Microphone Array

Abstract

Microphone arrays comprise several microphone capsules, the outputs of which being electronically combined for directional recording of sound. The directional and frequency properties of the microphone array depend on the number and positions of the microphone array. In order to obtain the smallest possible microphone array with only few microphone capsules, which, however, has an essentially uniform directional and frequency dependence over a speech frequency range, is scalable and robust against small incorrect positioning of the capsules, fifteen or twenty-one microphone capsules (K.sub.15,11-K.sub.15,35, K.sub.21,11-K.sub.21,37) are arranged on a carrier such that they lie on three similar branches, each with the same number of microphone capsules, which are rotated against each other by 120°. Each of the microphone capsules lies on a corner of a triangle of a grid in a flat isometric coordinate system with three axes rotated by 120° against each other and forming the grid of equilateral triangles.

Claims

1. A microphone array, comprising: fifteen or twenty-one microphone capsules (K.sub.15,11-K.sub.15,35, K.sub.21,11-K.sub.21,37), and a circuit arrangement, which is connected to the microphone capsules so as to receive microphone signals from the microphone capsules, and which is configured for processing the microphone signals together; wherein the microphone capsules are arranged in a plane on a carrier, characterized in that the microphone capsules are positioned on the carrier at the following positions: on three identical branches, each having the same number of microphone capsules, wherein the branches are rotated against one another by 120° around a common center; and wherein, in a planar isometric coordinate system with three axes rotated by 60° against each other and forming an isometric coordinate system of equilateral triangles, each of the microphone capsules lies on a corner of a triangle of the isometric coordinate system.

2. The microphone array according to claim 1, wherein the side length of each triangle of the isometric coordinate system corresponds to the smallest distance between two of the microphone capsules.

3. The microphone array according to claim 1, wherein the geometric center of the array is located in the middle of one of the triangles, which is the center triangle, and wherein positions within the isometric coordinate system are specified as multiples of the side lengths of the triangles in the format and relative to a reference point at that corner of the center triangle which lies opposite its side running parallel to one of the axes.

4. The microphone array according to claim 3, wherein the microphone array comprises fifteen microphone capsules (K.sub.15,11-K.sub.15,35), which, starting from the reference point, are arranged at the following positions, or at the corresponding positions in a mirrored arrangement: K.sub.15,11=(0,4,0), K.sub.15,12=(1,3,0), K.sub.15,13=(2,2,0), K.sub.15,14=(0,2,2), K.sub.15,15=(0,2,1), K.sub.15,21=(0,0,−3), K.sub.15,22=(0,−1,−2), K.sub.15,23=(0,−2,−1), K.sub.15,24=(2,0,−1), K.sub.15,25=(1,0,−1), and K.sub.15,31=(−3,0,1), K.sub.15,32=(−2,0,2), K.sub.15,33=(−1,0,3), K.sub.15,34=(2,−1,0), K.sub.15,35=(−2,0,0).

5. The microphone array according to claim 3, wherein the microphone array comprises twenty-one microphone capsules (K.sub.21,11-K.sub.21,37), which, starting from the reference point, are arranged at the following positions, or at the corresponding positions in a mirrored arrangement: K.sub.21,11=(0,5,2), K.sub.21,12=(0,4,3), K.sub.21,13=(0,6,0), K.sub.21,14=(0,0,6), K.sub.21,15=(−1,0,6), K.sub.21,16=(0,1,3), K.sub.21,17=(−1,0,2); K .sub.21,21=(−5,−1,0), K.sub.21,22=(−4,−2,0), K.sub.21,23=(−5,0,1), K.sub.21,24=(0,−5,0), K.sub.21,25=(0,−5,−1), K.sub.21,26=(−1,−2,0), K.sub.21,27=(1,−2,0); and K.sub.21,31=(2,0,−4), K.sub.21,32=(3,0,−3), K.sub.21,33=(0,0,−5), K.sub.21,34=(5,1,0), K.sub.21,35=(5,2,0), K.sub.21,36=(3,0,0), K.sub.21,37=(1,2,0).

6. The microphone array according to claim 1, wherein the microphone capsules are mounted on three similar circuit boards or groups of circuit boards rotated by 120° against each other.

