Acoustic set comprising a speaker with controlled and variable directivity

Abstract

An acoustic chamber includes a loudspeaker, which includes at least two membranes that each reproduce a different frequency band, and a filter that makes it possible to generate a plurality of activation signals from an audio signal source. The activation signals are each applied to an actuator of one of the membranes. The acoustic chamber has an operating range having a variable and controlled directivity, each frequency of which belongs to at least two frequency bands reproduced by the membranes. The acoustic chamber obtains a directivity control signal, and the filter makes it possible to dose, for each frequency of the operating range and depending on the directivity control signal, the contribution of each one of the at least two membranes reproducing the frequency.

Claims

1. An acoustic set comprising: a speaker comprising at least two membranes, each reproducing a different frequency band; and means for filtering a source audio signal, which generates a plurality of activation signals, each activation signal actuating one of the membranes; a frequency range of operation with variable and controlled directivity, wherein each frequency of the frequency range of operation belongs to at least two of the frequency bands reproduced by the membranes; means for obtaining a directivity control signal, wherein for each respective frequency of said frequency range, the means for filtering is configured to apportion a contribution of each of the at least two membranes reproducing the respective frequency according to the directivity control signal by applying: a first filtering having an amplitude that is weighted as a function of the directivity control signal; and a second filtering having a phase that is also weighted as a function of the directivity control signal.

2. The acoustic set according to claim 1, wherein the speaker comprises at least three membranes, each reproducing a different frequency band and said frequency range of operation is formed by at least two contiguous overlap bands between the frequency bands produced by said at least three membranes.

3. The acoustic set according to claim 1, wherein the means for obtaining the directivity control signal comprise means for setting the value of said directivity control signal by a user.

4. The acoustic set according to claim 1, wherein the means for obtaining the directivity control signal comprise: means for receiving a data stream containing said source audio signal and said directivity control signal; and means for extracting the directivity control signal contained in the data stream.

5. The acoustic set according to claim 1, wherein the speaker comprises at least three membranes forming at least three adjacent channels belonging to the group consisting of: a woofer, a low-mid channel, a high-mid channel and a tweeter.

6. The acoustic set according to claim 1, further comprising means for equalization, placed upstream to the means for filtering and acting according to the directivity control signal.

7. The acoustic set according to claim 1, wherein the means for filtering are implemented at least partially in the form of a processor executing at least one determined filtering program.

8. The acoustic set according to claim 1, wherein, for said respective frequency of said frequency range, the directivity control signal is proportional to an average directivity index desired for the acoustic set at said respective frequency.

9. The acoustic set according to claim 1, wherein said means for filtering comprise: at least two blocks each enabling generation, from a distinct source audio signal, of a plurality of activation signals each associated with one of the membranes; summing means enabling summing of the activation signals generated by said blocks, as a function of the membranes with which they are associated, to obtain resultant signals, each feeding one of said means for actuating one of the membranes; wherein the acoustic set comprises means for obtaining at least two directivity control signals each associated with one of the source audio signals, and wherein each block makes it possible, for a frequency of said frequency range of operation and as a function of the directivity control signal associated with the source audio signal received by said block, to apportion the contribution of each of said at least two membranes reproducing said frequency, for the reproduction of said source audio signal received by said block, each block being configured to make possible said apportion for each frequency of said frequency range of operation.

10. The acoustic set of claim 9, further comprising means for receiving a signal transporting a data stream containing at least two source audio signals each associated with a distinct directivity control signal, each directivity control signal configured to dynamically control and vary the directivity of an acoustic set, for the reproduction of the source audio signal associated with said directivity control signal.

11. The acoustic set according to claim 1, wherein said speaker is a coaxial speaker comprising coaxial membranes.

12. A method comprising: receiving a signal transporting a data stream containing a source audio signal and a directivity control signal, said directivity control signal configured to dynamically control and vary directivity of an acoustic set comprising a speaker having at least two membranes each reproducing a different frequency band; and filtering the source audio signal to generate a plurality of activation signals, each activation signal actuating one of the membranes, wherein the acoustic set has a frequency range of operation with variable and controlled directivity, wherein each frequency of the frequency range of operation belongs to at least two of the frequency bands reproduced by the membranes; and for each respective frequency of said frequency range of operation, apportioning a contribution of each of the at least two membranes reproducing said respective frequency according to the directivity control signal by applying: a first filtering having an amplitude that is weighted as a function of the directivity control signal; and a second filtering having a phase that is also weighted as a function of the directivity control signal.

