Abstract:
A system for directionally selective sound reception comprises an array of pressure sensors ( 120   a,    120   c ) each arranged to output a pressure signal indicative of pressure, and a processor arranged to receive the pressure signals. The sensor array comprises a support ( 130 ) supporting the four sensors. Two of the sensors are mounted on one side of the support and at least a third sensor is supported on an opposite side of the support. The sound pressure difference measured between the first sensor and the second sensor caused by sound arriving at the array from a direction parallel to the support ( 130 ) is dependent on the distance between the first and second sensors and the nature of material in the space between the first and second sensors. The sound pressure difference measured between the first and third sensors caused by sound travelling perpendicular to the support is dependent on the distance between the first and third sensors. The nature of material in the space between the first and third sensors, and the spacings and the materials are selected such that the sound pressure differences are substantially equal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority filing benefit of International PCT Application PCT/GB2014/051325 filed Apr. 29, 2014 and published under PCT 21(2) in the English language, Great Britain Patent Application No. 1307694.8 filed Apr. 29, 2013, and Great Britain Patent Application No. 1315134.5 filed Aug. 23, 2013. Each of the above listed applications is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to systems for the separation of a mixture of sounds from different sound sources, and in particular to the design of microphone arrays in such systems. 
     BACKGROUND TO THE INVENTION 
     The separation of convolutive mixtures aims to estimate the individual sound signals in the presence of other such signals in reverberant environments. As sound mixtures are almost always convolutive in enclosures, their separation is a useful pre-processing stage for speech recognition and speaker identification problems. Other direct application areas also exist such as in hearing aids, teleconferencing, multichannel audio and acoustical surveillance. 
     Our earlier patent application published as WO 2009/050487 discoses a system for separating a mixture of acoustic signals from a plurality of sources which comprises a sensor array comprising a plurality of pressure sensors and a processor arranged to receive signals from the sensors, and derive from them a series of sample values of directional pressure gradient, identify a plurality of frequency components of the signals, and define an associated direction for each frequency component. The system is then arranged to identify a subset of the frequency components with a source, thereby to define an accoustic signal for that source. Signals for several sources can be defined. In order to provide three dimensional source separation, a three dimensional array of sensors can be used, for example a tetrahedral array. 
     Our further earlier patent application no PCT/GB2013/050784 discloses a microphone array for a system similar to that of WO2009/050487, but in which the array is designed for ease of manufacture. This is achieved by forming the array of support means having two opposite sides and four sensors, with at least one of the sensors supported on each side of the support means, and the sensors facing in directions that are parallel to each other. Because the systems described in WO2009/050487 and PCT/GB2013/050784 work by measuring pressure gradient within the sound wave between different microphones in the array, the arrays described in WO2009/050487 and PCT/GB2013/050784 were designed to allow sound to travel between the microphones as easily as possible, so as to interfere as little as possible with the propagation of the sound wave past the array. 
     SUMMARY TO THE INVENTION 
     The formation of acoustic pressure gradients, and thereby ability of the algorithm of PCT/GB2013/050784 to localise sounds, can be manipulated by inserting acoustic barriers or other objects between microphone capsules that are located either in the same plane or in different planes. Such objects have the effect of modifying the directional sensitivity of each microphone capsule; this is interpreted by the algorithm as modified pressure gradients and a correspondingly different incident angle is deduced. 
     Accordingly the present invention provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means supporting the sensors, a first one and a second one of the sensors being mounted on one side of the support means and at least a third sensor being supported on an opposite side of the support means. The system may further comprise processing means arranged to determine the direction from which a sound component arrives at the array. The component may be a frequency component. The processing means be arranged, in determining the direction, to determine at least one pressure difference measured between two of the sensors. This may be a sound pressure difference, and may be an instantaneous sound pressure difference. 
