Method for operating an arrangement of sound transducers according to the wave field synthesis principle

Abstract

A method and a device for operating an arrangement of sound transducers according to the wave-field synthesis principle. In order to supply an extended audience region with the same signal, the same signal content is generated by at least two virtual sound sources, which are arranged such that the wavefronts thereof are directed only onto a part audience area, rather than generating only a single beam extending over the entire audience area. The wavefronts of the distributed virtual sound sources add up vectorially in the plane of the arrangement of sound transducers, whereby the effectiveness of the sound generation is increased.

Claims

1. A method for operating an arrangement of sound transducers according to the wave field synthesis principle to supply an audience area with an audio signal content, the method comprising: providing the arrangement of sound transducers; and operating the arrangement of sound transducers in such a manner that the arrangement of sound transducers radiates wave fronts toward the audience area, the radiated wave fronts corresponding to wave fronts of the same audio signal content that are generated in a model by at least two virtual sound sources, which are arranged behind the arrangement of sound transducers from the point of view of the audience area, and which direct the respective wave fronts of the same audio signal content only at a part of the audience area.

2. The method according to claim 1, comprising: lowering the signal level at the upper end of the frequency range to be transmitted only in a centre of the arrangement of sound transducers based on the wave field synthesis principle in order to enable greater efficiency in sound generation with the remaining frequency range due to the incoherent addition of the individual signals.

3. The method according to claim 1, comprising: locating the at least two virtual sound sources with the same signal content at a same distance from the point in the middle of a part of the audience area in which an overlap of the wave fronts is unavoidable in the model, or offsetting the at least two virtual sound sources with the same signal content temporally from each other to such a degree that their wave fronts reach this point at the same time.

4. The method according to claim 1, comprising: upon actuation of the sound transducers, subtracting the shortest travel time obtained from the calculation of the travel times between each pair of one of the virtual sound sources and one of the individual sound transducers from the calculated travel times.

5. The method according to claim 1, comprising: adjusting the levels of the virtual sound sources that supply the individual audience areas with the same audio signal content separately.

6. The method according to claim 1, comprising: equalizing the levels of the virtual sound sources that supply the individual audience areas with the same audio signal content separately.

7. The method according to claim 1, comprising: mixing individual signal content, which remains limited to the audience area supplied by a primary virtual sound source of at least two virtual sound sources, with the wave fronts of individual virtual sound sources that reproduce the signal content of said sound source from discrete positions.

8. The method according to claim 1, comprising: replacing two or more virtual sound sources, which supply the entire audience area with different signals from various positions in order to generate a spatial representation, by at least two respective virtual sound sources, which are arranged in the model such that their wave fronts are directed with smaller aperture angles at only a part of the audience area.

9. The method according to claim 1, comprising: measuring the temperature and/or wind direction and wind speed in the audience area so that scattering or deflection of the wave fronts can be counteracted by a corresponding adjustment of the parameters for generating the wave fronts in the model.

10. An apparatus for supplying an audience area, wherein the apparatus comprises: an arrangement of sound transducers designed to carry out a method a according to claim 1.

11. The apparatus according to claim 10, wherein a central area of the apparatus consisting of sound transducers is not equipped with sound transducers or equipped with sound transducers designed especially for the reproduction of the bass range, and/or wherein the arrangement of sound transducers is arranged as a frame around a corresponding image reproduction and/or surrounds the image reproduction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates one example of an arrangement of sound transducers.

(2) FIG. 2 illustrates one example of an arrangement of sound transducers from four virtual sound sources.

(3) FIG. 3 illustrates one example of phase relationships between individual signals in the plane of the arrangement of sound transducers.

(4) FIG. 4 illustrates one example of an arrangement of sound transducers based on wave field synthesis.

DETAILED DESCRIPTION

(5) According to the invention, therefore, a solution is to be described in which each individual emitter works more efficiently than an individual emitter of the same type in a conventional arrangement at the top end of the transmission range as well.

(6) Moreover, the advantage to the effect that almost the same sound pressure should be generated in the audience areas distant from the arrangement of sound transducers on the principle of wave field synthesis as in the areas directly in front of the stage should be preserved.

(7) The above objects and other objects which are made evident in the description are achieved by a method according to the features of claim 1. Further advantageous embodiments of the invention are defined in the dependent claims. A preferred embodiment of the present invention is represented in the following drawings and discussed in a detailed description, none of which is intended to be limiting of the present invention.

(8) The associated sound transducer arrangement typically comprises an arrangement of loudspeakers, typically dynamic loudspeakers, which are arranged in a flat surface. However, the use of other transducer principles, such as electrostatic or piezoelectric transducers or Micro Electro Mechanical Systems (MEMS) [1] [2] is also possible. A curvature of the surface or an angled arrangement of planar component surfaces is conceivable, even an irregular arrangement of transducers at defined points in space could produce a defined wave front according to the principle of wave field synthesis. A special case is the construction of the area as a single row of speaker. In this case, the method described is only partially effective.