7. The microphone array according to claim 6, wherein the microphone capsules are mounted on at least two groups of three similar circuit boards each, wherein each circuit board comprises at least two microphone capsules and wherein one circuit board from each group belongs to each branch.

8. The microphone array according to claim 6, wherein a middle region of the array, between the three rotated circuit boards or groups of circuit boards comprises no circuit board, or a circuit board without a microphone capsule.

9. The microphone array according to claim 1, wherein the signal processing performs beamforming.

10. The microphone array according to claim 1, wherein the microphone array is adapted for being mounted on a ceiling of a room; the carrier is a metal plate with a sound reflecting surface; and each of the microphone capsules is attached near a hole in the metal plate so as to acquire the sound through the hole.

11. The microphone array according to claim 1, wherein the side lengths of the triangles of the isometric coordinate system are in the range of 3-6 cm, in particular in the range of 4-5 cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Further details and advantageous embodiments are depicted in the drawings, showing in

[0013] FIG. 1 shows an arrangement of fifteen microphone capsules in a first embodiment of the invention;

[0014] FIG. 2 shows an arrangement of fifteen microphone capsules in a second embodiment of the invention which is mirror-symmetrical to the first embodiment;

[0015] FIG. 3 shows a coarray of an arrangement of microphone capsules according to the first or second embodiment;

[0016] FIG. 4 shows an exemplary arrangement of circuit boards for an arrangement of the microphone capsules according to the first embodiment;

[0017] FIG. 5 shows an arrangement of twenty-one microphone capsules in a third embodiment of the invention;

[0018] FIG. 6 shows an arrangement of six microphone capsules in a fourth embodiment of the invention; and

[0019] FIG. 7 shows a block diagram of a microphone array.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] In a first embodiment of the invention, FIG. 1 shows an arrangement of fifteen microphone capsules on a carrier T. The microphone capsules are arranged in three equal groups on congruent branches, each of which is rotated by 120° in relation to one another. The first branch comprises e.g. the capsules K.sub.15,11, K.sub.15,12, K.sub.15,13, K.sub.15,14 and K.sub.15,15. The second branch comprises the capsules K.sub.15,21-K.sub.15,25 and the third branch comprises the capsules K.sub.15,31-K.sub.15,35. A Cartesian coordinate system X,Y is indicated for orientation, but the capsules lie on the intersection points of an isometric coordinate system, which is also indicated in FIG. 1. The isometric coordinate system has three axes L0, L1, L2 offset by 60° in one plane and consists of equilateral triangles. The sides of each of these triangles are each parallel to one of the axes L0, L1 or L2. The center of the entire arrangement, around which the three congruent branches are rotated against each other, is the origin of the Cartesian coordinate system (i.e. X=0, Y=0). At the same time, it is also the center of gravity of a central triangle D.sub.M of the isometric coordinate system. That corner of the central triangle D.sub.M that is opposite its side parallel to the L0 axis is arbitrarily chosen here as the reference point R of the isometric coordinate system.

[0021] Positions in the isometric coordinate system are specified as multiples of the side lengths of the equilateral triangles. For example, the upper right corner of the center triangle D.sub.M is shifted from the reference point R by only one side length in the direction of the L1 axis, which is specified in isometric coordinates as position (L0,L1,L2)=(0,1,0). Correspondingly, the upper left corner of the center triangle D.sub.M is shifted from the reference point R by only one side length in the direction of the L2 axis, i.e. at the position (L0,L1,L2)=(0,0,1) in isometric coordinates. Starting from the reference point R, the microphone capsules are at the following positions:

TABLE-US-00001 Branch 1: Branch 2: Branch 3: K.sub.15, 11 (0, 4, 0) K.sub.15, 21 (0, 0, −3) K.sub.15, 31 (−3, 0, 1) K.sub.15, 12 (1, 3, 0) K.sub.15, 22 (0, −1, −2) K.sub.15, 32 (−2, 0, 2) K.sub.15, 13 (2, 2, 0) K.sub.15, 23 (0, −2, −1) K.sub.15, 33 (−1, 0, 3) K.sub.15, 14 (0, 2, 2) K.sub.15, 24 (2, 0, −1) K.sub.15, 34 (−2, −1, 0) K.sub.15, 15 (0, 2, 1) K.sub.15, 25 (1, 0, −1) K.sub.15, 35 (−2, 0, 0)