13. An acoustic set comprising: a speaker comprising at least three membranes, each reproducing a different frequency band; and means for filtering a source audio signal, which generates a plurality of activation signals, each activation signal actuating one of the membranes; a frequency range of operation with variable and controlled directivity, wherein each frequency of the frequency range of operation belongs to at least two of the frequency bands reproduced by the membranes, and said frequency range of operation is formed by at least two contiguous overlap bands between the frequency bands produced by said at least three membranes; means for obtaining a directivity control signal, wherein for each respective frequency of said frequency range, the means for filtering is configured to apportion a contribution of each of the at least three membranes reproducing the respective frequency according to the directivity control signal.

14. An acoustic set comprising: a coaxial speaker comprising at least two coaxial membranes, each reproducing a different frequency band; and means for filtering a source audio signal, which generates a plurality of activation signals, each activation signal actuating one of the membranes; a frequency range of operation with variable and controlled directivity, wherein each frequency of the frequency range of operation belongs to at least two of the frequency bands reproduced by the membranes; means for obtaining a directivity control signal, wherein for each respective frequency of said frequency range, the means for filtering is configured to apportion a contribution of each of the at least two membranes reproducing the respective frequency according to the directivity control signal.

Description

5. LIST OF FIGURES

(1) Other features and advantages of the invention shall appear from the following description, given by way of an indicatory and non-exhaustive example, and from the appended drawings, of which:

(2) FIG. 1A presents a block diagram of an acoustic set according to one particular embodiment of the invention;

(3) FIG. 1B presents an example of an embodiment of the filtering stage 3 appearing in FIG. 1A;

(4) FIG. 2 presents an example of a frequency response, in the axis of the coaxial speaker of FIG. 1A, for each of the three channels, without filtering;

(5) FIG. 3 presents a polar response at 1250 Hz of the low-mid and high-mid channels of the coaxial loud speaker of FIG. 1A without filtering;

(6) FIG. 4 presents a polar response at 5000 Hz of the low-mid, high-mid and tweeter channels of the coaxial speaker of FIG. 1A without filtering;

(7) FIG. 5 presents a polar response at 10000 Hz of the high-mid and tweeter channels of the coaxial speaker of FIG. 1A without filtering;

(8) FIG. 6 is a block diagram of an acoustic set according to a second particular embodiment of the invention;

(9) FIG. 7 presents a polar response at 1250 Hz of a combination of low-mid and high-mid channels of the coaxial speaker of FIG. 1A obtained with particular sets of weighting values;

(10) FIG. 8A illustrates a classic implementation of a recording and rediffusion system;

(11) FIG. 8B illustrates an implementation of a recording and redistribution system according to a first particular embodiment of the invention;

(12) FIG. 8C illustrates an implementation of a recording and redistribution system according to a second particular embodiment of the invention;

(13) FIG. 9A illustrates a classic implementation of a multichannel audio system in the particular case of a system 5.1;

(14) FIG. 9B illustrates an implementation of a multichannel audio system (in the particular case of a system 5.1) according to a first particular embodiment of the invention; and

(15) FIG. 10 illustrates an implementation of a multichannel audio system according to a second particular embodiment of the invention.

6. DETAILED DESCRIPTION

(16) In the particular embodiment of the invention presented in FIG. 1A, it is assumed that the acoustic set 10 is a three-way acoustic set: one low-mid channel, one high-mid channel, one tweeter channel. The invention however can also be applied with two channels or more than three channels.

