, caractéristique de la forme géométrique de la maille,
• et c_o est la célérité du son.

Indeed, according to a theory of operation of the active screen, it appears that the effect of continuous monolayer screening, is limited in the frequency domain due to the discrete distribution of these sources on the surface. It is a low-pass phenomenon, the cut-off frequency of which is: ∫ ₀ = α. c ₀ / a where:

• "a" designates the characteristic dimension of the source network mesh,
• α is a parameter slightly greater than 1, characteristic of the geometric shape of the mesh,
• and c _o is the speed of the sound.

Beyond this cutoff frequency, the incident waves are no longer only reflected by the screen, but also diffracted upstream and downstream of the screen, with for effect of inducing a pressure level twice the level of the incident wave, which makes then said screen not inoperative, but disruptive.

The choice of a sufficiently tightened mesh, of the order of magnitude of a half meter, provides a cutoff frequency of the order of a kiloHertz encompassing the majority of the power spectrum of the sound wave of an airplane for example.

Thus, the different sources are arranged according to surfaces which can be produced by a trellis which is itself raised, arranged above the area to be covered or above the buildings adjoining it.

By continuous surface is meant a surface which has geometric regularity such that all sources can be considered, with respect to a noise wave, as equivalent in their contribution to mitigation, to the effect near their orientation.

Such surfaces can be flat, or for example belong to the family quadrics, and in particular cylinders.

In practice, we examined that a hexagonal mesh allowed to have the best compactness of sources, and therefore better coverage in a frequency band for identical source density.

According to another characteristic of the invention, the device according to the invention comprises several sets of electro-acoustic sources arranged in several surfaces offset relative to each other by translation normal to their surface, of so as to form complex, multi-layered electroacoustic sources, which allows to increase their transverse spacing, with equal bandwidth.

So when the speakers that constitute the electro-acoustic sources are associated according to surfaces which are close to each other and substantially parallel, these speaker sets have the effect of a larger section speaker, without occupy the surface.

Indeed, a single speaker with an identical useful surface would occupy a proportion Too much mesh, which would reduce the visual transparency of the screen.

In particular configurations, several devices can be combined so that these devices are juxtaposed next to each other in the space of the area to protect, to cover a particular geometry area, such as a intersection of streets. These devices can be combined, in continuity with surfaces of the same nature constituting passive screens, in particular glazed structures, for architectural and functional reasons.

The different screens are controlled by a microphonic capture system located upstream near the screen. This noise wave capture system has the ability to separate and characterize these waves respectively in direction and in signal, so as to allow the counter-noise sources to counteract them additively.

In the case of a single source of noise, all the echoes carry practically the same signal, namely that of the direct wave. It is therefore the signal of the first wave captured with an amplitude factor and a time lag.

The control means are able, using the appropriate algorithms, to extract the common reference signal as well as the amplitude and delay parameters specific to each echo signal, from a set or base of sensors microphones placed upstream of the screens.

The minimum number of sensors to be used in the microphone base is at least equal to the number of signals to be discriminated, but practically greater for get rid of the effect of parasitic noise of near origin.

In the case of more complex figures the sources of noise are multiple and independent, such as for noise generated by means of transport on land, such as vehicles, cars or wagons.

In this case, the number of eigen signals, independent sources, is greater than the ten. We will then use numerous complementary microphone bases and directives, preferably arranged as close as possible to the sources.

For example, the microphone bases can be arranged along the roadway or railway track so as to selectively acquire, by proximity, the various reference signals specific to independent sources, such as wheel sets, bogies, and air boundary layers.

In this way, it is facilitated by prior knowledge, the separation of the different noise waves at the screen.

But in this case, the signals that propagate from the microphone base to the screens, are subject to the vagaries of the atmospheric propagation of sound which must take into account in the signal separation algorithms in the database microphone close to the screen.

In all cases, the algorithmic principles used for signal selection require great precision. This precision of the algorithms is determined by the overall precision of the counter-noise waves, which is evaluated in the manner next.

Assuming that the active screen is intended to oppose a noise wave of amplitude b (t), by generating a counter-noise wave of amplitude cb (t)., The amplitude of the residual noise is : e (t) = b (t) - cb (t). The quadratic norm, or its energy evaluated over the characteristic duration of the auditory perception of noise signals (of the order of a tenth of a second) is as follows: e ² = bx ² - 2 cb.b + cb ² where the upper bar designates the time averaging effect.

