FR2817648A1 - ACTIVE SOUND REFLECTOR - Google Patents

Abstract

The invention relates to an active device for reflecting sound waves, comprising one or more sound sensors (1) and their preamplifiers (2), and one or more loudspeakers (5) located near the sensors (1) connected to the sensors. (1) via a control circuit (3) and power amplifiers (4). The control circuit (3) comprises an echo cancellation filter reducing the acoustic coupling between loudspeakers (5) and sensors (1), a control device for capturing and transmitting directivity, and a reverberation filter. The active sound reflector improves room listening conditions by effectively enhancing the sound from the stage, and increasing the reverb time if necessary.

Description

<Desc / Clms Page number 1>

The present invention relates to the active correction of the acoustics of theaters, auditoriums, conference rooms and amphitheatres, etc. The idea of the active sound reflector (where the active sound suppressor) derives from that of the traditional (passive) lampshades, a sort of reflector generally placed above the stage to send back to the audience of the energy. sound coming from the scene. This early energy, arriving shortly after the direct sound (sound propagating directly from the source to the listeners), reinforces the intelligibility and clarity of the messages perceived by the listeners. When active sound reflectors are placed on the sides of a room, the early energy reinforcement also results in a pleasant feeling of widening the source on stage. Thanks to the use of electroacoustic techniques (microphones, filters, amplifiers, loudspeakers), the active lamps are much more efficient than passive lamps. Their dimensions can be much smaller, and their behavior (for example the intensity with which they reflect the sound) can be adjusted by the user according to the requirements of each type of show. A set of active sound reflectors will also increase the reverberation of the room in which they are placed, with or without the aid of reverberant electronic filters.

Here are some typical applications of active sound reflectors - Direct sound reinforcement, with active sound reflectors placed above the front stage, or near the stage frame.

- Active stage conch. Improvement of acoustics on stage thanks to sound reflectors placed around the musicians.

- Increase the reverberation time of a room thanks to a set of active sound reflectors.

- Creation of a room effect in an outdoor show.

<Desc / Clms Page number 2>

- Local reinforcement of the sound level in the room, through the use of one or more active sound reflectors placed near the area to be reinforced.

Active sound reflectors are radically different from the sound system in that they essentially change the acoustics of the room rather than a massive diffusion of sound energy. Thus, in a room equipped with a set of well-adjusted active sound reflectors, the listener has the impression of unamplified natural acoustics.

Passive lamps have been used in theaters for many years. They are in the form of reflective panels, planes or curved, usually suspended above the stage or the stage. Their typical dimensions are of the order of one to a few square meters.

The idea of an active sound reflector is described in the French patent application FR 2 449 318 filed on 11-02-1980 by Philips Gloeilampenfabrieken NV where it is spoken of a sound amplifier reflector comprising a microphone, an amplifier and a speaker whose direct radiation is poorly captured by the associated microphone, all being assembled into a mechanical unit. In said patent, we do not speak explicitly of the amplification of early energy, but rather the use of a number of these reflectors to reinforce the reverberation in a room. Moreover, since the 1960s, a number of commercial achievements of reverberation reinforcement electroacoustic systems have emerged, a brief description of which can be found in M.

acoustics and eiectroacoustics, Proc. of ACTIVE 95, Newport Beach, CA, USA, 1995, 39-54. Citons entre autres les systèmes MCR (Multichannel Réverbération de Philips décrit dans S. Kleiner, P. Svensson, Review of Active Systems in Room

acoustics and eiectroacoustics, Proc. of ACTIVE 95, Newport Beach, CA, USA, 1995, 39-54. These include the MCR systems (Multichannel Reverb Philips described in S.

H. De Koning, The MCR system: multiple-channel amplification of reverb, Philips Tech. Rev. , flight. 41, no. 1, pp. 12-23, 1983/84), ACS (Acoustic Control System, described in A.