     The spacing of the sensors, and the materials located between the sensors may be selected so that the error in the measured direction of the sound component is no more than 45°, preferably no more than 30°. This may be for sound coming from any direction over a 360° range, or it may be for sound coming from any direction in three dimensions. 
     The sound pressure difference measured between the first sensor and the second sensor caused by sound arriving at the array from a first predetermined direction, for example the direction parallel to the support means, may be dependent on the distance between the first and second sensors or the nature of the material between the first and second sensors, or both. The sound pressure difference measured between the first and third sensors caused by sound travelling in a second predetermined direction, which may be the direction perpendicular to the support means, may be dependent on the distance between the first and third sensors, or the nature of material in the space between the first and second sensors, or both. The spacings, or the materials, or both, may be selected such that the sound pressure differences are substantially equal. The spacing of the sensors, or the materials located between the sensors, or both, may be selected so that the sound pressure differences are substantially equal. This may be the case for sound coming from any direction over a 360° range, or it may be for sound coming from any direction in three dimensions. 
     The present invention further provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means supporting the four sensors, a first one and a second one of the sensors being mounted on one side of the support means and at least a third sensor being supported on an opposite side of the support means, wherein the spacing between the first and second sensors, which may be in the direction parallel to the plane of the substrate, is greater than the distance between the first and third sensors, which may be in the direction perpendicular to the plane of the substrate. 
     For example the spacing between the first and second sensors may be at least 10%, or at least 25%, or at least 50%, or at least 100% greater than that between the first and third sensors. 
     The present invention further provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means supporting the four sensors, a first one and a second one of the sensors being mounted on one side of the support means and at least a third sensor being supported on an opposite side of the support means, wherein the sound pressure difference measured between the first sensor and the second sensor caused by sound arriving at the array from a direction parallel to the support means is dependent on the distance between the first and second sensors and the sound pressure difference measured between the first and third sensors caused by sound travelling perpendicular to the support means, and the attenuation of sound travelling from the first sensor to the third sensor is dependent on the distance between the first and third sensors and the nature of material in the space between the first and second sensors, and the spacings and the materials are selected such that the sound pressure differences are substantially equal. 
     As the sound pressure differences are frequency dependent, the sound pressure differences may be those at at least one audible frequency. The audible frequency range is from 20 Hz to 20 kHz. For example frequency may be 1000 Hz, or 256 Hz. Substantially equal may be that one of the sound pressure differences is no more than 50%, or no more than 25%, or no more than 10%, or no more than 5% higher than the other. 
     The present invention further provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means supporting the four sensors, a first one and a second one of the sensors being mounted on one side of the support means and at least a third sensor being supported on an opposite side of the support means, wherein the acoustic attenuation of sound travelling from the first sensor to the second sensor is substantially the same as the acoustic attentuation of sound travelling from the first sensor to the third sensor. Substantially equal may be that one of the attenuations is no more than 50%, or no more than 25%, or no more than 10%, or no more than 5% higher than the other. 
     The attenuations, or pressure differences, may be achieved by having a greater spacing between the first and second sensors than the first and third sensors, in which case the material between the first and second sensors is typically air, or by providing a barrier between the first and third sensors, typically a solid barrier, which will generally have a higher density, and acoustic impedence, than air and hence provide more attenuation, or by both one or more barriers and greater spacing. 
     The spacing and/or the barrier or barriers may therefore be arranged at least partially to compensate for the directional asymmetry produced by the presence of the support means. 
     The system may comprise processing means arranged to receive pressure signals from each of the sensors. 
     The present invention further provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means having two opposite sides and four sensors, at least two of sensors being supported on one side of the support means, and a barrier located between said two of the sensors. 
     The present invention also provides a system for directionally selective sound reception comprising an array of pressure sensors each arranged to output a pressure signal indicative of pressure, and processing means arranged to receive the pressure signals, the sensor array comprising support means supporting the four sensors, at least two of sensors being arranged to face in one direction, the system including a barrier located between said two of the sensors. The barrier may comprise part of the support means, or may be a separate component. 