- "r" est le coefficient de défaut à l'unité de corrélation des signaux b(t) et cb(t), donné par la formule suivante : 1 - r = b.cb b 2·cb 2

- et "M" est le rapport des énergies, selon la formule suivante : 1+ M = cb 2 b 2

The mitigation factor AT lt = e 2 b 2 is expressed according to the following formula: AT tt = 2r (1+ M 2 ) + (1- r) M 2 4 in which:

- "r" is the default coefficient to the unit of correlation of the signals b (t) and cb (t), given by the following formula: 1 - r = b.cb b 2 · cb 2

- and "M" is the ratio of energies, according to the following formula: 1+ M = cb 2 b 2

To obtain an attenuation factor of the order of 20 decibels, it is therefore necessary the correlation coefficient of the counter-noise signal to the noise signal is 0.995, value which reflects the very great similarity to be obtained between noise signals, with large bandaged.

This value generally implies that the various elements involved in the attenuation device must have a quadratic precision of the order of 2.10 ^-3 , not achieved in the field of usual sound reproduction.

Brief description of the figures

The manner of carrying out the invention, as well as the advantages which flow therefrom will emerge clearly from the description of the particular embodiments which follow, the support of the attached figures in which:

Figure 1 is a summary perspective view of a residential area equipped with several screens according to the invention.

Figure 2 is a summary perspective representation of a house located near a traffic lane, and fitted with screens in accordance with the invention.

Figure 3 is a schematic representation of a screen according to the invention as well as different control blocks for each of the active elements of the screen.

Figure 4 is a schematic representation of an electro-acoustic source monolayer used in a screen according to the invention, in a mesh of the network.

Figure 5 is a schematic view illustrating a two-layer association of sources. In these last two figures, we have drawn the lines tangent to the main axis most along acoustic particulate hodographs.

Figure 6 shows a window frame protected by a plurality of electroacoustic sources arranged in accordance with the invention.

Figure 7 is a schematic perspective view of a set of sources associated in three parallel planes.

Way of realizing the invention

As already said, the invention relates to an improvement of the device noise reduction as described in patent EP 0 787 340.

Such a device includes a number of surfaces grouping sources electroacoustics. According to a characteristic of the invention, these surfaces are continuous from so as to cover an area of up to several hundred meters square, these surfaces are arranged at altitudes of the order of 10 to 15 m, or even more, above places to protect.

These surfaces are produced for example by trellis of tubes or cables to the intersection of which the sources of counter-noise are fixed.

As seen in Figure 1, the area to be protected may include a number flat screens A, B, C, D, intended to ensure this protection.

The screens A, B, C are arranged above the street to protect R1 while the screen D is placed high, across the street R1, and is intended to block the guided waves, by multiple reflections on the facades along the street.

In the particular case of FIG. 1, the device makes it possible to protect part of the metropolitan area adjacent to an airport, with regard to aircraft noise on takeoff and landing along a trajectory substantially parallel to the street R1 illustrated in FIG. 1, at a horizontal distance of the order of half a kilometer.

More specifically, screens A and C are placed immediately above the building facades, in front of the area to be protected. They are arranged in such a way as to be inclined to the vertical.

Screen B is placed across a street perpendicular R2 to the street main R1 parallel to the path of the aircraft. This screen B closes the gap offered to aircraft noise, when this one overlooks this perpendicular street R2.

Screen D is placed in the same way, across street R1 so to reflect the noise which reaches the area to be protected in a guided way, by reflection preliminary and multiple along the facades in the street R1.

Screens B and D are also tilted to improve the efficiency of the device mitigation.

In another embodiment, as illustrated in FIG. 2, the device is intended to protect an isolated dwelling (10) located at the edge of a motorway (11) with regard to traffic noise.

More specifically, the device consists of a cylindrical screen (12) adapted to the protection of the main facade of the house (10) exposed to noise.

We also note in Figure 2, the presence of a plurality of microphones (15) arranged immediately beside the roadway (11), and intended to pick up noise clean of vehicles (16).

The signals generated by the microphones (15) are routed to the control of the screen (12) by an appropriate means and in particular by wire connection (not shown).

As shown, the screens consist of pylons (20-22) of form suitable supporting panels (24) of regular lattice with triangular, square meshes or preferably hexagonal, in the center of which are fixed the sources (25) of noise counter. These sources can be single-layers, or preferably multi-layers, that is to say made up of the association of several speakers offset one compared to the others, and according to the normal to the reference surface.