J. Berkhout, D. De Vries, P. Vogel, Acoustic control by

<Desc / Clms Page number 3>

 wave field synthesis, J. Acoust. Soc. Am. , vol. 93, no. 5, pp. 2764-2778,1993, and in European Patent Application EP 0 335 468 A1 filed by Birch Wood Acoustics Nederland BV,), SIAP (System for Improved Acoustic Performance, described in WCJM Prinssen, M. Holden, System for improved acoustic performance, Proc IOA, pp 93-101, 1992, and in European Patent Application EP 0 386 846 Al filed by Prinssen In Bus Raadgevende Ingenieurs VOF), LARES (Lexicon Acoustic Reverberation Enhancement System, described in D. Griesinger , Improving room acoustic through time-varying synthetic reverberation, A udio Eng S 90th Convention, Preprint 3014 (B-2), 1991, and in U.S. Patent 5,109,419 to Lexicon Inc.), and VRA (Variable Room Acoustic, described in MA Poletti, On Controlling the Apparent Absorption and Volume Assisted Reverberation System, Acta Acustica, Vol 78, pp 61-73,1993, and in US Patent Nos. 5,729,613 and 5,862,233 to Industrial Research Limited). All these systems consist of a number of microphones associated with loudspeakers via a multichannel electronic amplification and filtering chain. They are looped systems since the signals picked up by the microphones are sent to the loudspeakers, whose acoustic radiation is picked up by these same microphones. The microphones are distributed in the room and / or near the stage, and the speakers are distributed in the room (and sometimes also on the stage).

However, in these systems the sound picked up by a microphone is sent by the electronic chain to a speaker located far from this microphone. In fact, in the opposite case, the significant energy of the direct acoustic wave propagating from the loudspeaker towards the microphone would impose a very low gain of electronic amplification so that the looped system does not become unstable (Larsen effect), thus compromising the effectiveness of the system. Only the Carmen system (CSTB-France) uses the concept of proximity reaction. In this system, the signal from a microphone is sent (via an electronic chain) to a speaker located near the microphone, and the energy of the wave

<Desc / Clms Page number 4>

 Acoustic direct-speaker Microphone is limited by an acoustic decoupling principle The directivities and positions of the microphone and the associated speaker are chosen so that the microphone captures only little direct energy from the speaker. We speak here of a proximity reaction when the distance between the microphone and the associated loudspeaker is small compared to the typical propagation distances in the rooms, but still higher than the smallest wavelengths considered (a few centimeters to the very high frequencies). Although this definition is not explicitly mentioned in patent application FR 2 449 318, it can be considered to be implicit in the concept of sound reflection mentioned therein. When the microphone-speaker distance is smaller than the smallest wavelengths considered, it is called a local reaction, as is the case in active impedance control applications such as those described in X. Meynial, Active Materials for room acoustics applications, 3rd ICIM / ECSSM'96, Lyon '96, 968-973. In the case of the active sound reflector, said distance is typically of the order of 20cm to 2m.

Figure 1 illustrates the block diagram of the active sound reflector. The microphone (1) is connected to the input of a preamplifier (2) whose output is connected to a control circuit (3). The output signal of the control circuit (3) is amplified by a power amplifier (4) charged by a loudspeaker (5). The microphone (1) and the loudspeaker (5) are subject to a screen (6). The distance separating the microphone (1) from the loudspeaker (5) is of the order of 20cm to 2m typically. The active sound reflector may comprise a plurality of microphones (1) and several preamplifiers (2), and several loudspeakers (5) and several power amplifiers (4). In this case, the active sound reflector is characterized by the average of the distances between all the pairs (loudspeaker - microphone) of the reflector; this distance being between 20 cm and 2 m. Operation

<Desc / Clms Page number 5>

 of each active reflector is independent insofar as it does not receive electrical signals from other active reflectors.

The device according to the invention corresponds to the description above and is characterized by the functions provided by the control circuit (3). It can be electronic decoupling between the speakers (5) and the microphones (1), directional filtering, and reverb. These functions will be explained later.

Microphones (1) may be more generally sound sensors, but we will keep the name of microphone in the suite. The term transfer function indicates in the following a function of the frequency, although the variable is not mentioned so as not to burden the explanations.

The active sound reflector can only work if the acoustic coupling between the loudspeakers (5) and the microphones (1) is limited, otherwise the system will become unstable (Larsen effect) even for low chain gain values. amplification (2,3,4). In the following, we discuss the problem of acoustic coupling for a single speaker and a single microphone, the problem is essentially the same when several microphones and / or several speakers are used.