(17) The acoustic set comprises: a block 7 for obtaining a directivity control signal 13; an analog/digital conversion stage comprising an analog/digital converter (ADC) 2, which receives a source audio signal 1, should the input signal 1 not be digital; a filtering stage 3, which receives a signal 8 output from the analog/digital converter (ADC) 2, as well as the directivity control signal 13. It generates three signals 9.sub.1, 9.sub.2 and 9.sub.3, corresponding to the three above-mentioned channels. An exemplary embodiment of this filtering step 3 is described here below with reference to FIG. 1B; a digital/analog conversion stage 4 comprising three digital/analog converters (DAC) 41.sub.1, 41.sub.2 and 41.sub.3, each receiving one of the signals 9.sub.1, 9.sub.2 and 9.sub.3 generated by the filtering stage 3; an amplification stage 5 comprising three amplifiers 51.sub.1, 51.sub.2 and 51.sub.3, each receiving one of the signals 11.sub.1, 11.sub.2 and 11.sub.3 generated by the digital/analog conversion stage 4; a transduction stage 6 comprising three transducers 61.sub.1, 61.sub.2 and 61.sub.3, each comprising a membrane and a means of electrodynamic actuation (typically a coil, magnetic circuit and basket assembly). Each transducer receives signals 12.sub.1, 12.sub.2 and 12.sub.3 generated by the amplification stage 5.

(18) In one alternative embodiment, the amplification stage 5 is digital (i.e. it comprises amplifiers capable of receiving and processing the digital signals 9.sub.1, 9.sub.2 and 9.sub.3 generated by the filtering stage 3) and the acoustic set does not comprise the digital/analog conversion stage 4.

(19) The transducers of the low-mid and high-mid channels comprise for example two membranes (one in each transducer) made in the form of a concentric circular rings. The transducer of the tweeter channel comprises for example a dome.

(20) The block 7 for obtaining the directivity control signal 13 comprises for example an adjustment potentiometer enabling the user to easily vary the directivity of the acoustic set. In one variant, the directivity control signal 13 is conveyed at the same time as the source audio signal 1 (for example in the form of “metadata”) and block 7 for obtaining this directivity control signal 13 comprises means for receiving a data stream (containing the source audio signal 1 and the directivity control signal 13) and means for extracting the directivity control signal 13 contained in this data stream.

(21) In the example of an embodiment illustrated in FIG. 1B, the filtering step 3 comprises: an amplitude equalization stage comprising an equalizer filter 31 which receives a signal 8 output from the analog/digital converter (ADC) 2 as well as the directivity control signal 13; a stage for putting the channels into phase (adjusting delays due to axial shifts of the membranes) comprising three delay lines 32.sub.1, 32.sub.2 and 32.sub.3, each receiving the output signal from the equalizing filter 31. Each of the delay lines corresponds to one of the three channels mentioned here above; a cross-over filtration and directivity control stage comprising three filters 33.sub.1, 33.sub.2 and 33.sub.3, each corresponding to one of the above-mentioned three channels. Each of the filters receives the output signals from one of the delay lines 32.sub.1, 32.sub.2 and 32.sub.3, as well as the directivity control signal 13. The filters of the different channels are therefore a function of the bandwidth of use of each transducer but also of the directivity desired by the user.

(22) The equalizing filter 31 corrects the differences in the balance and thus guarantees the tonal balance whatever the directivity chosen. In practice, this equalizing filter 31 can be integrated into the channel cross-over filter 33.sub.1, 33.sub.2 or 33.sub.3 of each of the channels. This reduces the computation load of a DSP implementing the filtering stage 3.

(23) FIG. 2 shows an example of a frequency response in the axis of the coaxial speaker of FIG. 1A for each of the three channels without filtration. In this example: the non-filtered low-mid (LM) channel covers a frequency band of about 80 Hz-8 kHz (frequency response curve referenced 71); the non-filtered high-mid (HM) channel covers a frequency band of about 250 Hz-10 kHz (frequency response curve referenced 72); and the non-filtered tweeter (TW) channel covers a frequency band of about 2 kHz-20 kHz (frequency response curve referenced 73).

(24) The non-filtered low-mid and high-mid channels therefore have a common bandwidth (overlap band) extending from 250 Hz to 8 kHz. In this overlap band, it is therefore possible to combine their contributions in order to modify the filtered directivity of the acoustic set.