(9) Various audience areas can also be supplied by a shared arrangement of sound transducers having different signal content or also with adjusted level and equalization values for the same signal content. This makes it possible to create sound pressures in remote audience areas that are almost the same as in areas immediately in front of a major event stage.

(10) According to the invention, in order to supply a wide audience area the device does not emit a single wave front which then spreads out over a wide radiation angle to cover the entire audience area, but instead the audience area is supplied by a plurality of individual virtual sound sources, which are generated by the arrangement of sound transducers according to the principle of wave field synthesis, in a narrow radiation angle. All these virtual sound sources have the signal content of the one virtual sound source, which would otherwise have to supply the entire audience area.

(11) This has the advantage on the one hand that the sound pressure of the individual wave fronts with the small aperture angle hardly decreases as the distance increases. On the other hand, because of the incoherent addition of the individual signals in the plane of the loudspeaker arrangement, the level of each of these virtual sound sources can be much higher than is reflected in their share of the wide radiation angle that is otherwise necessary for one virtual sound source.

(12) With a large number of virtual sound sources having the same signal content, it is unavoidable that the coverage regions of the different areas overlap. To the extent that the starting points of the wave fronts in question are at different distances from the listener, the signals are then subtract and added according to their phase positions relative to each other. Comb filter effects are created in the resulting frequency response. This problem is solved according to the invention in that the individual virtual sound sources are generated with the same signal content at those positions that are equidistant from a point in the middle of the overlap region.

(13) In another embodiment of the inventive solution, the signals of virtual sources with the same signal content are delayed with respect to each other in such a way that their signals arrive at the point in the middle of the overlap region at the same time. This also helps to minimise the comb filter effects in this area. Further, the covered area may be better adapted to the audience area. This is due to the greater freedom when positioning the virtual sound sources.

(14) When the wave fronts are radiated in narrower angles, the requirement that the audience areas distant from the sound transducer arrangement should receive almost the same sound pressure level as those areas immediately in front of said arrangement may be fulfilled by separately adjusting the levels of the individual wave fronts.

(15) With the arrangement of transducers based on the principle of wave field synthesis, it is possible to separate the intended coverage areas both in the azimuthal plane and in the elevation plane. For example, wave fronts with a lowered level may be generated and directed downwards for audience areas near the stage, while the wave fronts above these are radiated with a higher level for the audience areas at the back. Further, a separate equalization of the frequency response, to compensate for the amount of high-frequency roll-off caused by airborne sound reduction for more distant audience areas is possible with the inventive solution.

(16) The further object, according to which each individual transducer should function more efficiently with an arrangement of sound transducers based on the principle of wave field synthesis, even in the upper frequency range of the playback spectrum, compared with the reproduction of a single, widely propagated wave front, is achieved with the inventive solution. For this, the effect described below is applied.

(17) One can imagine the one virtual sound source, which has to supply a large audience area in a wide radiating angle, as an addition of n virtual sound sources at a common point. In principle, these n virtual sound sources might then also be spatially distributed in such manner that they might supply original area again with single wave fronts emitted in a narrow angle. If the level of each virtual sound source then formed the nth part of the level of the original one virtual source, in principle nothing would have changed with regard to the ratios.

(18) However, this inventive solution can now benefit from the physical effect according to which the levels of multiple virtual sound sources with the same signal content in the individual sound transducers are only added linearly if they are in the same phase position. As long as all n starting points of the virtual sound sources are at the same geometric position, all of their signal components are added together linearly as coherent signal components at each point of the sound transducer arrangement.

(19) However, if the same individual signals emanate from different spatial positions, they are incident at each sound transducer with different travel times. Consequently, their fractions are added and subtracted. Unlike the addition of in-phase signals, there is no longer a doubling of the signal level for the addition of two non-phase-correlated signal components, but only a vector addition to the value of the root of 2=1.414, corresponding to a level increase of just +3 dB. This difference from linear addition becomes more pronounced with a large number of virtual sources with the same signal content at different positions. For example, the addition of 256 coherent signal sources yields a level increase of +48 dB, whereas the addition of 256 incoherent sources only yields a level rise of +24 dB. According to the invention, the level of the spatially distributed virtual sound sources with the signal content of the one original virtual sound source can now be raised by the difference between the two values, in the exemplary case, by +24 dB, without overloading the individual sound transducers.