• un bloc (41) de caractérisation des ondes de contre-bruit ;
• un bloc (42) de commande des sources de contre-bruit ;
• des blocs (43) de commande intégrée.

As shown in FIG. 3, each panel (24) is associated with a microphone recording base (30) and an electronic control system (40) which comprises the following functional blocks:

• a block (41) for characterizing the counter-noise waves;
• a block (42) for controlling the counter-noise sources;
• integrated control blocks (43).

i More specifically, the block (41) for characterizing noise waves makes it possible to determine the main characteristics of the direct incident waves reflected by the ground and various obstacles.

This characterization block (41) determines the respective directions of the normals to these waves, the acoustic signals specific to each of them as well as their positions relative in time.

The delay of each of these signals with respect to the direct wave signal is determined with respect to a single reference point Oi, called "reference point of the microphone base ", generally located at its barycenter.

The counter-noise source control block (42) provides linear filtering identical for each of the characteristic signals of the noise waves coming from the blocks (41) of the aforementioned characterization.

The purpose of this filtering is to equalize the group times of the electroacoustic sources. over the range of the screen action frequency band.

Each of the filtered signals is then routed, for example by multiplexing, over a common bus (44) to the integrated control blocks (43) of the noise reduction sources.

At the same time, and in a clocked manner, the characteristic delays of the signals are also transmitted on this bus (44). These deadlines evolve continuously according to the movement of the noise source, and with the vagaries of sound propagation.

• de positionner dans le temps les signaux propres aux différentes ondes, en leur appliquant par des "lignes à retard" réglables, les délais qui correspondent à leur position géométrique. Ainsi, les sources de contre-bruit doivent délivrer des signaux en concomitance stricte avec ceux que portent les différentes ondes balayant leurs surfaces actives. Les délais sont calculés à partir des délais de référence transmis sur le bus, selon la position géométrique de la source vis à vis du point de référence de la base microphonique M ;
• de sommer tous les signaux ainsi recalés dans le temps ;
• de les appliquer, après un décodage numérique-analogique, à des amplificateurs propres à chaque source élémentaire de contre-bruit.

Each counter-noise source is itself endowed with its own integrated control unit (43), the dual function of which is:

• to position in time the signals specific to the different waves, by applying to them by adjustable "delay lines", the delays which correspond to their geometric position. Thus, the sources of counter-noise must deliver signals in strict concomitance with those carried by the different waves sweeping their active surfaces. The delays are calculated from the reference delays transmitted on the bus, according to the geometric position of the source with respect to the reference point of the microphone base M;
• summing all the signals thus readjusted in time;
• to apply them, after digital-analog decoding, to amplifiers specific to each elementary source of counter-noise.

The sources of noise control around the edges of the screens are subject to orders similar to the whole, but adjusted in a specific way in level and time to regularize the side effects.

• la supervision du fonctionnement local du système avec un ajustage permanent par une boucle de retour, ayant une constante de temps de quelques secondes, permettant de pallier les dérives paramétriques et d'assurer ainsi la meilleure conformité des signaux acoustiques de contre-bruit vis à vis des signaux de bruit;
• le réglage adaptatif fin des lois de commande des sources de contre-bruit et particulièrement des sources de contour vis à vis des effets de bord des écrans en fonction du mouvement de l'avion, et ce, selon une constante de temps de l'ordre de la seconde,
• la détection d'anomalies de fonctionnement avec une possible mise en arrêt éventuel et une signalisation ;
• la possibilité d'exécuter des procédures automatiques de test.

It is also advantageous to have inside the volume located downstream of the screen, that is to say under its acoustic protection, one or more microphones (32) for monitoring residual noise, the signals of which are returned to the control unit (42) against noise sources, so as to provide additional functions such as:

• the supervision of the local operation of the system with a permanent adjustment by a feedback loop, having a time constant of a few seconds, making it possible to mitigate the parametric drifts and thus to ensure the best conformity of the acoustic counter-noise signals with respect noise signals;
• the adaptive fine adjustment of the control laws of the noise-reduction sources and in particular of the contour sources with respect to the edge effects of the screens as a function of the movement of the aircraft, and this, according to a time constant of the order of the second,
• detection of operating anomalies with possible shutdown and signaling;
• the ability to run automatic test procedures.