(25) By way of an example, FIG. 3 shows the polar response (a directivity pattern) of the low-mid channels (curve referenced 81) and high-mid channels (curve referenced 82) at 1250 Hz, without filtering. As expected, it is noted that the directivity is greater for the high-mid channel comprising the smallest membrane or membranes and narrower for the low-mid channel comprising the biggest membrane or membranes.

(26) If the user wishes to obtain the narrowest directivity (represented by the curve referenced 81) at this frequency of 1250 Hz, the filtering must be such that all the contribution of the acoustic set to this frequency is provided by the low-mid transducer and that the contribution of the high-mid transducer is zero (therefore completely filtered) at this frequency.

(27) Conversely, if the user wishes to obtain the greatest directivity (represented by the arrow referenced 82) at this frequency of 1250 Hz, the filtering must be such that all the contribution of the acoustic set to this frequency is provided by the high-mid transducer and that the contribution of the low-mid transducer is zero (therefore completely filtered) at this frequency.

(28) The table here below summaries the situation:

(29) TABLE-US-00001 Weighting by the Weighting by the Directivity obtained low-mid filter 33.sub.1 high-mid filter 33.sub.2 (after filtering) 1 0 Narrowest curve (referenced 81) 0 1 Widest curve (referenced 82)

(30) Naturally, between these two extreme values of directivity, the weightings can be intermediate between 0 and 1 on each of the channels so as to obtain intermediate patterns of directivity between these two extremes.

(31) Similarly, FIG. 2 shows that the high-mid and tweeter channels have a common bandwidth (overlap band) extending from 2 kHz to 10 kHz. In this overlap band, it is therefore possible to combine their contributions in order to modify the directivity of the filtered acoustic set.

(32) By way of an example, FIG. 5 shows the polar response (pattern of directivity) of the high-mid channels (curve referenced 101) and tweeter channel (curve referenced 102) at 10000 Hz without filtering. As expected, it is noted that the directivity is wider for the tweeter channel comprising the smallest membrane (dome) and narrowest for the high-mid channel comprising the biggest membrane.

(33) If the user wishes to obtain the narrowest directivity (represented by the curve referenced 101) at this frequency of 10000 Hz, the filtering must be such that the entire contribution of the acoustic set at this frequency is provided by the high-mid transducer and that the contribution of the tweeter transducer is zero (hence completely filtered) at this frequency.

(34) Conversely, if the user wishes to obtain the widest directivity (represented by the curve referenced 102) at this frequency of 10000 Hz, the filtering must be such that all the contribution of the acoustic set at this frequency is provided by the tweeter transducer and that the contribution of the high-mid transducer is zero (therefore completely filtered) at this frequency.

(35) The table here below summarizes the situation:

(36) TABLE-US-00002 Weighting by the Weighting by the Directivity obtained low-mid filter 33.sub.1 high-mid filter 33.sub.2 (after filtering) 1 0 Narrowest curve (referenced 101) 0 1 Widest curve (referenced 102)

(37) Naturally, between these two extreme values of directivity, the weighting can be intermediate between 0 and 1 on each of the channels so as to obtain intermediate patterns of directivity between these two extremes.

(38) It will be noted that, in FIG. 2, the 2 kHz-8 kHz band can be covered by all three transducers (i.e. all three three channels). In this overlap band, the polar patterns of the three channels can be combined (in the same way as explained here above for two channels) in order to obtain a more extensive panel of variation of directivity. For example, FIG. 4 shows the polar response (pattern of directivity) of the low-mid channel (curve referenced 91), high-mid channel (curve referenced 92) and tweeter channel (curve referenced 93) at 5000 Hz, without filtering. As expected, it is noted that by decreasing order of width of directivity, we have the directivity of the original channel, that of the high-mid channel and finally that of the low-mid channel.

(39) In short, in the example of FIG. 2, we have several overlap bands, namely: 250 Hz to 2 kHz, for the overlap band between the non-filtered low-mid and high-mid bands; 2 kHz to 8 kHz, for the overlap band between the non-filtered low-mid, high-mid bands and tweeter channels; 8 kHz to 10 kHz for the overlap band between the non-filtered high-mid bands and tweeter channels.