(20) In this way, it becomes possible to provide a sufficiently high sound level in an expansive audience area even with sound transducers of low power when they are used in an arrangement of sound transducers based on the principle of wave field synthesis. Using the method, only the signals from the distributed virtual sound sources in the centre of the transducer arrangement remain in phase, because the requirement that the wave fronts arrive in the overlap area with the same phase position cannot be fulfilled otherwise.

(21) However, this area at the upper end of the transmission range includes only a few transducers near the centre point, the surface only becomes larger with the wavelength of the signal. Here, however, the better adjustment of the sound transducer arrangement ensures increased efficiency.

(22) It may also be necessary to supply the entire audience area from several spatially separate virtual sound sources, so that a spatial impression is created in the entire audience area. Thus for example, virtual sound sources can also be generated behind the arrangement of sound transducers based on the principle of wave field synthesis that functions as an acoustic curtain and radiate the signal that is otherwise supplied to the stereo speakers. In order to utilise the advantages of the method described, the respective signal content thereof may also be radiated according to the method described from any two or more virtual sound sources at different positions.

(23) The method is illustrated in FIGS. 1 to 4. It will be explained with reference to these drawings.

(24) FIG. 1 shows the radiation from an arrangement of sound transducers based on the principle of wave field synthesis (1) in which the virtual sound source (2) would supply the entire audience area (3). The consequence of this would be that the sound pressure would decline rapidly as distance increased from the arrangement of sound transducers (1) because the energy of the wave front is distributed over surface that is growing rapidly with increasing distance.

(25) The problem is solved in that the signal is distributed from a plurality of virtual sound sources (5), (6), (7) with the same signal content, instead of one single virtual sound source (2).

(26) This distribution of the same signal to multiple starting points is made possible according to the invention by the fact that all virtual sound sources generate their wave fronts from such positions, from which they located at an equal distance from the centre of the respective, unavoidable overlap region (9), (10) and (11) in the audience. For this purpose, the overlapping virtual sound sources are positioned on a common radius around the centre of the overlap region. In another arrangement of multiple virtual sound sources with the same signal content, clearly audible comb filtering effects would be the unavoidable result in the overlap region due to the superposition of identical signals having different travel times.

(27) Because of the narrow aperture angle of radiation, the surface of the wave fronts emanating from the virtual sound sources (5), (6), (7) and (8) rise significantly more slowly in front of the arrangement of sound transducers (1) as the distance from (1) increases, than the surface of a wave front that would emanate from individual virtual sound source (2). The level thereof falls correspondingly more slowly as distance increases. Moreover, level and equalization can now be controlled separately for each sub-region.

(28) In FIG. 2, the audience area (3) is again supplied by the arrangement of sound transducers (3) from the four virtual sound sources (5), (6), (7) and (8). In the illustration, however, the subareas for the supply have been given different sizes. Because of the different aperture angles of the wave fronts emanating from virtual sound sources (5) and (6), these starting points can no longer be arranged on a common radius around the centre of their overlap region (9).

(29) In practice, however, the requirement for different aperture angles does exist. On the one hand, the radiation can be adjusted better to the prevailing conditions. And on the other hand, better use can also be made of the available sound output. Distant audience areas are supplied in a very narrow angle, while for the nearby areas the sound output is also sufficient if it is distributed in a wide radiation angle.

(30) Thus, the signals have to be shifted closer to each other temporally so that the wave fronts of adjacent virtual sound sources still arrive in their overlapping region at the same time.

(31) In the example, the signal from the virtual sound source (6) has to be delayed by the time it takes for the sound to travel over path (dt). In this context, the speed of sound corresponding to the current outdoor temperature has to be used to calculate the travel time, so that travel times in the virtual and real parts of the radiation match. The current temperature in the audience area has therefore to be measured and the speed of sound calculated therefrom has to be updated regularly for all calculations. A measurement of wind direction and speed in the audience area can increase the accuracy of the individual wave fronts in the spectator areas.

(32) The virtual sound source (7) have then also to be delayed correspondingly, so that the wave fronts from this source and from virtual sound source (6) reach their region of overlap (10) at the same time. Accordingly, the travel times from (7) to each individual transducer are calculated first. Then, the time difference compared with virtual sound source (6) plus travel time (dt 5) is added to each of the calculated values. In this way, the curvature of the wave front is preserved, it is only radiated correspondingly later.

(33) After all travel times from all virtual speakers to all virtual sound sources have been calculated in accordance with this procedure, the smallest calculated travel time in the entire system runtime can be subtracted from all calculated running times in the system that record the final values. In this way, any unnecessary latency anywhere in the system is avoided.

(34) FIG. 3 illustrates the phase relationships between the individual signals in the plane of the arrangement of sound transducers. The geometrical relationships are the same as in FIG. 1.