As explained above, to obtain an attenuation performance of around 20 decibels, it is necessary that the overall precision of the measurement and restitution chain be 5.10 ^-3 in linearity, which imposes precision on each component of the system on the order of 2. 10 ^-3 .

Consequently, with regard to the counter-noise sources, namely the various speakers and the associated analog amplifiers, the non-linear distortion rate must be less than 2.10 ^-3 , at the maximum level delivered.

This requirement for precision requires special attention for the design speakers and their control circuits.

With regard to the microphones which constitute the basis for capturing the different noise waves, the effect of the physical parameters of the environment, such as temperature, atmospheric pressure, humidity rate, are compensated so as not to affect the linearity of the response. beyond the rate of 2.10 ^-3 required.

The effects of wind on microphones, with a low time constant, typically less than the second, are aerologically limited, using for example bodies porous profiles as protective envelopes, as well as electronically for do not disturb the counter-noise control signals in the frequency band of system operation.

As regards the block (41) for characterizing the signals specific to the incident waves to be processed by the screen, the precision and the extraction of the signals must be of the order of 10 ^-3 , which implies in particular a rate of crosstalk less than this value, and thus fixes the overall performance of the algorithms designed to ensure, in real time, this discrimination.

In addition, the requirements concerning the linearity of the different processing devices, a special requirement is required for the resolution of timing timing signals control of noise-reducing sources, necessary to ensure their concomitance with respect noise wave signals, and therefore in inverse function to the high frequency limit of the system operating band.

More specifically, to obtain the attenuation rate of 20 decibels, it was determined that it was necessary for the correlation coefficient of the counter-noise signal to the noise signal to be greater than 0.995, which implies a maximum phase shift between their spectral components of 6 °, or 1/60 ^th of the period.

As a result, the temporal resolution of the signals must typically be better than 17 micro seconds for the frequency of one kiloHertz.

Thus, the cadence of the clock which fixes the timing step of the signals in the speaker control blocks, will be greater than 60 kiloHertz. Translated into wavelength, this temporal resolution corresponds to a resolution in position geometric of the sixtieth of the maximum wavelength, i.e. 5 mm for a maximum frequency of 1 kiloHertz.

This value corresponds to the rigidity requirement of the support structure, which links the microphone base on the noise reduction sources panel.

Its deformation, in particular under the load due to the wind, must not therefore induce relative displacements greater than this value, to maintain an attenuation rate of around 20 decibels.

Operation of the active noise canceling screen as an attenuation device sound waves in free space, is already described in patent EP 0 787 340 of Applicant. It is therefore a system close to sources of noise canceling, arranged and signal-driven to generate counter-noise waves algebraically opposite to tangent noise waves.

To better understand the operation of the invention, it may be useful to give a direct and effective physical description of the operation, explaining the necessary spatial and temporal concomitance at the level of the counter-noise sources.

So, being an incident wave reaching the system in the form fundamental of a noise wavefront, i.e. a jump in particle acceleration, to linearly filter in the useful frequency band, the action of the counter-noise sources with respect to this front consists, for each source, in interacting with this front at the exact moment of its passage, and in such a way that this front does not propagate beyond the source, in direction of the downstream area to be protected.

The sources of counter-noise therefore simultaneously create boundary conditions adapted to reflect or absorb the incident wavefront. Sources of noise control thus constitute screens realizing, in acoustic terms, boundary conditions particular.

Insofar as these sources of counter-noise are constituted by loudspeakers electrodynamic, they behave naturally, in the frequency range where we uses them as sources of variation in sound flow.

To the current injected into the speaker coil, corresponds a Laplace force which meets as main reaction the inertia force of the moving part of the loudspeaker. This mobile equipment takes an acceleration proportional to said current.

By the control system, we adjust this acceleration, and therefore the variation of i acoustic flow delivered by the diaphragm of the loudspeakers, and concomitantly, double the normal noise acoustic wave rate on the clean mesh surface to the loudspeaker, this source of counter-noise realizing, on said mesh, a condition for limits of total reflection of the wave.

The pressure is indeed zero on the screen surface, and the acoustic load is therefore zero at the source.

This is the theoretical operating mode of an active screen consisting of a single layer of speakers. However, as already mentioned, such a mode of operation with a single source has a cutoff frequency which is not high enough to counteract the annoying part of the spectrum of the waves emitted by conventional means of transport, insofar as the surface density of the speakers to preserve the visual transparency of the screen.