     In each case, the processing mean may be arranged to derive from the pressure signals a series of sample values of directional pressure gradient. The processing means may be arranged to identify a plurality of frequency components of the signals, and define an associated direction for each frequency component. 
     The processing means may be arranged to identify a plurality of frequency components of the signals, identify at least one source direction, and identify at least one of the components as coming from the source direction. 
     The barrier may be mounted on the support means. The support means may be planar. The barrier may be planar and/or may have two parallel sides. The barrier, or the sides of the barrier, may extend perpendicular to the plane, or the sides, of the support means. 
     Two of the sensors may be supported on one of the sides of the support means and two of the sensors may be supported on the other of the sides. In this case there may be two barriers, each located between the two sensors on a respective side of the support means. The barrier, or each of the barriers, may have two perpendicular surfaces facing each of the two sensors that it is located between. These surfaces may each be perpendicular to the plane of the surface of the support on which the sensor is mounted. This can result in one of, or each of, the microphones being located in a cavity defined between three perpendicular surfaces. 
     Alternatively one of the sensors may be supported on one of the sides and three of the sensors may be supported on the other of the sides. In this case the barrier may be located between at least two of said three sensors. 
     Each of the sensors may have a sensing centre point. This may be the point at which the sensor nominally measures the pressure. It may be the centre of a region over which the sensor is arranged to sense pressure. For example it may be the centre of a diaphragm of the sensor. The sensing centre points of the four sensors may be arranged such that each of them is equidistant from each of the other three. The sensing centre points may be arranged so that they lie at the corners of a regular tetrahedron. 
     The height of the barrier above the surface of the support needs to be sufficient to have an appreciable effect. For example a part of the barrier may be located on a straight line between the sensing centre points of two of the sensors. This may be the case even if the barrier is not mounted on the support means. For example the system may include a housing in which the array is housed. The barrier may be mounted on the housing. There may be a gap between the barrier and the support means. 
     Regardless of where the barrier is mounted, the housing may have one or more apertures though it each associated with at least one of the sensors. Preferably there are four apertures, one associated with each of the sensors. The apertures may be of any shape, and may be all of the same shape, or may be of different shapes. The apertures may be spaced apart by a greater distance than the sensing centre points of the sensors. Where the sensors, or the sensing centre points of the sensors, are each equidistant from the other three, and therefore also equidistant from a centre point, the apertures may be arranged so that each of them lies on a line from the centre point extending outwards through the sensor, or the sensing centre point. The apertures may also each be equidistant from the other three. 
     The attenuation of sounds in the direction away from the array centre point, the ‘preferred’ direction, is preferably at least 1 dB lower in the preferred direction than in any perpendicular direction, and more preferable for it to be at least 2 dB lower, or even 3 dB lower. This is preferably for all sounds in the acoustic range, but may be just at one frequency, for example 250 Hz. 
     The system may be arranged to separate a mixture of acoustic signals from a plurality of sources, or it may be a directional listening system arranged to receive sounds only from one or more selected directions. 
     Each of the sensors may have a central axis. The central axis may be an axis about which the sensor has at least a degree of rotational symmetry. The central axis may be a line through the centre of the sensor, and may extend in a direction in which the sensor faces. The sensor may have a rear side which is closest to the support means and a front side, opposite the rear side, which faces in said direction. The two, or three, sensors on one side of the support means may be parallel to each other. For example their central axes may be parallel to each other, or they may face in the same direction. Where there are two sensors on each of the surfaces, the two sensors on the other side of the support means may also be parallel to each other. 
     The processing means may be arranged to define a series of time windows; and for each time window:
         a) generate from the pressure signals a series of sample values of measured directional pressure gradient;   b) identify different frequency components of the pressure signals;   c) for each frequency component define an associated direction;   d) from the frequency components and their associated directions generate a separated signal for one of the sources.       

