EP3868127A1

EP3868127A1 - Loudspeaker enclosure and modulation method for a loudspeaker enclosure

Info

Publication number: EP3868127A1
Application number: EP19808867.6A
Authority: EP
Inventors: Thibault NOWAKOWSKI; Ilan KADDOUCH
Original assignee: Akoustic Arts
Current assignee: Akoustic Arts
Priority date: 2018-10-17
Filing date: 2019-10-17
Publication date: 2021-08-25
Also published as: FR3087608A1; FR3087608B1; WO2020079377A1; US11490195B2; US20210377656A1

Abstract

The invention relates to a loudspeaker enclosure (10) comprising: - at least two sources (11) suitable for producing ultrasound signals (SU), and - supply means (12) designed to process and amplify at least one input signal (SE) so as to produce, for said sources (11), supply signals (SAn) of the same frequency and of different phases, characterised in that the supply means are configured to apply different gains and/or phase shifts to at least two different frequency components of at least one of the supply signals. The invention also relates to a method for signal modulation for such an ultrasonic loudspeaker enclosure.

Description

Title of the invention: Acoustic enclosure and modulation method for an acoustic enclosure

The present invention relates generally to the field of directional loudspeakers, and in particular those which use the properties of acoustic non-linearities of air to recreate audible sound from ultrasound.

It relates in particular to an acoustic enclosure comprising at least two sources capable of producing ultrasonic signals, and supply means suitable for processing and amplifying an input signal so as to produce for said sources supply signals of different amplitudes and phases.

The invention finds a particularly advantageous application when a sound is to be broadcast to a single listener or a reduced number of listeners, in a limited volume, away from the speakers.

The acoustic non-linearity properties of the air make it possible to recreate audible sound from ultrasound only. Indeed, when two ultrasonic waves, emitted at a high sound level (typically above 100 dB), propagate in the air, they interact by converting part of their energy to form two new waves whose frequencies are, on the one hand, the difference between the two ultrasonic frequencies and, on the other hand, the sum between the two ultrasonic frequencies. If the "sum" wave is located in the ultrasonic range and is therefore inaudible, the "difference" wave is located in the audible range as soon as the frequency difference between the two ultrasonic waves is less than 20 kHz. This non-linear phenomenon, occurring in the air, is called "self demodulation". This acoustic effect occurs at each point of the ultrasonic beam emitted by the loudspeaker as long as the residual energy of the ultrasonic waves is high enough to generate it. All the audible waves demodulated at a point in the beam propagate along the latter and interact in a constructive manner with the audible waves demodulated at the next point (their amplitudes add up, we speak of virtual antenna). As a result, a new beam, called a secondary beam, appears. Its directivity is similar to that of the ultrasonic beam, since the demodulated audible waves interact little outside the ultrasonic beam.

In order to set up an ultra-directional transmission system for broadcasting audio signals, the first step to be carried out is to translate the audio signals, between 20 Hz and 20 kHz, in the ultrasonic field. For this, amplitude modulation, a method from the telecommunications field, is used. An ultrasonic carrier (or ultrasonic carrier signal) is modulated by the input audio signal. The result is a modulated signal whose bandwidth, of around 20 kHz, lies exclusively in the ultrasonic field. This modulated signal is transmitted to piezoelectric transducers which enter into vibration and emit in the air the corresponding ultrasonic waves according to a diffusion cone.

We know several theoretical and experimental studies through the document "

Parametric audible sounds by phase cancellation excitation of primary waves ”, by T. Kamakura, S. Sakai, H. Nomura and M. Akiyama, presented at the Acoustics'08 Paris conference in 2008. These laboratory studies have proposed methods in order to considerably reduce the level of the ultrasonic carrier in the center of the ultrasonic beam. The simplest method is to consider a circular emission surface. By adding a second independent emitting surface in the form of a ring around the circular surface, so that the two surfaces have the same area, it is possible to modulate the signals to be transmitted with two carriers of the same frequency but of different phases. on each surface. Thus, by properly choosing the phase of the carrier emitted on the annular surface, the two carriers interact destructively at a certain propagation distance and therefore cancel each other out. The dimensioning of the transmitting antenna then makes it possible to vary the distance at which the beams emitted by each surface cancel each other out. Studies have shown that demodulated audible levels are only slightly affected by the cancellation of the carrier.