(40) The solution proposed therefore enables the use of this coaxial speaker on an extended band of 250 Hz to 10 kHz for example, as a speaker whose directivity is variable and controlled (as a function of the directivity control signal 13).

(41) Advantageously, it is preferable to choose membrane sizes with different dimensions (provided that a desired bandwidth is arrived at for each of the speakers), so as to be able to obtain patterns of directivity that are themselves very different and to therefore obtain a wider directivity adjusting panel.

(42) In a more elaborate variant, the pattern of radiation of each transducer (each filter channel), i.e. the contribution of each transducer to a given frequency, can be modulated: not only by means of a weighting of the amplitude of the filter of the channel of this transducer at this frequency (weighting done by means of the gain of the filter at the frequency considered) as is the case in the above example, but also through a weighting of the phase of the filter of the channel of this transducer at this same frequency (weighting done by means of the phase of the filter at this same frequency).

(43) Taking a particular example of the complex weighting of amplitude and phase according to the above-mentioned variant, it is possible to choose one of the two following sets of weighting values, each bringing into play negative coefficients −1 (amplitude 1, phase 180°):

(44) TABLE-US-00003 Weighting on the low- Weighting on the high- Directivity obtained low-mid filter 33.sub.1 high-mid filter 33.sub.2 (after filtering) 1 −1 Curve 82a (FIG. 7) −1 1 Curve 82b (FIG. 7)

(45) FIG. 7 gives a view, like FIG. 3, of the polar response (pattern of directivity) of the low-mid channel (curve referenced 81) and high-mid channel (curve referenced 82) at 1250 Hz without filtering. It also shows the polar response (pattern of directivity) 82a obtained with each of the two above-mentioned sets of weighting values. Indeed, the signal phase is inverted in one case relative to the other but the pattern of directivity is the same. It can be noted in this pattern of directivity 82a that the contribution in this axis is cancelled and that the maximum contributions (lobes) are +90° and −90°.

(46) The filter synthesis giving the gain and the phase of the filters of the different channels can implement the optimal algorithm set forth in the article [1] mentioned here above. In other words, constraints are fixed (as a function of cost integrating for example the axial response, the radiation pattern and the index of directivity) and, through the algorithm used, a filtering vector is obtained specifying the amplitude and phase to be applied by each filter in each overlap band to obtain a filter fulfilling the constraints to the utmost efficiency.

(47) The directivity control signal 13 can for example be proportional to the parameter DI.sub.av, i.e. the mean directivity index sought (see equation (14) of the above-mentioned article).

(48) The filtering stage 3 (see FIG. 1A) is implemented for example at least partially in the form of a DSP (or other processor) executing at least one determined filtering program.

(49) More generally, this filtering stage 3 can be obtained equally well: according to a software solution, i.e. on a reprogrammable computing machine (PC computer, processor, DSP, microcontroller, etc) executing a program comprising a sequence of instructions, or according to a hardware solution, i.e. on a dedicated computation machine (for example an FPGA (field programmable gate array) or ASIC (application-specific integrated circuit) comprising a set of logic gates.

(50) Should the invention be implanted on a re-programmable computation machine, the corresponding program (i.e. the sequence of instructions) could be stored on a storage medium (such as for example a floppy disk, a CD ROM or a DVD ROM) which may or may not be detachable, this storage medium being readable by a computer or a processor.

(51) In the example described here above, the coaxial speaker comprises three channels (low-mid, high-mid and tweeter), the transducer of each of the low-mid and high-mid channels comprises a ring-shaped membrane and the transducer of the tweeter channel comprises a dome-shaped membrane.

(52) It is clear that many other embodiments of the invention can be envisaged. It is possible especially to provide for a speaker comprising only two channels or more than three channels.