[0022] In Cartesian coordinates (X,Y), this results in approximately the following values, depending on the scale (for example for a side length of the triangles, or isometric length unit respectively, of 0.05 m, as shown in FIG. 1; unit: meter):

TABLE-US-00002 K.sub.15, 11 (0.100, 0.144) K.sub.15, 21 (0.075, −0.159) K.sub.15, 31 (−0.175, 0.014) K.sub.15, 12 (0.125, 0.101) K.sub.15, 22 (0.025, −0.159) K.sub.15, 32 (−0.150, 0.058) K.sub.15, 13 (0.150, 0.058) K.sub.15, 23 (−0.025, −0.159) K.sub.15, 33 (−0.125, 0.101) K.sub.15, 14 (0.000, 0.144) K.sub.15, 24 (0.075, −0.072) K.sub.15, 34 (−0.125, 0.072) K.sub.15, 15 (0.025, 0.101) K.sub.15, 25 (0.125, −0.072) K.sub.15, 35 (−0.100, −0.029)

[0023] The scale is to be chosen such that the smallest distance between two microphone capsules corresponds to a side length of a triangle of the isometric coordinate system. Thus, the microphone array depicted in FIG. 1 has a diameter of about 35 cm.

[0024] The positions apply to the coordinate systems indicated in FIG. 1 and, naturally, deviate numerically when the coordinate systems or the array are rotated or when another reference point is selected. Moreover, the positions can be reached in different ways in the isometric coordinate system (since the axes are not orthogonal to each other), which leads to different, but equivalent coordinates. For example, (1,1,0), (0,2,−1), (2,0,1), (3,−1,2) and other further coordinates define the same point. Some equivalent variants can be mapped onto the arrangement shown in FIG. 1 by rotating around the center point.

[0025] FIG. 2 shows an arrangement of fifteen microphone capsules on a carrier T′ in a second embodiment of the invention which is mirror-symmetrical to the first embodiment. It has the same acoustic and geometric properties as the arrangement of the first embodiment and is equivalent to it. The mirror symmetry exists here along the Y-axis. However, identical but rotated variants can be produced by mirroring the arrangement according to the first embodiment on any axis. Because the sensitivity of the array can be adjusted largely uniformly in all directions, all of these variants are equivalent, i.e. they result in the same coarray, and are therefore identical either with the first or the second embodiment. The isometric coordinate system can be determined, for example, for a given arrangement or a given base area in that three positions on the outer edge lie on a straight line (e.g. K.sub.15,11, K.sub.15,12, K.sub.15,13 or K′.sub.15,21, K′.sub.15,22, K′.sub.15,23) that is parallel to one of the axes L0,L1,L2 of the isometric coordinate system. The distances between two adjacent positions in this group of three correspond to the side lengths of the triangles and thus to a unit of the isometric coordinate system.

[0026] FIG. 3 shows the coarray of the arrangement according to the first as well as the second embodiment, which is in principle known from R. Haubrich's theory but adapted here for sound waves of speech sound, or speech frequencies respectively. The points represent the relative position of two microphone capsules of the array to one another, that is, the distances between them and the directions of these distances. In other words, each point in the coarray means that there is at least one pair of microphone capsules in the array whose position relative to one another is the same as the position of this coarray point relative to the coarray center point C.sub.M. Each point thus also represents a possible direction of incidence and wavelength of sound waves, which can be processed by the microphone array exactly according to their direction, i.e. which can be used to locate a sound source and generate a directional effect. It is important here that no holes occur in the coarray at points of the isometric coordinate system. This is true here. A hole in the coarray would mean that the microphone array could not process sound waves of the corresponding direction of incidence and wavelength in a directionally accurate manner. Usually, however, it is not possible to infer an unambiguously associated microphone arrangement directly from a coarray.