This method has drawbacks. On the one hand, the reduction in amplitude of the carrier is only valid at the center of the total ultrasonic beam, which is uncomfortable and impractical for the listener. On the other hand, the secondary lobes of the beam are increased, which can modify the directivity of the enclosure and cause exposure of the listener to unwanted ultrasound levels.

The invention aims to remedy these drawbacks so as to guarantee the quality and level of the sound signal perceived by the listener by reducing the power levels of the ultrasonic waves emitted.

In order to achieve this objective, the invention relates to an acoustic enclosure of which at least one of said supply signals has a level of amplification different from the other supply signals. Thus, the invention relates to an acoustic enclosure comprising:

- at least two sources capable of producing ultrasonic signals, and

supply means suitable for processing and amplifying at least one input signal so as to produce for said sources supply signals of the same frequency and of different phases,

characterized in that the supply means are configured to apply separate gains and / or phase shifts to two separate frequency components of at least one of the supply signals.

Thus thanks to the invention, in particular a better control over the beam is obtained.

ultrasound, and in particular on the sizes of the main lobes of the various frequency components of the ultrasonic beam, and therefore on those of the demodulated beam, that is to say of the audible beam, as well as a reduction in the level of the ultrasonic carrier on a wider area than those obtained so far, thus widening the listening area.

Advantageously, applying a separate gain to different frequency components of the signal makes it easier to enlarge the size of the main lobe, while reducing the secondary lobes, while applying a separate phase shift to different frequency components makes it possible to shrink more easily. the size of the main lobe.

A combination of gains and distinct phase shifts makes it possible to obtain more easily and more effectively a compromise between the size of the main lobe and that of the secondary lobes.

Advantageously, this improved control prevents the emergence of side lobes of too high levels. Another notable advantage is the control of health effects due to prolonged exposure to ultrasonic waves. Indeed, although no standard has been established in France for prolonged exposure to ultrasonic waves, different organizations and countries offer tables of exposure levels according to frequency bands and listening times, the invention makes it possible to respect these tables.

The gains and the phase shifts can be apodization functions dependent on the frequency of the frequency component.

The gains and the phase shifts can be apodization functions dependent on the position of the ultrasonic source. Apodization functions are particularly advantageous because they allow an effective reduction of the secondary lobes.

According to a characteristic of the invention, the ultrasonic sources are concentric and extend annularly around a central source. This characteristic makes it possible, when implementing a differentiated power supply according to the invention, to obtain better performance than the speakers according to the prior art. By efficiency is understood the relationship between the average level of the audible signal resulting from the constructive demodulation of the ultrasonic waves and the average level of the ultrasonic waves.

According to one embodiment of the invention, the supply means are configured for

- receive a first channel signal and a second channel signal,

- summing the frequency components located in a lower frequency band of the two channel signals so as to form a low frequency signal,

- summing the frequency components located in a higher frequency band of the first, respectively second, channel signal with the low frequency signal so as to form a first, respectively second, input signal,

- generating a first supply signal for a first source from the first input signal, a second supply signal for a second source from the second input signal and a third supply signal for the central source from the low frequency signal.

It is thus possible to generate, by the same enclosure, two audible signals forming for example a stereophonic audio signal. Here, only the higher frequency components of the channel signals are transmitted stereophonically, the lower frequency components being combined and transmitted by all sources. This advantageously makes it possible to optimize the sound power; indeed, although the audible level, that is to say the sound power of the signal, is proportional to the surface of emission, it is also proportional to the square of its frequency. Thus, for the highest frequencies, the attenuation generated by the distribution of the high frequency components of each of the channel signals over only some of the ultrasonic sources is negligible.

Advantageously, the first source and the second source can together form the same ring extending around the central source.

Thus, the same ring of the audio speaker is used to generate the stereophonic signal. The enclosure thus remains compact. According to another characteristic of the invention, said at least two sources are sets of at least two piezoelectric transducers adjacent in pairs to define a substantially continuous surface. The transducers are juxtaposed and, since they are generally cylindrical in shape, they cannot cover a completely continuous surface, they form a substantially continuous surface. The use of piezoelectric transducers to emit ultrasonic signals is simple and inexpensive, and known for this type of enclosure. The choice of piezoelectric transducer model can be made according to the size of the enclosure, as well as the desired listening area.