Indeed, and as illustrated in FIG. 4, it has been observed that it is established between the wave of incident noise and the wave reflected by the sources, in the vicinity of these said sources, a interference field whose "streamlines" (50) are shown diagrammatically by the tangents in each point to the major main axis of the acoustic particulate hodographs.

This field is spatially organized in a network, by tubular cells which repeat themselves periodically according to the meshes of the screen, as soon as these meshes are sufficient numerous, so that the organization of the interference field is practically invariant from one stitch to another.

i In each cell, the "current lines" allow you to define acoustic current which converges on the active surface of the counter-noise source, that is to say the diaphragm of the speaker (25).

These tubes constitute as many fictitious waveguides inside which is established the interference field.

The diagram in Figure 4 illustrates the following phenomena.

Indeed, compared to the reference phase wave surface Φ, the difference of guided wave walk increases as the tubes move away from the axis of revolution to become practically equal to the diameter "a" of the cell.

The cut-off phenomenon occurs for: λ _0/2 = C / 2 f ₀ = a / 2, when the steady state along the extreme tube presents a half-wavelength of difference in path compared to the central tube, and is therefore in phase opposition with the flow rate of the counter-noise source.

The cut-off frequency f _o is therefore close to c / a , as stated above.

Beyond this frequency, the incident traveling progressive wave guided in the extreme tube can no longer be controlled by the noise reduction source, and crosses the screen giving birth of refracted, oblique waves, which obstruct the protective role conferred on the screen.

According to another characteristic of the invention, the sources can be advantageously arranged in sub-assemblies according to screens parallel to one another others, and the operation is then illustrated in FIG. 5.

Indeed, the operation of multi-layer sources makes it possible to increase the cut-off frequency of the system for a given dimension of the mesh of the network. More precisely, and according to the diagram illustrated in FIG. 5, the different sources (27, 28) of the same mesh are controlled, with a determined phase offset, to act on the end tube (51) so as to avoid it to escape above the frequency f ₀ , continuing to keep the "current lines" channeled towards the source of the appropriate layer.

Indeed, and with reference to FIG. 5, along the axis (52) of the sources (27, 28), and therefore under the primary source (27), a series of secondary sources (28), ordered with the desired phases and modules, makes it possible to capture and reflect the acoustic flow of the incident wave, for the current lines which escape the primary source (27), in respecting the interference structure of the sound field, close to the loudspeaker, such as previously described.

In a simplified manner, FIG. 5 represents a single secondary source.

We observe on the traced current lines, a running delay to reach the second source (28) of the order of d + a, where "d" represents the distance between the membranes speakers (27, 28), and "a" the half mesh of the screen.

In accordance with the invention, this running delay is compensated by the control device, which supplies this second source (28) with a signal delayed substantially by d + a / c , this value being able to be adjusted with more precision: so as to ensure strictly the orthogonality of the source field to the oblique modes of the network.

The cutoff frequency is thus brought to twice the initial frequency, itself close to c / a. In this spirit, the flow rates of the sources are adjusted in proportion to the surfaces of the controlled current tubes.

The principle can be extended to a higher number of sources, and the table below presents the cut-off frequency for different numbers of layers of counter-noise sources, for two cases of surface area density of sources. Number of layers 1 2 3 4 Cutoff frequency in Hertz A multiple source for 1 m ² 370 700 1000 1300 A multiple source for 5 m ² 170 300 450 600

The sources as represented in FIG. 5, by axis loudspeakers can be advantageously carried out by contiguous sets of sources smaller dimensions, properly assembled and ordered with delays appropriate to ensure the best regularity in the acoustic flow field.

It appears from the above that the device according to the invention provides a noise attenuation in a frequency band covering the majority of the spectrum noise waves from means of transport such as airplanes or railways.

The operation of the screen described is an active reflector operation with respect to against incident noise waves, it is nevertheless possible to envisage an operation as a perfect absorber of these waves insofar as the reflected waves could present, in certain situations, a harmful effect with respect to the surrounding site.

• p désigne la pression acoustique,
• V_n la vitesse acoustique normale à l'écran,
• ρ ₀ c ₀ l'impédance acoustique de l'air.