The limitation of the acoustic coupling can be done in two ways acoustic decoupling, and electronic decoupling.

One or the other is used, or a combination of both in the active sound reflector. The final decoupling shall be such that the energy of the direct loudspeaker (5) # microphone (1) is less than the energy reverberated by the room from the loudspeaker (5), and picked up by the microphone ( 1). It is on this condition that a set of some active sound reflectors can reflect on the audience of sound waves whose amplitude is comparable to that of the direct wave reaching listeners from the scene.

<Desc / Clms Page number 6>

Acoustic decoupling involves playing on the directivity characteristics of the loudspeaker (5) and / or the microphone (1) and on the symmetry of the device, so that the direct sound picked up by the microphone coming from the loudspeaker is of low amplitude. A number of combinations are possible, such as those illustrated in Figure 2 using either a bidirectional microphone (Figure 2a), an omnidirectional microphone (Figure 2b), or a cardioid microphone (Figure 2c). The reflector is seen from the front in FIG. 2a, and in section in FIGS. 2b and 2c. In Figures 2a and 2c, the dashed arrow represents the direction of the maximum sensitivity of the microphone. The idea of acoustic decoupling is presented in the patent application FR 2,449,318, which describes the implementation corresponding to FIG. 2c. It is important to note that some of these solutions use directional microphones, which makes it possible, if we point the axis of maximum sensitivity of these to the stage, to favor the capture of the direct sound coming from the scene in relation to the reverberation of the sound reverberated by the room, especially as the reflector is close to the stage. The efficiency of the reflector in terms of reinforcement of the early energy is increased.

The judicious use of several microphones (forming a microphone antenna) can allow an even more selective capture of the source on stage. Likewise, the use of several loudspeakers may make it possible to privilege the direct propagation between the loudspeakers and the listeners with respect to the propagation reverberated by the room.

In the device according to the invention, the electronic decoupling optionally completes the acoustic decoupling to reduce the influence of sound reflections from obstacles located near the active reflector, diffracted waves on the edge of the screen (6). or waves reflected by other neighboring active sound reflectors. Electronic decoupling consists of attenuating the electrical signal delivered by the microphone (1)

<Desc / Clms Page number 7>

 from the loudspeaker (5) by means of an echo canceller filter (7) included in the control circuit (3), and such as that shown in Figure 3. In this filter, the transfer function K = -Vm / Vhp must be estimated (in the absence of feedback X = G = 0) while the reflector is installed in the room, and the estimate X injected into the echo canceller filter (7). The transfer function K can be either previously measured by applying a test signal to the input of the power amplifier (4), or estimated continuously during the operation of the reflector according to the well-known adaptive techniques in sound or telephony .

The echo canceller (7) cancels out the most energetic part of the loudspeaker response (5) microphone (1), ie the beginning of the corresponding impulse response (typically the first milliseconds). The transfer function G of FIG. 3 determines the characteristics of the sound reflected by the active reflector.

The use of several loudspeakers (5) excited by delayed signals and suitably filtered by the control circuit (3) can make it possible to control the directivity diagram (ie the sound level emitted as a function of the considered direction) of the loudspeaker network (5), according to well-known antennal principles. For example, a vertical linear array of loudspeakers separated by a distance d (assumed to be less than the half-wavelengths considered) and indexed from 1 to p from the top, radiates a maximum of energy in the direction # if we assign a delay # n = n.sin (#), d / c to the signal sent to the nth speaker, where c is the velocity of the sound waves, and # = 0 is the horizontal. This elementary principle easily extends to a two-dimensional network. We can then define the transmission direction of the network as the angular direction corresponding to the maximum energy emitted. Thus, the device according to the invention provided with several loudspeakers (5) makes it possible to generate several reflections