According to a characteristic of the invention, the supply means comprise a signal processor adapted to generate, from an electrical audio input signal, supply signals resulting from the modulation of a carrier of a frequency substantially greater than 20 kHz by said input signal. The carrier, whose characteristics are its frequency fp, its phase 0h and its amplification level 0n, corresponds to the modulation of the input signal to the nth source. A signal processor can include different modulation elements and tools, known to those skilled in the art and easy to install.

According to another characteristic of the invention, the supply means are adapted to generate a differentiated supply signal for each of the ultrasonic transducers of the enclosure and constituting the ultrasonic sources.

According to another characteristic of the invention, the enclosure comprises means for locating in real time a listener, and the supply signals are processed and amplified as a function of the position of said listener. Thus it is possible to define a listening area which changes according to the position of the listener. These localization means may include a position sensor of the infrared camera type which detects the position of the listener in real time. The listener has freedom of movement, while keeping the same sound level. In addition, this adaptability makes it possible to guarantee that the listener can never be exposed for a long period to excessively high ultrasonic levels in the area close to the enclosure. Another advantage is that if a third party is placed between the listener and the speaker, the sound level will adapt to this third party and therefore the latter will not be exposed to too high levels of ultrasound either. According to another characteristic of the invention, the supply signal results from an amplitude or frequency or pulse width modulation of a carrier by the input signal. These modulation methods are known to those skilled in the art.

The invention also relates to a modulation method for an acoustic enclosure comprising the steps:

- a step of defining at least two sources of the emitting surface of the enclosure;

a step of supplying each of the sources with a supply signal resulting from the modulation of a carrier of the same frequency by an input signal, for each of said sources, said carrier having a different level of amplification and a different phase for at least one source

a step of applying separate gains and / or phase shifts to at least two frequency components of at least one of the supply signals

The gains and phase shifts can be apodization functions dependent on the frequency of the frequency component.

The gains and the phase shifts can be apodization functions dependent on the position of the ultrasonic source.

The method may include a step of applying separate phase shifts to at least two frequency components of at least one supply signal.

According to one embodiment, the enclosure comprises a first source and a

second semi-annular source together forming a single ring extending around a central source, the method comprising

a step of receiving a first channel signal and a second channel signal,

a step of summing the frequency components located in a lower frequency band of the two channel signals so as to form a low frequency signal,

a first, respectively second, summation of the frequency components situated in a higher frequency band of the first, respectively second, channel signal with the low frequency signal so as to form a first,

respectively second, input signal,

a step of generating a first supply signal for the first source from the first input signal, a second supply signal for the second source from the second input signal, and a third power supply signal for the central source from the low frequency signal. According to a characteristic of the method according to the invention, the sources are assemblies composed of at least two adjacent piezoelectric transducers in pairs to define a substantially continuous surface.

According to another characteristic of the invention, the adjustment of the amplification level and of the carrier phases of the sources is such that the ultrasonic level of the carrier is reduced by destructive interference, on at least the listening area.

Of course, the different variants and embodiments of the invention can be combined with each other in various combinations as long as they are not incompatible or mutually exclusive of each other.

Similarly, the different characteristics, variants and forms of implementation of the method according to the invention can be associated with each other in various combinations as long as they are not incompatible or mutually exclusive of each other.

In addition, various other characteristics of the invention emerge from the appended description made with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[Fig. 1] represents a block diagram of an acoustic enclosure according to the invention comprising n ultrasonic sources;

[Fig.2] a schematic front view of an enclosure according to the invention comprising eight concentric ultrasonic sources each comprising a plurality of ultrasonic transducers;

[Fig.3] shows a schematic view of the cone of acoustic perception of the ultrasonic enclosure illustrated in Fig. 2 when powered;

[Fig. 4] represents, in axial section, the intensity of the ultrasonic field emitted by the enclosure illustrated in FIG. 2 when all the ultrasonic sources of the enclosure are all supplied with the same signal resulting from the modulation of the carrier by the acoustic signal.

[Fig. 5] represents a table of normalized phase values and amplification levels of the supply signals from ultrasonic sources of the enclosure according to FIG. 2 in the context of the implementation of the method according to the invention; [Fig. 6] represents, in axial section, the intensity of the ultrasonic field emitted by the enclosure illustrated in FIG. 2 when all the ultrasonic sources of the enclosure are all supplied by the supply signals according to the table in Fig. 5.