[0027] The coarray of the microphone arrangement according to the invention has the advantageous property that (at least in the inner region of the coarray) each coarray point has six neighboring points arranged evenly around it, each at the same distance. This allows the size of the microphone arrangement to be scaled to the wavelengths of interest. The coarray points with the smallest distance to the origin (smallest inter-element distances) indicate the highest frequency that is spatially clearly resolvable, before undersampling begins, i.e. below the so-called spatial aliasing. The coarray points with the greatest distance to the origin correspondingly determine the beamformer's performance for low frequencies. As a result, the smallest inter-element spacing of the microphone arrangement can be scaled to the smallest wavelength or highest frequency of interest, while the closest possible coverage of all wavelengths is maintained for all larger inter-element spacings or larger wavelengths, respectively. For example, scaling the microphone arrangement of the first or second embodiment to a diameter of 35 cm (L=5 cm) results in a highest frequency (below spatial aliasing) of approximately 6.9 kHz.

[0028] One advantage of the invention is that the microphone capsules are not evenly distributed over the entire area of the array, but rather from groups. This means that relatively large parts of the surface do not have to be covered by circuit boards or printed circuit boards for contacting the capsules. In particular, it is not necessary to provide a circuit board or group of circuit boards in the size of the entire arrangement. This further reduces the manufacturing costs for the array, which are relatively low due to the small number of microphone capsules, and its weight. In addition, since the microphone capsules are distributed over three congruent branches, equal circuit boards can also be used for each branch.

[0029] FIG. 4 shows an exemplary arrangement of three identical circuit boards P1,P2,P3 for an arrangement of the microphone capsules according to the first embodiment. The circuit boards P1-P3, each containing five microphone capsules of a branch, are each rotated by 120° and arranged on a carrier. Further components such as a processing unit with one or more processors, AD converters etc. may also be arranged on these circuit boards. In particular, however, it is also possible to accommodate at least some of these additional components on an additional circuit board (not shown) which is located in the middle and is connected to the circuit boards P1-P3 carrying the capsules. For this, it is another advantage of this arrangement that there is no microphone capsule in the middle, so that there is enough space for a central circuit board. This means that no stacking of circuit boards is necessary in this area, which would make the array thicker, more complex to manufacture and therefore more expensive. Further, the fundamentally symmetric structure ensures that the center of gravity of the entire array is in the middle, which makes assembly easier. In addition, it is possible to replace each of the three boards P1-P3 with, for example, two sub-boards P1.sub.1 ,P1.sub.2,P2.sub.1,P2.sub.2,P3.sub.1,P3.sub.2 each, in order to reduce the total board area. This is advantageous if the total area of the array and thus of the (sub) boards is large compared to the area required for the components. Here, too, at least three sub-boards are identical, e.g. P1.sub.1, P2.sub.1 and P3.sub.1. Depending on the space requirements for the further components, it is possible that the circuit boards (or at least the capsule-carrying circuit boards) all together make up less than half of the total area of the array.

[0030] FIG. 5 shows an arrangement of twenty-one microphone capsules, in a third embodiment of the invention. To this embodiment, the same applies as to the first and second embodiments described above. In particular, it has similar advantages. However, the quality of the sound recording may be even better due to the larger number of microphone capsules. Starting from the reference point R, which is defined as described above, and with the coordinate systems indicated in FIG. 5, the microphone capsules are at the following positions (wherein here, too, equivalent variants can be generated by mirroring on an axis and/or by rotating):

TABLE-US-00003 Branch 1: Branch 2: Branch 3: K.sub.21, 11 (0, 5, 2) K.sub.21, 21 (−5, −1, 0) K.sub.21, 31 (2, 0, −4) K.sub.21, 12 (0, 4, 3) K.sub.21, 22 (−4, −2, 0) K.sub.21, 32 (3, 0, −3) K.sub.21, 13 (0, 6, 0) K.sub.21, 23 (−5, 0, 1) K.sub.21, 33 (0, 0, −5) K.sub.21, 14 (0, 0, 6) K.sub.21, 24 (0, −5, 0) K.sub.21, 34 (5, 1, 0) K.sub.21, 15 (−1, 0, 6) K.sub.21, 25 (0, −5, −1) K.sub.21, 35 (5, 2, 0) K.sub.21, 16 (0, 1, 3) K.sub.21, 26 (−1, −2, 0) K.sub.21, 36 (3, 0, 0) K.sub.21, 17 (−1, 0, 2) K.sub.21, 27 (1, −2, 0) K.sub.21, 37 (1, 2, 0)