For this, it is theoretically appropriate to associate with the sources of variation of acoustic flow, sources of variation of acoustic pressure, so as to achieve on the surface of the screen the mixed boundary condition of adaptation: ∂ p / ∂ t = ρ ₀ . c ₀ . ∂ Vn / ∂ t , where

• p denotes the sound pressure,
• V _n the normal acoustic speed on the screen,
• ρ ₀ c ₀ the acoustic impedance of the air.

These two types of sources being controlled concomitantly from the same signals specific to incident noise waves.

Having practically no sources of sound pressure, we are led to realize the same condition from a double distribution of sources of variation of flow, placed on two parallel surfaces distant from e, e being less than half minimum wavelength.

Under these conditions, a simple model shows that by controlling the downstream source in quadrature with the incident wave, and the upstream source shifted in time by e / c _o , at the same amplitude, the required adaptation condition is ensured on a decade of frequency, with a source flow variator of the order of three times higher than for a single screen.

In this operation, the downstream source is acoustically discharged and it is the upstream source which absorbs the power of noise.

On the other hand, the active reflective acoustic screens described can be combined in their operation on contiguous passive screens. These passive screens can be constituted by pre-existing built surfaces (roofs and facades of buildings according to the figure 1). They can also be installed for acoustic reasons, in complementarity of active screens, and produced using architectural techniques adapted, in particular using glazed surfaces of suitable thickness, according to arguments of use or aesthetics, specific to the development of particular sites.

Such passive screens then cause a reflection of the dual type of the type described. for active screens, namely that acoustically "hard", they create the conditions for limits approaching the cancellation of the normal acoustic speed, and the doubling of the sound pressure on their surface.

Care must be taken at the junction of these two types of screens to avoid the resulting strong pressure gradients locally altering the eigen effects of reflection sought, thereby causing acoustic leaks to the volume at protect.

The recommendation of this patent is to soften these gradients by passing more progressively active control of "hard" reflective screen type (normal zero speed) towards that described of "soft" reflecting screen (characterized by zero pressure).

To make an active "hard" screen, sources with variation control are used. pressure. These are sources that we do not have a priori, the various types of speakers on the contrary approach sources of flow variation, and their delay response makes their control in pressure variation illusory.

Such sources are produced by association, by pair of sources of variation of opposite flow rates, mounted back to back, constituting an acoustic dipole.

Such sources are in particular to be used for producing active screens on openings in front of buildings, windows or bays, so as to ensure the condition reflection in an "open window" situation, thus preventing external noise from entering the interior of dwellings.

For this, multiple sources of bipolar type are produced, for example four sources (26) at the four corners of the doorway, according to the assembly described in FIG. 6.

Such multiple sources can be realized, as illustrated in FIG. 7, by the association of several elementary sources (27) deposited in parallel planes. Each elementary source (27) is a bipolar source having two speaking faces in opposition.

Claims (8)

Active device for attenuating the sound intensity at the level of a determined zone, by emission of counter-noise waves, of the type comprising: a set of sensors (30) capable of determining the signals and the directions of the waves emitted by the distant noise sources; means for processing the signals b (t) from said sensors, and for generating signals cb (t) corresponding to the counter-noise waves; a set of electro-acoustic sources (25), said sources being installed in the space close to the area to be protected, and connected to said processing means, and capable of emitting counter-noise waves in the same direction and in the same direction as the incident waves, the sensors and the electro-acoustic sources being placed in such a way that the incident waves reach the sensors beforehand,
characterized in that the electro-acoustic sources (25) are arranged on a continuous surface (24), and on a regular mesh, this surface constituting a reflecting screen, possibly absorbing, with respect to noise waves. Device according to claim 1, characterized in that the electro-acoustic sources (25) are arranged in a hexagonal mesh. Device according to claim 1, characterized in that the mesh has a pitch of less than two meters. Device according to claim 1, characterized in that it comprises several sets of electro-acoustic sources arranged on several surfaces offset relative to each other by translation, according to their normal, so as to limit the surface density of the sources for a frequency high cutoff given. Device according to claim 1, characterized in that the continuous surfaces have a plane geometry, or a quadric geometry, in particular cylindrical. Device according to one of claims 1 to 5, characterized in that it is associated with a rigid structure forming a solid screen. Device according to one of claims 1 to 6, characterized in that some of the electroacoustic sources are associated in pairs to constitute acoustic dipoles. Assembly composed of several devices according to one of claims 1 to 7, in which the different devices are juxtaposed in the space of the area to be protected.

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