[Fig. 7] shows a schematic front view of a variant of the enclosure according to the invention comprising eight concentric ultrasonic sources each comprising a plurality of ultrasonic transducers.

[Fig. 8] shows a block diagram of an acoustic enclosure according to an embodiment of the invention, comprising n ultrasonic sources;

[Fig. 9] represents certain components of an audible beam from a directional speaker.

[Fig. 10] represents certain components of an audible beam of an enclosure according to an embodiment of the invention.

[Fig. 11] is a schematic front view of a stereophonic acoustic enclosure according to an embodiment of the invention comprising seven concentric annular ultrasonic sources and two semi-annular sources, each comprising a plurality of ultrasonic transducers;

[Fig. 12] shows a block diagram of a stereophonic acoustic enclosure according to an embodiment of the invention and comprising n ultrasonic sources.

It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references.

A unidirectional enclosure 10 according to the invention as shown schematically in FIG. 1 comprises a series of ultrasonic sources SU1 to Sun designated as a whole by the reference 11. The sources 11 are supplied by supply means 12 ensuring the processing of an input audio signal SE.

The supply means 12 are adapted to generate from the input audio signal

SE as many supply signals SA1 to SAn as the enclosure comprises ultrasonic sources SU1 to SUn. The supply means can in particular be constituted by a dedicated signal processing processor but also by any other suitable signal processing system resulting from the assembly of discrete and / or integrated electronic components. The input audio signal SE is generated from an audio source, such as a telephone, a computer, a hi-fi system, connected to the speaker, for example by an audio cable. It is also possible to envisage a Bluetooth or WI-FI box to recover this audio input SE signal. The input audio signal can also come from a suitable system integrated into the enclosure 10 comprising means for reading a removable or non-removable memory and means for generating an input audio signal corresponding substantially to the signal capable to power a sound transducer such as for example a headset.

The supply means 12 are then adapted to modulate ultrasonic P carriers from the input audio signal SE to generate the supply signals SAn. The carriers P all have the same relatively high frequency fp in the ultrasonic field, greater than 20 kHz and are generated from a reference carrier. However, the supply means 12 are adapted to apply, according to the needs of the method according to the invention, different amplification levels and phases relative to the reference carrier to the supply signals SA1 to SAn supplying respectively the ultrasonic sources SU1 to SUn. Thus, the supply signal SAn supplying the source SUn results from the modulation of the reference carrier by the input signal SE with an amplification or gain associated ïn and an associated phase shift cpn. There will therefore be n levels of amplification T and n phases cp.

According to an essential characteristic of the invention, on the one hand, at least one amplification level associated with an ultrasonic source has a value different from that of at least one amplification level associated with another ultrasonic source and, on the other hand, at least one phase shift associated with an ultrasonic source has a value different from that of at least one phase shift associated with another ultrasonic source. In other words, not all amplification levels have the same value and not all phase shifts have the same value.

In order to implement this method of controlling or modulating the signals for a directional acoustic enclosure, the invention proposes, to make and arrange the ultrasonic sources as illustrated in FIG. 2.

Thus according to this embodiment of the invention, the enclosure 10 comprises two hundred piezoelectric transducers 15 which are here of cylindrical shape of the same diameter, distributed over a substantially regular hexagonal surface inscribed in a circle C. The transducers of cylindrical shape are arranged so as to best pave the surface hexagonal knowing that no transducer is centered on the center of the hexagonal surface.

According to the example illustrated, the two hundred transducers are divided into eight groups each forming an ultrasonic source. The eight sources 11 are arranged so as to be concentric and therefore all centered on the center of the hexagonal surface and of the circle C. Thus, a first source occupies a central region of the hexagonal surface and comprises four transducers inscribed in a hexagon. The other seven sources consist of concentric rings of substantially hexagonal shape, each ring being paved by transducers and having a width substantially equivalent to the diameter of an ultrasonic transducer.

The sources 11 are controlled the supply means 12 which supply for each source 11 a supply signal SA1 to SA8 respectively.