[0031] The microphone capsules can be distributed very compactly, for example on two boards per branch. One option for one of the circuit boards P.sub.21,1 with five capsules K.sub.21,11-K.sub.21,15 of the first branch is depicted in FIG. 5. The other two capsules K.sub.21,16,K.sub.21,17 are close together and can therefore also be mounted very compactly on a second circuit board (not shown). The necessary further electronic components (processor, AD-converter etc.) may be accommodated on one of the two boards and/or on a possible further, central board (not shown) in the middle of the array. Also in this case, the other two branches are congruent, rotated by 120° each, and can use the same type of boards (i.e., boards having the same layout) as the first branch. An optional central board can be jointly used. Also in this embodiment it is possible that at least the capsule-carrying circuit boards make up less than half of the total area of the array (depending on the space required by the other components).

[0032] Note that only a relative scale is indicated in FIG. 5. This results from the fact that the arrays according to the invention (in all embodiments) are scalable in size without the occurrence of disruptive and difficult to calculate non-linear effects. For the embodiment shown in FIG. 5, the radius r.sub.max of the outermost capsules is approximately L*6.11, with L being the side length of the isometric triangles, and the maximum inter-element spacing is approx. d.sub.max=L*11.79. For example, with a scale of L=5 cm and a circular shape of the array, a diameter D of approx. 61.1 cm results (d.sub.max=58.95 cm); with a scale of L=4 cm, a diameter D of approx. 48.9 cm results (d.sub.max=47.16 cm), and with a scale of L=3.5 cm, a diameter D of approx. 42.8 cm results (d.sub.max=41.27 cm). Vice versa, the diameter of the array or the maximum inter-element spacing may be given. For example, to obtain a diameter of the array of approx. 55 cm, the scale L=4.5 cm is to be chosen, and a maximum inter-element spacing of e.g. d.sub.max=40 cm results for approx. L=3.39 cm. Arrays of different sizes do not differ fundamentally in their frequency behavior, but only the frequency range is slightly shifted. As is well known, the maximum inter-element spacing is important for the localizability of sound sources and the generation of directivity at low frequencies, while the minimum inter-element spacing (i.e., the scale L) is important for the localizability and the generation of directivity at high frequencies. Overall, depending on the embodiment, a scale of L=3-6 cm can be useful for speech frequencies, in particular in the range L=4-5 cm.

[0033] Corresponding relationships with regard to scalability also apply to the other embodiments. For example, r.sub.max=L*3.512, D=L*7.024 and d.sub.max=L*6,557 (rounded) applies to the first and second embodiment.

[0034] FIG. 6 shows an arrangement of six microphone capsules, in a fourth embodiment. This variant is particularly suitable for very small microphone arrays that can be placed, for example, on a conference table, while the embodiments described above are well suited for mounting on ceilings and walls. For this variant, the quality of the directivity and the localization of the sound sources are not as good as for the above-described variants due to the small number of microphone capsules, but better than for other comparable arrangements with only six capsules. Starting from the reference point R, which is defined as described above, and with the coordinate systems indicated in FIG. 6, the microphone capsules lie on the following positions (wherein equivalent variants can be generated here, too, by mirroring on an axis and/or by rotating):

TABLE-US-00004 Branch 1 Branch 2: Branch 3 K.sub.6, 11 (1 , 0, 0) K.sub.6, 21 (−1 , 0, 0) K.sub.6, 31 (0, −1 , −1 ) K.sub.6, 12 (0, 1 , 0) K.sub.6, 22 (−1 , −1 , 0) K.sub.6, 32 (0, 0, −2)

[0035] The microphone capsules can in this case be distributed to one circuit board per branch or, because of the small overall size, they can all be mounted on a single circuit board P.sub.6. Resulting values are (rounded) r.sub.max=L*1.527, D=L*3.054 and d.sub.max=L*2.646.