<Desc / Clms Page number 8>

 characterized by delay, attenuation, and transmission direction FIG. 4 shows an embodiment of the invention consisting of a device using a microphone (1), a speaker array (5), and a n number of reverbers (9) whose output signals are assigned to as many directions of emission, through a directional matrix (10). The electronic decoupling corresponding to the transmission direction i is ensured by the echo cancellation filter (7) of transfer function X1, by noting Vm the voltage delivered by the preamplifier (2) of the microphone (1), and VR1 the output voltage of the reverberator i, then the transfer function H1 = Vm / VR1 represents the emission of the reflector in the direction i and the acoustic propagation in the room up to the microphone (1). Note that this function takes into account the filter echo canceller X, H, = K, + X ,. The open-loop gain of the system {reflector + room} is therefore written n T = # H1. R1 t = 1 where R2 is the voltage transfer function of the reverberator i. However, it is well known in room acoustics that the frequency distributions of the modules of the H2 functions are governed by the Rayleigh law if the direct sound of the speakers (5) microphone (1) is of low energy compared to the sound reverberated by the room (condition which is ensured by the acoustic and electronic decoupling).

The higher the radiation of the speaker array (5) for a given transmission direction (corresponding to one of the reverberators) differs from the radiation corresponding to the other transmission directions (corresponding to the other reverberators), the less the H2 functions are correlated. between them. In addition, the distributions of the modules of the transfer functions R2 are also governed by the Rayleigh law since they also reflect a reverberation phenomenon. Then, by applying the law of large numbers, the variance of # T # decreases and its distribution tends to a Rayleigh law as the number n of

<Desc / Clms Page number 9>

 reverberators (9) increases, especially since the H functions are decorrelated between them This is of great interest in practice, since the gain of the amplification chain (2,3,4) can be all the more stronger than # T # distribution approaches Rayleigh's law. At the limit, the gain will be the same as if there was no reverb, but the use of these will effectively extend the reverberation of the room. Such an active sound reflector thus makes it possible both to generate early reflections of high amplitude (by taking advantage of the directivity of the speaker array), and to increase the reverberation time of a room. The filtering performed by the directional matrix (10) can also be slowly varied so as to change the emission direction assigned to each reverberer over time, which will make it possible to improve the stability of the system and to reduce the coloration associated with it. acoustic looping of the loudspeakers (5) to the mi crophones (1). An effective implementation of this principle is to perform a slow rotation of the assignments of the outputs of the reverberators to the directions of emission. The speaker network (5) can take various forms, for example the plane network or the linear network. A particularly advantageous combination is obtained with a vertical linear array of loudspeakers (5) and a bidirectional microphone (1) whose membrane is located in a plane containing the loudspeakers (5), thereby ensuring excellent acoustic decoupling according to the principle of Figure 2a. A simplified version of the device consists in deleting the directional matrix (10), and assigning the output of each reverberator (9) directly to one or more speakers of the network (5).

In addition to the interest that has just been discussed, the fact of using several loudspeakers (5) makes it possible to distribute the sound energy emitted by the reflector over a larger area, and thereby to avoid The listener is not bothered by too much sound energy density from a single loudspeaker (5). If we remove the reverberators (9), we keep this last advantage, but we lose the benefit of

<Desc / Clms Page number 10>

 these for increasing the reverberation time of the room. However, the reflector will always tend to increase the reverberation by simply acoustic looping of the speakers (5) to the microphone (1) via the room.

In the following, the case where there is no reverberer is considered as a particular case of unit transfer function reverb R = 1.

FIG. 5 illustrates another embodiment of the device according to the invention. In a manner similar to the above, the use of a microphone array (1) and a directional matrix (10) placed between the microphone preamplifiers (2) and the reverberators (9) makes it possible to pick up the incident waves according to several directivity diagrams (we will talk about sensor channels), each of these sensor channels being assigned to a separate reverberator (9). The echo cancellation filter has been omitted for clarity. The outputs of the reverberators (9) are sent to a summator (10) whose output excites the loudspeaker (5) via the power amplifier (4). The open-loop transfer function of the system is of the same form as that given in the previous paragraph, and therefore has the same advantages in terms of reinforcement of the reverberation. As in the previous case, a simplified implementation of the device consists in eliminating the directional matrix (10) and assigning the output of each microphone (1) directly to one or more reverbers (9). The directivity of the capture is then determined by the specific directivity of each microphone (1).