When the ultrasonic sources are supplied by the supply means 12, acoustic waves are emitted into the air with a perception cone shown diagrammatically in FIG. 3. This cone corresponding to the region of space in which the ultrasonic waves have a sufficient intensity to generate by self-demodulation an acoustic signal perceptible by the human ear, in other words, audible. This perception cone has an angle at the top less than 50 ° and whose axis AA passes through the center of the enclosure, this is the reason why we speak of a directional enclosure. There is a listening area in the perception cone, hatched in FIG. 3, in which a listener is capable of perfectly hearing the acoustic signal resulting from the self-demodulation of the ultrasonic waves. The distance from the listening area to the speaker results from the characteristics of the modulation of the carrier P by the input audio signal SE.

When all the ultrasonic sources and therefore all the transducers are supplied with a single signal resulting from the modulation of the carrier P by the input signal SE as proposed by the prior art, that is to say when all the sources are supplied with the same level of amplification and the same phase shift relative to the carrier, a section of the ultrasonic field generated by the enclosure is illustrated in FIG. 4. This field corresponds to that necessary to allow listening of satisfactory quality in the listening area. However, it appears that the darkest area in which the intensity is the greatest is relatively large so that a subject in the perception cone is subjected to a relatively large ultrasonic intensity which may not comply with the recommendations. in terms of exposure level. The invention overcomes this drawback by ensuring a differentiated supply of ultrasonic sources. Thus, in accordance with the modulation method according to the invention, the supply means are adapted to apply a different phase shift and amplification to each of the supply signals SA1 to SA8 including the phase and the amplification level of the ultrasonic carrier. P are different (the carrier frequency being identical for each source 11).

According to the example illustrated, the amplification levels of the carrier P for the various ultrasonic sources SU1 to SU8, as defined above, change in normalized values between OdB and -60 dB. The carrier phases for these different sets of transducers 15 vary between - p rad and p rad. Fig. 5 is a table showing the characteristics of the power supply signals of each of the ultrasonic sources in phase shift and in amplification with respect to the carrier.

When supplying sources SU1 to SU8 constituting the enclosure according to the invention with such signals, this results in the emission of an ultrasonic field whose intensity seen in axial section of the perception cone corresponds to Fig. 6. It appears that the regions of high intensity are less extensive than in the case of the uniform supply of the ultrasonic sources even though the quality of perception of the sound signal by a listener is substantially identical.

The invention therefore reduces the levels of exposure to ultrasonic waves while maintaining listening comfort identical to that provided by the prior art. In this regard, comparative measurements were made in order to evaluate the ultrasonic and audible (demodulated) levels, at a distance of 1.5 m from the emitting surface of the enclosure as illustrated in FIG. 2 in three distinct modes of implementation.

In a first mode of implementation, mode 1, all the transducers of the enclosure are supplied with an identical signal so that the enclosure behaves like a single ultrasonic source.

In a second embodiment, mode 2, the enclosure is implemented in accordance with the method according to the invention so as to differentiate there from eight ultrasonic sources, a central and seven concentric annulars supplied by supply signals presenting a phase shift and a differentiated gain. In this second mode of implementation, the respective phase shifts and gains were selected so as to obtain an attenuation of -10dB, relative to the first mode, of the ultrasonic level measured at 1.5m from the emission face of the enclosure defined by the substantially coplanar emission faces of the transducers.

In a third embodiment, mode 3, the enclosure is supplied as in the case of mode 1 so as to form a single ultrasonic source but amplification or gain of the single amplification signal is chosen to so as to obtain an attenuation of −10 dB, compared to the first mode, of the ultrasonic level 1.5 m from the emission face of the enclosure, ie the same level of attenuation as in the second mode.

For the three modes of implementation, measurements were carried out at several measurement points distributed in a circle 20 cm in diameter facing the enclosure. The table below indicates the average of the measurements with for reference in the case of the ultrasonic level the first mode and in the case of demodulated sound (audible) the average ultrasonic level at the level of the speaker in the considered mode.

[Table 1]

It appears that in mode 2, corresponding to the implementation of the method according to the invention, it is obtained, with the same ultrasonic level, an audible signal of a level higher than that obtained in mode 3, c that is to say with a single-source ultrasonic enclosure. The invention therefore makes it possible to reduce the level of ultrasonic exposure by reducing the impact of this reduction on the level of the demodulated and therefore audible signal. The invention therefore makes it possible to increase the efficiency of the speaker, namely the ratio between the ultrasonic power emitted and the power of the audio signal audible by a user in the listening area.