(35) In the plane of the sound transducer arrangement (1), the spherical sectors of the wave fronts, which are directed toward audience area (2) and emanating from the virtual sound sources (3), (4), (5) and (6) located equidistantly from the overlapping areas are only in phase at a single point in the centre of the sound transducer arrangement. Only there are the membrane excursions of the transducer in question added linearly for all virtual sound sources. With the requirement that adjacent virtual sound sources have to be located at the same distance from the centre of the overlap area of their wave fronts in the audience, this condition is always met. Only in the centre of the arrangement of sound transducers are the signals from all the virtual sound sources with the same signal content in phase up to the highest frequencies of the transmission range. A corresponding reduction in this area prevents overloading. Because of the relatively small affected area, the loss of level in the upper transmission range can easily be compensated for corresponding equalization of the overall signal.

(36) It would also be conceivable to arrange special sound transducers for the bass range in this area, or to set up the arrangement of transducers as a framework about a centrally arranged image reproduction.

(37) FIG. 4 shows the arrangement of sound transducers (1) based on the principle of wave field synthesis, behind which two virtual sound sources (2r) and (2l) for generating a spatial playback are shown. It would also be possible to divide the arrangement of sound transducers, to arrange the virtual sound sources (2l) and (2r) on a broader baseline.

(38) Regardless of whether such a split installation is selected, the process described for a single source can then be applied for each partial surface. In the sketch, this is shown only for the left channel of stereo reproduction. Again, (3) represents the audience area. The virtual sound sources (5), (6), (7) and (8) then emit the signal from the left source from their starting points on the radii about the overlapping areas (9), (10) and (11). The right channel is a mirror image split into separate virtual sound sources, and is not shown in greater detail in the drawing for reasons of clarity.

(39) According to one embodiment, a method for allocating virtual sound sources behind an arrangement of sound transducers based on the principle of wave field synthesis is provided, wherein in order to supply an extensive audience region with the same audio signal content not just a single wave front, propagating from a virtual sound source until it covers an entire expansive audience area, is used but rather that the same signal content is generated by at least two virtual sound sources, which are arranged so that their wave fronts are only directed toward a portion of the audience area.

(40) In a further development, the method is performed such that the signal level at the upper end of the frequency range to be transmitted is lowered in the centre of the arrangement of sound transducers based on the principle of wave field synthesis so that sound can be generated more efficiently with the remaining area because of the incoherent addition of the individual signals.

(41) In a further development, the method is performed such that the virtual sound sources with the same signal content are located at an equal distance from a point in the middle of the section in the supply area, in which an overlap of the wave fronts thereof is unavoidable, or that they are temporally delayed with respect to each other to such an extent that the wave fronts thereof reach this point at the same time.

(42) According to yet a further development, the method is performed such that the shortest travel time resulting from the calculation of travel times between all virtual sound sources and all individual sound transducers, is subtracted from all calculated travel times.

(43) According to yet a further development, the method is performed such that the level of virtual sound sources that supply individual audience areas with the same signal content, can be controlled separately, and/or that the levels of the virtual sound sources that supply individual audience areas with the same signal content, can be equalized separately.

(44) According to yet a further development, the method is performed such that individual signal content, which remains limited to the audience area supplied by primary virtual sound source, can be mixed with the wave fronts of individual virtual sound sources that reproduce the signal content from discrete positions.

(45) According to yet a further development, the method is performed such that two and more virtual sound sources, which supply the entire audience area with different signals from various positions in order to generate a spatial representation, are also each replaced by at least two respective virtual sound sources, which are arranged such that the wave fronts thereof are directed with smaller aperture angles at only a part of the audience area.

(46) According to yet a further development, the method is performed such that the temperature and/or wind speed and direction in the audience area is measured so that scattering or deflection of the wave fronts can be counteracted by a corresponding adjustment of the parameters for generating the wave fronts.

(47) According to one embodiment, an apparatus consisting of sound transducers based on the principle of wave field synthesis is designed to implement the methods described above.

(48) According to a development, a central area of the apparatus consisting of sound transducers is not equipped with sound transducers or equipped with sound transducers designed especially for the reproduction of the bass range, so that the arrangement of sound transducers may form a frame around an assigned area used for image reproduction.

(49) The features of the various embodiments described herein can also be combined with each other.

REFERENCE LITERATURE

(50) [1] John J. Neumann, Jr. and Kaigham J. Gabriel, CMOS-MEMS Membrane for Audio-Frequency Acoustic Acuation, Electrical and Computer Engineering Dept., Carnegie Mellon University, 2001, pp. 236-239, XP-002240602.

(51) [2] U.S. Pat. No. 6,936,524

(52) [3] Berkhout, A. J. (1988): A holographic approach to acoustic control. Journal of the Audio Engineering Society, Vol. 36, No. 12, December 1988, pp. 977-995.

(53) [4] DE 10 2005 001 395 A1