Finally, it is of course possible to combine, as shown in FIG. 6, the systems described in the last two paragraphs. An array of microphones (1) connected via their preamplifiers (2) to a directional capture matrix (10a) is then used. and a network of loudspeakers (5) excited via their power amplifiers (4) by the outputs of a directional transmission matrix (10b) the reverberators (9) being placed between the outputs of the directional capture matrix ( 10a) and the inputs of the directional emission matrix (10b). With

<Desc / Clms Page number 11>

 this latter embodiment of the device according to the invention, it is possible to assign to each wave picked up along a given channel any number of waves possibly delayed and attenuated with respect to each other at each radiation path.

Figure 7 illustrates in a simple way this type of possibility, with a reflector based on 2 cardioid microphones (1) pointing in the directions + # and - #, and a network of speakers (5) separated by a distance d, reflecting the waves in a pseudo-specular manner thanks to a directional emission matrix (10b) consisting of summators (8) and delays (11) # = d / c.sin (#) to the incident direction + # corresponds to the reflected direction - # (and vice versa), but only two directions are considered.

In this example, there is no capture directional matrix because the directivities of two capture channels result simply from the directivity of the microphones (1).

Reverbers may optionally be introduced between the outputs of the microphone preamplifiers (2) and the inputs of the directional emission matrix (10b).

In FIGS. 5 to 7, the echo cancellation filter has been omitted for clarity. It is clear that its structure and response tend to become more complex as the number of microphones or loudspeakers is increased and the number of reflections emitted for each incident wave increases. specify that, except for any consideration related to the directivity of a microphone or speakers, the multiplication of the number of speakers (which is not accompanied by a multiplication of the total power transmitted because the gain in open loop mean <T> must remain constant) tends to decrease the energy of the direct sound picked up by the microphone from all speakers if the added speakers are located farther from the microphone than the existing ones.

In addition to the electronic decoupling function and operations related to the use of multiple microphones

<Desc / Clms Page number 12>

 and / or loudspeakers, the control circuit (3) performs various filtering functions, such as gain control, equalization (frequency correction), compression of the signal dynamics to limit the maximum amplitude of the signals sent to the loudspeaker (5) via the power amplifier (4), or a time-varying filtering (slow modulation of the phase or the delay of the signal, or more complex modulation) which possibly makes it possible to further increase the gain threshold of the control circuit corresponding to the instability (Larsen effect). Most of the filtering operations of the control circuit (3) is performed by one or more digital signal processors (DSP). These filtering functions, which have not been explicitly mentioned in the preceding paragraphs and the related figures for the sake of clarity, can be considered as part of the reverberators (9).

The user adjusts the behavior of the active sound reflector by modifying (using a remote control) the filtering parameters of the control circuit (3).

The entire electronics {preamplifier (2), control circuit (3), power amplifier (4)} can be physically integrated in the reflector, for example on the back of the screen (6), or remote. The preamplifiers (2) may optionally provide the power function for the microphones (1), especially if they are of the electrostatic type. They may possibly be part of the same mechanical unit as the control circuit (3), just like the power amplifiers (4). Depending on the type of directivity desired for the loudspeakers (5), they can be mounted in a closed enclosure or bass-reflex, or simply placed in a screen, or any other type of acoustic load. The microphones (1) are either placed on the screen or remote (as is the case in Figure 2c) using a perch for example. The screen (6) can be of various sizes and shapes (rectangular, elliptical, or other), provided that the decoupling is sufficient. If the speakers are mounted in a

<Desc / Clms Page number 13>

 speaker, the screen may be reduced to the front of the speaker. It is not necessarily plan.

Although the operation of each active reflector is autonomous, several active sound reflectors will often be associated, as is the case in the first four examples of use mentioned above. In this case, the control parameters control is common to all the associated sound reflectors, and transmitted by a network shared by these sound reflectors The shape of the sound reflectors may be designed for the assembly of several of they, for example to form a large active reflector above the scene frame.