According to an alternative embodiment of the invention more particularly illustrated in FIG. 7, the transducers constituting the ultrasonic sources are arranged in the hexagon corresponding to the active surface of the enclosure 10 so that a transducer is perfectly concentric with the center of the hexagon. The first ultrasonic source SU1, occupying a central position, then comprises seven ultrasonic transducers placed in staggered rows. The seven annular sources SU2 to SU8 then surround the central source SU1 each having a width of a transducer while retaining the staggered arrangement. Such a configuration makes it possible to obtain a greater density of transducers 15. Thus it is possible to place 217 transducers while according to the shape illustrated in FIG. 2 there are 200 transducers arranged in a hexagon of the same surface.

Furthermore in another embodiment of the invention and so as to allow angularly offset the axis of the perception cone relative to the axis AA of the enclosure, the supply means 12 are adapted to apply a time phase shift to the input signal located in the acoustic spectrum before the modulation of the carrier by the input signal thus phase shifted. More precisely, the supply means are adapted to induce a time difference in phase of the differentiated input signal for each of the transducers constituting an ultrasonic source and this for all the ultrasonic sources of the enclosure. This time shift is then determined as a function of the distance from each transducer to a plane passing through a target point located in the listening zone or as a function of the distance of each transducer to the so-called target point. The sound signals corresponding to each of the transducers are then used to modulate the carrier so that as many ultrasonic signals are obtained as the enclosure comprises transducers. In accordance with the method according to the invention, all of the ultrasonic signals of the transducers constituting the same ultrasonic source are applied the phase shift and the gain corresponding to said source and likely to be different from those associated with other ultrasonic sources. Thus the ultrasonic beam is oriented in space while retaining the assurance that the phase and the amplification level of each carrier allow destructive interference reducing the ultrasonic level to the listening position of the user.

In the context of this embodiment, the enclosure comprises or is associated with locating means 13 of the listener. These locating means 13 may comprise a position sensor 14 shown in FIG. 3. The setting of the sources 11 is then adjusted as a function of these data in real time. The position sensor 14 can be a camera or any other device making it possible to know the position of the listener.

According to an alternative embodiment of the invention illustrated in Figure 8, the power supply means 12 are configured to apply a separate gain to different frequency components of the power supply signals SA. For example, the supply means 12 are configured to apply to each supply signal a gain y (f, z) whose value is a function of the frequency f and of the position z of the source SU receiving the signal d 'food. In particular, the value of the gain y (f, z) is here defined by an apodization function. As a variant, the gain could depend only on the frequency f or only on the position z of the source SU.

The supply means 12 are further configured here to apply a separate phase shift to different frequency components of the supply signals. For example, the supply means 12 are configured to apply to each supply signal SA a phase shift cp (f, z) whose value is a function of the frequency f of the supply signal SA and of the position z of the ultrasonic source SU. As a variant, the phase shift could depend solely on the frequency f of the supply signal SA or only on the position z of the ultrasonic source SU.

Alternatively, the supply means could be configured to apply only a separate gain or only a separate phase shift to the different frequency components.

For example, the apodization functions can be chosen from conventional functions, such as the Hamming window, the Hann window, the Nuttall window, the Blackman window, a rectangular window, or even a combination of these functions.

In addition, the apodization functions could be tailor-made, depending on the application envisaged. For example, the definition of the apodization function can be done during an experimental phase comprising a measurement of the directivity of the demodulated frequency components, that is to say of the attenuation of the demodulated frequency components according to their position relative to the enclosure and a corresponding modification of the gain and phase shift values of the ultrasonic frequency components of the power supply signals SA so as to standardize the directivity of the demodulated (audible) frequency components and thus obtain a sharper listening area. As a variant, it is also possible to use, during the experimental phase, a non-linear acoustic model making it possible to predict the directivity of the demodulated frequencies as a function of the directivity of the carrier and of the ultrasonic frequency component (modulated).

Figures 9 and 10 are polar diagrams representing the directivity of four frequency components C1, C2, C3, C4 of the demodulated beam emitted by two speakers and corresponding respectively to the frequencies 500Hz, 1000Hz, 4000Hz, 8000Hz. For the sake of simplification of the figures, only these four components have been represented although the demodulated beam includes others. In FIG. 9, these components are those of a demodulated beam emitted by a directional enclosure in which the supply means do not apply gain or phase shift to the various frequency components of the supply signals.