(53) FIG. 6 is a block diagram of an acoustic set 60 according to a second particular embodiment of the invention. This second embodiment can be distinguished from the first one (described here above with reference to FIGS. 1A and 1B) in that it enables the processing of two input signals 1 and 1a, each as a function of a distinct directivity control signal 13, 13a. More specifically, the acoustic set 60 in this second embodiment comprises a part 61 that is an addition compared with the acoustic set 10 according to the first embodiment.

(54) This additional part 61 comprises: a block 7a for obtaining a directivity control signal 13a. This block 7a is identical to the block 7 of FIG. 1A; an analog/digital conversion stage comprising an analog/digital converter (ADC) 2a which receives a source audio signal 1a, should this input signal 1b be not digital. This stage 2a is identical to the stage 2 of FIG. 1A; a filtering stage 3a that receives the signal 8a output from the analog/digital converter (ADC) 2a, as well as the directivity control signal 13a. It generates three signals 9.sub.1a, 9.sub.2a and 9.sub.3a, corresponding to the three channels mentioned here above.

(55) Furthermore, the acoustic set 10 is slightly modified in that it comprises three summing means, enabling the adding of the signals generated by the filtering stage 3a with those generated by the filtering stage 3a as follows: 9.sub.1+9.sub.1a, 9.sub.2+9.sub.2a and 9.sub.3+9.sub.3a.

(56) The digital/analog conversion stage 4 comprises, as in the first embodiment, three digital/analog converters (DAC) 41.sub.1, 41.sub.2 and 41.sub.3, but in the present second embodiment each of these receives one of the signals generated by the summing means.

(57) Thus, by injecting another input signal 1a associated with another directivity control signal 13b and another filtering stage 3b (another set of filters) it is possible to obtain a different directivity for the two input signals 1 and 1a at the same instant t.

(58) The principle of this second embodiment can be extended to N input signals. In this case, N−1 additional parts 61 will be used.

(59) Here below, referring to FIGS. 8A, 8B and 8C, two examples are presented of application of acoustic sets with variable directivity according to the invention, in the context of a system of recording and redistribution.

(60) FIG. 8A illustrates a classic implementation of a method of recording and redistribution.

(61) In a recording phase, two microphone capsules 81, 82 possessing different levels of directivity are used simultaneously: one of them 81 to pick up the main sound from an instrument or a voice (this microphone capsule 81 possesses an omnidirectional or cardioid directional pattern), and the other 82 to pick up the ambient sound that does not contain the main sound (this microphone capsule 82 has an figure-of-8 shaped directivity pattern). The signals (referenced Mic1 and Mic2 in FIG. 8A) coming from the two microphone capsules 81, 82 are mixed and added up by means of a mixing panel 83 which gives only one signal S that can be registered in a storage/reading means 84.

(62) In a reproduction phase, the signal S preliminarily recorded is read in the storage/reading means 84 and presented (after amplification by an amplifier 85) to a classic acoustic set 86 (i.e. an acoustic set with fixed directivity, that is permanently fixed at the time of its construction). It is therefore not possible to control the way in which the classic acoustic set 86 radiates the acoustic wave in the reproduction room.

(63) FIG. 8B illustrates an implementation of a system for recording and redistribution according to a first particular embodiment of the invention.

(64) In a recording phase, as in the classic technique of FIG. 8A, the two microphone capsules 81, 82 possessing different levels of directivity, are used. However, contrary to the classic technique, the mixing panel 83 does not mix and does not add up the signals Mic1 and Mic2 coming from the two microphone capsules 81, 82. The mixing panel 83 is used in the present case only to modify each of the signals Mic1 and Mic2 (for example adjustment in level and in frequency content, with possible effects: reverberations, delay, etc), the resulting signals being referenced S1 and S2 respectively. The mixing panel 83 therefore gives two signals S1, S2 (and not a single signal S) capable of being recorded in the storage/reading means 84.