[0036] FIG. 7 shows an exemplary block diagram of a microphone array which may, for example, correspond to the first or second embodiment. Other embodiments have a different number of microphone capsules per branch and/or a further subdivision of the circuit boards into sub-boards. Three circuit boards P1,P2,P3 are each structured identically and, rotated by 120° against each other, arranged on the carrier T, as shown in FIG. 1 and FIG. 4. Each of these boards comprises the same number of microphone capsules K.sub.15,11-K.sub.15,15, the signals of which are provided to the respective analog-to-digital converters AD.sub.1-AD.sub.5, which are also located on the same board. This makes the connection of the sensitive analog microphone signal to the AD-converter very short. Optionally, individual digital processing blocks DP.sub.1-DP.sub.5 and/or common processing blocks SP.sub.1 may be present on the board, e.g. processors. These may filter the digitized microphone signals, for example. Digital output signals S.sub.1-S.sub.3 of the boards are provided to a central board CP, where a processing unit performs the audio processing AP of the array, in particular the beamforming. Further, the audio processing AP of the array may perform an acoustic search of a (main) sound source in real time in order to align the resulting beam of the array in the direction of the (main) sound source. For this purpose, it may optionally report signals SD.sub.1-SD.sub.3 back to the boards P1-P3. The resulting digital output signal S.sub.A,out of the array is output. Optionally, also an analog output signal may be output.

[0037] Because all microphone capsules of a branch are attached together on a circuit board or group of circuit boards and the positioning of the circuit boards on the carrier T can also take place with very little deviation, the relative position of the capsules to one another is very accurate. The carrier may comprise, e.g., one or more solid or sound-reflecting plates made of metal, plastic or the like. In an embodiment, the carrier is a metal or plastic plate with holes through which the sound can reach the microphone capsules (in the ceiling microphone from the bottom when installed). The plate in that case is sound reflecting, so that the sound pressure level at the microphone capsules is increased by up to 6 dB and the array works as a boundary microphone. On the other hand, the arrangement of the microphone capsules according to the invention allows small deviations from the predefined position of up to 0.5 mm, for example, which makes assembly easier and therefore cheaper. Conventionally, a higher degree of accuracy is necessary here in order to achieve a certain audio quality. The microphone capsules can also be mounted on at least two groups of three identical (sub-) boards PCB.sub.1,1-PCB.sub.3,2 each, with one board of each group belonging to each branch. Each (sub-) board may comprise at least two microphone capsules. A middle region of the array between the three rotated boards or groups of boards may comprise no board, or a board without a microphone capsule. Alternatively, there may also be an additional microphone capsule in the middle, which increases the total number of capsules. The other positions remain unchanged. Thus, the modified first and second embodiments have sixteen microphone capsules, the modified third embodiment has twenty-two capsules and the modified fourth embodiment has seven capsules. Such center capsule has the advantage that it acquires a sound signal at the position of the highest sound pressure (dynamic pressure) and thus improves the directivity and the SNR for the entire array. However, such additional central capsule is not located on a point of the L2-lattice and therefore leads to an unsymmetric coarray with holes, so that the array gets an uneven directivity and is not easily scalable in size anymore.

[0038] Electret capsules are particularly suitable as microphone capsules. Each microphone signal may be corrected or normalized individually, e.g. by means of filtering in the individual digital processing blocks DP.sub.1-DP.sub.5. The corresponding filtering parameters depend on characteristics of the respective microphone capsule, for example its phase response and frequency response. Therefore, in particular such electret capsules are well suited that have an internal memory element with corresponding correction data from which filter parameters may be determined. In addition, the filter parameters can be influenced by the examined or detected direction of the sound source (i.e. the localization of the sound sources or the beamforming). The localization of sound sources and the actual recording of sound from the main sound source can be two separate processes. It is possible to use only some of the microphone capsules for the localization in order to keep the processing effort low while using all capsules for the actual sound recording.

[0039] An advantage of the microphone arrays according to the invention is the good directivity and the high SNR, i.e. a good noise suppression. Noise suppression is the more difficult the less microphone signals are available. However, this relationship is non-linear, depending, among other things, on the positions of the microphone capsules, and therefore difficult to predict. In particular the microphone arrays according to the invention that have fifteen or twenty-one microphone capsules show a good and uniform directivity over all relevant frequency components and directions of incidence of the sound, or a very good noise suppression given the small number of microphone capsules, and are particularly well-suited for ceiling mounted microphones.