It clearly appears that the different demodulated frequency components have different shapes. Thus, a listener located at a first position PI facing the enclosure would hear a good quality signal comprising all these frequency components.

However, by placing himself in a second position P2, the listener would perceive only some of these frequency components only, that is to say that he would perceive a degraded sound and therefore annoying.

In FIG. 10, the components C1 to C4 are those of an audible beam emitted by an enclosure according to the embodiment described above in connection with FIG. 8. It clearly appears that the different frequency components C1 to C4 have similar dimensions and geometries.

Thus, in the first position PI, the listener always hears a good quality sound comprising all of the frequency components C1 to C4. At the second position P2, the listener no longer hears the audio signal at all. Thus, since the geometries of the components of the audible beam are similar, the audible beam is better defined and the area of the spaces in which the listener risks perceiving a degraded sound is greatly reduced.

According to an alternative embodiment illustrated in Figure 11, the enclosure 10 is a stereophonic enclosure. Thus, the arrangement of the sources is analogous to that described in connection with FIG. 2 and the enclosure 10 comprises the seven sources SU1 to SU7, and two semi-annular sources in place of the source SU8 described previously.

The enclosure 10 comprises a left semi-annular source SU8g and a right semi-annular source SU8d forming a ring extending around the sources SU1 to SU7, called central sources, and specifically along the seventh source SU7.

FIG. 12 schematically illustrates the stereophonic enclosure according to the invention in which the supply means comprise a block 21 of filters separators by frequency bands.

According to this embodiment, the supply means 12 are configured to receive a first channel signal SCg, or left signal, and a second channel signal SCd, or straight signal, and for summing the frequency components BFg and BFd located in a lower frequency band of the two channel signals SCg and SCd, for example but not limitatively a frequency band between lOOFIz and 4KFIz, so as to form a low signal frequency BF.

The supply means 12 are further configured to perform a first summation of the frequency components HFg located in a higher frequency band of the first channel signal SCg, for example but not limited to a frequency band between 4KHz and 16Khz , with the low frequency signal BF so as to form a first input signal SEg, or left input signal, and to carry out a second summation of the frequency components HFd located in the upper frequency band of the second channel signal SCd, with the low frequency signal BF so as to form a second input signal SEd, or right input signal.

It would be perfectly possible to choose different frequency bands, for example a lower frequency band between 100 Hz and 8 kHz and a higher frequency band between 8 kHz and 16 kHz. The choice of frequency bands may for example depend on the configuration of the enclosure, in particular the number and / or the nature of the transducers, the size of the emitting surface of the enclosure, etc.

The supply means 12 are configured to generate a supply signal SAg for the left semi-annular source SU8g from the first input signal SEg, a second supply signal SAd for the semi-annular source right SU8d from the second input signal SEd and a third power supply signal SEc from the sources SU1 to SU7 from the low frequency signal BF.

Although it has been described here a stereophonic enclosure comprising only two semi-annular sources, it would be possible for the enclosure 10 to include more semi-annular sources, and in particular only semi-annular sources. It is also possible that the stereo speaker has a different number of central sources.

According to the examples described above, the ultrasonic sources are constituted by ultrasonic transducers all of the same model and characteristic. However, the sources 11 can be composed of different models of transducers 15 having different acoustic characteristics such as a different resonant frequency, a different carrier frequency, a different bandwidth or a different frequency response. The settings will be made according to these new parameters.

Of course, various other modifications or variants of the enclosure or of the method according to the invention can be envisaged in the context of the appended claims.

Claims

[Claim 1] Acoustic enclosure (10) comprising:

a first source and at least a second source (11) capable of producing ultrasonic signals (SU), and

supply means (12) suitable for processing and amplifying at least one input signal (SE) so as to produce for said sources (11) supply signals (SAn) of the same frequency and of different phases,

characterized in that the supply means (12) are configured to apply separate gains and / or phase shifts to at least two

separate frequency components of at least one of the signals

feed.

[Claim 2] A loudspeaker according to claim 1, in which the gains and the phase shifts are functions of apodization dependent on the frequency (f) of the frequency component to which they are applied.

[Claim 3] Enclosure according to claim 1 or 2 in which the gains and the phase shifts are functions of apodization dependent on the position (z) of the ultrasonic source to which they are applied.