(65) In a reproduction phase, the pre-recorded signals S1 and S2 are read in the storage/reading means 84 and presented (after amplification of each by a distinct amplifier 85.sub.1, 85.sub.2) to the two inputs of a variable-directivity acoustic set 60 according to the invention (for example the one described further above with reference to FIG. 6). The directivity associated with the first input is adjusted with a first directivity control signal 13 given by the block 7. Similarly, the directivity associated with the second input is adjusted with a second directivity control signal 13a given by the block 7b. As described further above, each block 7, 7a comprises for example an adjusting potentiometer enabling the user to easily vary the directivity of the acoustic set for the signal received at the concerned input (first and second input). In one variant, the directivity control signal 13, 13a is determined during the mixing and is stored and conveyed at the same time as the source audio signal 1, 1a (for example in the form of “metadata”) and the block 7, 7a comprises appropriate extraction means. Thus, the acoustic set 60, simultaneously and with different and adapted directivities (omnidirectional/cardioid directivity referenced 87 and figure-of-eight shaped directivity referenced 88), renders each of the two input signals 1 and 1a. Each of these two directivities corresponds to the utmost to the directivity of the microphone 81 or 82 used during sound pick-up operations. Through the invention, the characteristics of diffusion of the sound wave in the listening room are therefore complied with.

(66) FIG. 8C illustrates an implementation of a system of recording and redistribution according to a second particular embodiment of the invention

(67) In a recording phase, a monophone signal Mic1 is picked up by a microphone 91 (for example omnidirectional) or is synthesized by a sound generator (synthesizer, sound track, sampler, etc), and then is mixed with a mixing panel 92. This mixing consists for example in creating effects of broadcasting of the sound wave, that are variable depending on the results sought in the listening room. This can be done through a directivity control button 93 which is present for example on the mixing panel 92. In this case, the mixing panel 92 gives two signals to a storage/reading means 94: the signal Mic1 coming from the microphone and a directivity control signal 95. In the example illustrated, an example of a signal Mic1 is presented during two time slots t1 and t2 along with an example of a directivity control signal 95 associated by the same time slots t1 and t2.

(68) In a reproduction phase, the pre-recorded signal Mic1 is read in the storage/reading means 84 and is presented (after amplification by an amplifier 96) to the input of an acoustic set 97 with variable directivity according to the invention (for example identical to the acoustic set 10 described further above with reference to FIG. 1A). This acoustic set 97 also receives the directivity control signal 95 (for example identical to the signal referenced 13 in FIG. 1A) which gives the result sought (rendering of the diffusion effects) in the listening room, by the real-time modification of the acoustic set (dynamically). For example, a directivity referenced 99 is obtained during a first time slot (corresponding to t1 during the recording phase) and then the directivity referenced 98 is obtained in a second time slot (corresponding to t1 during the recording phase). The directivity referenced 98 corresponds to a pattern of directivity illustrated in FIG. 7 and obtained with weighting coefficients +1 and −1.

(69) Here below, with reference to FIGS. 9A, 9B and 10, two examples of application of variable directivity acoustic sets according to the invention are presented in the context of a multichannel audio system.

(70) FIG. 9A illustrates a classic mode of implementation of a multichannel audio system in the particular case of a system 5.1 comprising a left acoustic set

(71) (L), a center acoustic set (C), a right acoustic set (R), a left surround acoustic set (LS), a right surround acoustic set (RS), and a subwoofer (S) acoustic set.

(72) Usually, for the front acoustic sets (right R, left L and center C), it is desired to have diffusion that is more directional. For the front acoustic sets R, L and C, therefore, identical acoustic sets of a first type are used. This is illustrated in FIG. 9A by the pattern of directivity (frontally directive pattern) represented near each of the acoustic sets R, L and C and by the “type 1” label attached to each of these acoustic set R, L and C.

(73) By contrast, for the rear (left LS and right RS) it is sought rather to obtain an enveloping effect. Unfortunately, if the same type of acoustic set is used for these rear acoustic sets LS and RS as for the front acoustic sets R, L and C, especially for aesthetic reasons, this enveloping effect cannot be obtained. Indeed, all the acoustic sets then necessarily have the same directivity. This is illustrated in FIG. 9A by the directivity pattern represented near each of the rear acoustic sets LS and RS and by the “type 1” label attached to each of the rear acoustic sets LS and RS.