Claims (11)

CLAIMS 1- Active device reflecting the sound waves and for improving the listening conditions in theaters and congresses, theaters and amphitheatres, recording studios etc, comprising one or more sound sensors (1) and their preamplifiers (2), and one or more loudspeakers (5) and their power amplifiers (4), characterized in that the average of the distances between sensor and loudspeaker for all the couples (sensor - loudspeaker) is between 20 centimeters and two meters, and in that the inputs of the amplifiers (4) are connected to the outputs of the preamplifiers (2) via an electronic control circuit (3) ensuring in particular at least one of the following functions - echo cancellation reducing the acoustic coupling between the loudspeakers (5) and the sound sensors (1), - controlling the directivity of all sound sensors (1), - controlling the sound ectivity of the set of loudspeakers (5), - reverberating filter, 2- Device according to claim 1, comprising an array of several loudspeakers (5) and a control circuit (3) provided with an input and of several outputs connected to the loudspeakers (5) via a plurality of power amplifiers (4), characterized in that said control circuit (3) allows from a wave picked up by the one or more sensors (1). ) to emit any number of waves according to several directivity diagrams of the loudspeaker network (5), and possibly delayed and attenuated relative to each other. 3- Device according to claim 1, comprising a network of several sound sensors (1) and a circuit of <Desc / Clms Page number 15>  control unit (3) provided with an output and with several inputs connected to the sensors (1) via a plurality of preamplifiers (2), characterized in that said control circuit (3) allows a wave capture according to several diagrams of directivités of the sensor array each corresponding to a sensor path, and emitting any number of waves possibly delayed and attenuated relative to each other for each wave captured by each sensor path. 4- Device according to claim 1, comprising - an array of several sound sensors (1), - a network of several loudspeakers (5), - and a control circuit (3) provided with several inputs connected to the sensors (1). ) via several preamplifiers (2), and several outputs connected to the loudspeakers (5) via a plurality of power amplifiers (4), characterized in that said control circuit (3) allows a capture waves according to several diagrams of directivities of the sensor array (1) each corresponding to a sensor channel, a wave emission according to several directivity diagrams of the loudspeaker array (5) each corresponding to a radiation path, and comprises a matrix of reverberators whose inputs come from the sensor channels and the outputs are assigned to the radiation paths. 5- Device according to claim 2, characterized in that it uses a plurality of speakers (5) aligned vertically and mounted in a column-type enclosure, a sound sensor (1) consisting of a bidirectional microphone attached to the column enclosure and placed so that its membrane describes a plane containing the loudspeakers (5). Device according to Claim 2, characterized in that the control circuit (3) contains several filters <Desc / Clms Page number 16>  reverberators excited by the same signal from the preamplifier (2), is provided with as many outputs as there are reverberant filters, each of these outputs being connected to a power amplifier (4) charged by one or more amplifiers. speakers (5), the allocation between the outputs of the reverberators and the outputs of the control circuit (3) may possibly vary slowly over time. 7- Device according to claim 5, characterized in that the control circuit (3) contains several reverberator filters excited by the same signal from the preamplifier (2), and is provided with as many outputs as there are filters each of these outputs being connected to a power amplifier (4) charged by one or more loudspeakers (5), the allocation between the outputs of the reverberators and the outputs of the control circuit (3) possibly varying slowly over time. 8- Device according to claim 2, characterized in that the control circuit (3) contains several delays excited by the same signal from the preamplifier (2), is provided with as many outputs as there are delays, each of these outputs being connected to a power amplifier (4) charged by one or more loudspeakers (5), the allocation between the delay outputs and the outputs of the control circuit (3) possibly varying slowly during the time. 9- Device according to claim 4 characterized in that the characteristics of the sensor channels and radiation paths, as well as the properties of the reverberator matrix may optionally vary slowly over time. 10- Device according to claim 9, characterized in that the sensor channels are each constituted by <Desc / Clms Page number 17>  by the signal from a sound sensor or a group of sound sensors (1) specific. 11- Device according to claim 9, characterized in that the radiation channels are each constituted only by a speaker or a group of speakers (1) specific. 12- Device according to claim 10, characterized in that the radiation paths are each constituted only by a speaker or a group of speakers (1) specific. 13- Device according to claim 10 characterized in that it uses a planar array of loudspeakers (5) and two sound sensors (1) consisting of two cardioid microphones pointing in two directions symmetrical with respect to the normal to the top plane speakers (5).