[Claim 4] The enclosure of claim 1 or 2, wherein the ultrasonic sources are concentric and extend annularly around a central source.

[Claim 5] An enclosure according to claim 4, wherein the supply means (12) are configured to

- receive a first channel signal (SCg) and a second channel signal (SCd),

- summing the frequency components (BFg, BFd) located in a lower frequency band of the two channel signals (SCg, SCd) so as to form a low frequency signal (BF),

- summing the frequency components (HFg, H Fd) located in a higher frequency band of the first, respectively second, channel signal (SCg, SCd) with the low frequency signal (LF) so as to form a first, respectively second, input signal (SEg, SEd),

- generate a first supply signal (SAg) for a first source (SU8g) from the first input signal (SEg), a second supply signal (SAd) for a second source (SU8d) from the second input signal (SEd) and a third power signal (SEc) for the central source from the low frequency signal (LF).

[Claim 6] Enclosure according to claim 4 or 5, in which the first source (SU8g) and the second source (SU8d) are semi-annular and together form a single ring extending around the central source

[Claim 7] Enclosure (10) according to any one of the preceding claims, in which said at least two sources (11) are sets of at least two piezoelectric transducers (15) adjacent in pairs to define a substantially continuous surface .

[Claim 8] Enclosure according to the preceding claim, in which the means

supply are adapted to generate a differentiated supply signal for each of the ultrasonic transducers of the enclosure and constituting the ultrasonic sources.

[Claim 9] Enclosure (10) according to any one of the preceding claims, in which the supply means (12) comprise a signal processor (16) adapted to generate, from an electrical audio input signal (SE), supply signals (SAn) resulting from the modulation of a carrier (P) of a frequency substantially greater than 20 kHz by said input signal (SE).

[Claim 10] A loudspeaker (10) according to any one of the preceding claims, which comprises means for locating (13) in real time a listener, and said supply signals (SAn) are processed and amplified according to the position of said auditor.

[Claim 11] Enclosure (10) according to claim 10, wherein the locating means

(13) include a position sensor (14).

[Claim 12] Enclosure (10) according to any one of the preceding claims, in which the power signal (SA) results in part from an amplitude or frequency or pulse width modulation of a carrier (P) by an input signal (SE).

[Claim 13] Modulation method for an acoustic enclosure (10) comprising the steps:

- a step of defining a first source and at least a second source (11) of the emitting surface of the enclosure (10);

a step of supplying each of the sources with a supply signal resulting from the modulation of a carrier (P) of the same frequency by a signal input (SE), said carrier (P) having a different amplification level and a different phase for at least one source (11)

a step of applying separate gains and / or phase shifts to at least two distinct frequency components of at least one of the supply signals (SAn).

[Claim 14] Method according to claim 13, in which the gains and the phase shifts are functions of apodization dependent on the frequency of the frequency component.

[Claim 15] Method according to claim 13, in which the gains and the phase shifts are functions of apodization dependent on the position of the ultrasonic source.

[Claim 16] Method according to one of claims 13 to 15, wherein the enclosure comprises a first source (SU8g) and a second source (SU8d) semi-annular together forming a same ring extending around a source central, the method comprising,

a step of receiving a first channel signal (SCg) and a second channel signal (SCd),

a step of summing the frequency components (BFg, BFd) situated in a lower frequency band of the two channel signals (SCg, SCd) so as to form a low frequency signal (BF),

- a first, respectively second, summation of the frequency components (HFg, H Fd) located in a higher frequency band of the first, respectively second, channel signal (SCd, SCg) with the low frequency signal (LF) so as to form a first, respectively second, input signal (SEg, SEd),

a step of generating a first supply signal (SAg) for the first source (SU8g) from the first input signal (SEg), a second supply signal (SAd) for the second source (SU8d ) from the second input signal (SEd) and a third supply signal (SEc) for the central source from the low frequency signal (BF).

[Claim 17] Method according to any one of claims 13 to 16, in which said sources (11) are assemblies composed of at least two piezoelectric transducers (15) adjacent in pairs to define a surface

substantially continuous. [Claim 18] Method according to any one of claims 13 to 17, in which the adjustment of the amplification level and of the carrier phases (P) of said sources (11) is such that the ultrasonic level of the carrier is reduced by destructive interference, on at least the listening area.