(74) A known alternative solution consists in the use, for the rear acoustic sets LS and RS, of dipolar acoustic sets 100 which necessarily have different aesthetic features, and are more bulky because they require at least twice the number of speakers per acoustic set. This alternative solution is illustrated at the bottom of FIG. 9A by the eight-shaped directivity pattern (enabling the creation of an enveloping effect) represented close to a dipolar acoustic set (that can be used for each of the rear acoustic sets LS and RS) and by the “type 2” label attached to this dipolar acoustic set 100.

(75) FIG. 9B illustrates an implementation of a multichannel audio system (in the particular case of the system 5.1) according to a first particular embodiment of the invention.

(76) The front acoustic sets R, L and C and the rear 6 acoustic sets LS and RS are identical: they are all acoustic sets with one input and with variable directivity according to the invention (for example identical to the acoustic set 10 described further above with reference to FIG. 1A). This is illustrated in FIG. 9B by the “type 3” label (meaning “type according to the invention”) attached to each of these five acoustic sets R, L, C, LS and RS.

(77) Through the invention, the rear acoustic sets LS and RS are identical to the front acoustic sets R, L and C but however have radically different directivities because they receive different directivity control signals. This is illustrated in FIG. 9B by the “frontally directive” type pattern represented close to each of the rear acoustic sets LS and RS and by the “laterally directional” type pattern (enabling the creation of an enveloping effect) represented close to each of the rear acoustic sets LS and RS.

(78) Optionally, the directivity of each acoustic set is optimized relative to the place of listening and the effect sought. This directivity is coupled for example to a room compensation algorithm considering the directivity to be an input that is variable (through the concept of the present invention) and no longer unchanging (as described in the patent document FR2965685).

(79) The directivity of each acoustic set is adjusted for example through a potentiometer (at the height of the acoustic set) or else by means of a directivity control signal sent by the multichannel decoder, given the channel addressed to this acoustic set (right channel, left channel, center channel, left surround channel or right surround channel).

(80) FIG. 10 illustrates an implementation of a multichannel audio system according to a second particular embodiment of the invention.

(81) This multichannel audio system comprises a decoder 5.1 to which front acoustic sets R, L and C are connected (and a “woofer” acoustic set S, not shown), but no rear acoustic sets LS and RS.

(82) Each of the acoustic sets, namely the left front acoustic set L and right front acoustic set R is a two-input acoustic set with variable directivity according to the invention (for example identical to the acoustic set 60 described further above with reference to FIG. 6). More specifically, the rear left acoustic set LS receives, at an “input 1”, the “left surround channel” signal at an “input 2” the “left channel” signal. Similarly, the right rear acoustic set RS receives, at an “input 1”, the “right surround channel” signal and at an “input 2” the “right channel” signal. At each input (“input 1” or “input 2”) of each rear acoustic set LS or RS there is a distinct directivity control signal associated: the signal at the “input 2” (i.e. the “right channel” signal or the “left channel” signal) is associated with a directivity control signal making it possible to obtain a directivity pattern of the “frontally directivity” type (referenced 101 in FIG. 10); and the signal at the “input 2” (i.e. the “right surround channel” signal or the “left surround channel” signal) is associated with a directivity control signal that can be used to obtain a directivity pattern of the “laterally directive” type (referenced 102 in FIG. 10), enabling the diffusion of the laterally acoustic energy in order to create an enveloping effect.

(83) The front center acoustic set 6 is an acoustic set with one input and variable directivity according to the invention (for example identical to the acoustic set 10 described further above with reference to FIG. 1A). More specifically, it inputs the “center channel” signal with which there is associated a directivity control signal used to obtain a directivity pattern of the “frontally directivity” type (referenced 101 in FIG. 10).

(84) It is clear that the applications presented here above with systems comprising one or more acoustic sets to one or more inputs and variable directivity according to the invention, are given by way of an illustration and are not exhaustive.

(85) An embodiment provides a technique enabling the control and variation of the directivity of a coaxial multiway acoustic set.

(86) An embodiment provides a technique of this kind that is simple to implement and costs little.

(87) An embodiment provides a technique of this kind requiring, on the part of the user, either a very simple action or no action at all.

(88) Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.