     The processing means may be arrange to define from the pressure signals a series of values of a pressure function. A directionality function may be applied to the pressure function to generate the separated signal for the source. For example, the pressure function may be, or be derived from, one or more of the pressure signals, which may be generated from one or more omnidirectional pressure sensors, or the pressure function may be, or be derived from, one or more pressure gradients. 
     The separated signal may be an electrical signal. The separated signal may define an associated acoustic signal. The separated signal may be used to generate a corresponding acoustic signal. 
     The associated direction may be determined from the pressure gradient sample values. 
     The directions of the frequency components may be combined to form a probability distribution from which the directionality function is obtained. 
     The directionality function may be obtained by modelling the probability distribution so as to include a set of source components each comprising a probability distribution from a single source. 
     The probability distribution may be modelled so as also to include a uniform density component. 
     The source components may be estimated numerically from the measured intensity vector direction distribution. 
     Each of the source components may have a beamwidth and a direction, each of which may be selected from a set of discrete possible values. 
     The directionality function may define a weighting factor which varies as a function of direction, and which is applied to each frequency component of the omnidirectional pressure signal depending on the direction associated with that frequency. 
     The system may further comprise, in any combination, any one or more features of the preferred embodiments of the invention which will now be described by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a system according to an embodiment of the invention; 
         FIG. 2  is a diagram of a microphone array forming part of the system of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the orientation of microphones in a known microphone array; 
         FIG. 4  is a schematic diagram of the orientation of the microphones in the array of  FIG. 2 ; 
         FIG. 5  is a perspective view of the microphone array of  FIGS. 2 and 4 ; 
         FIG. 6  is a diagram of the array of  FIG. 5  showing barriers which are not shown in  FIG. 5 ; 
         FIG. 7  is a diagram of a microphone array according to a second embodiment of the invention; 
         FIG. 8  is a diagram of a microphone array according to a third embodiment of the invention; 
         FIG. 9  is a diagram of a microphone array according to a fourth embodiment of the invention; 
         FIG. 10  is a front view of a phone unit including a microphone array according to a fifth embodiment of the invention; 
         FIG. 10 a    is a rear view of the phone of  FIG. 10 ; 
         FIG. 11  is an enlargement of part of  FIG. 10 ; 
         FIG. 11 a    is an enlargement of part of  FIG. 10   a;    
         FIGS. 12 a  and 12 b    are sections on lines A-A and B-B of  FIG. 11 ; 
         FIGS. 13 a , 13 b , 13 c , 13 d , 13 e , 13 f    show modifications to the embodiment of  FIG. 10  with different shaped apertures in the phone casing; 
         FIG. 14  is a schematic view of a microphone array according to a further embodiment of the invention; 
         FIG. 15  is a schematic side view the array of  FIG. 14 ; 
         FIG. 16  is a schematic plan view of the array of  FIG. 14 ; 
         FIG. 17  is a section through an array according to a further embodiment of the invention; 
         FIG. 18  is a section through an array according to a further embodiment of the invention; 
         FIG. 19  is a section through an array according to a further embodiment of the invention; 
         FIG. 20  is a section through an array according to a further embodiment of the invention; 
         FIGS. 21 a  and 21 b    are plots showing the deduced direction (angle) of the source as a function of actual angle for a system without barriers and the same system with barriers; 
         FIG. 22 a    is a schematic front view of a microphone array corresponding to that of  FIG. 6 ; 
         FIG. 22 b    is a schematic plan view of the microphone array of  FIG. 22   a;    
         FIG. 23 a    is a schematic front view of a microphone array according to a further embodiment of the invention; and 
         FIG. 23 b    is a schematic plan view of the microphone array of  FIG. 23   a.    
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an audio source separation system according to a first embodiment of the invention comprises a microphone array  10 , a processing system, in this case a personal computer  12 , arranged to receive audio signals from the microphone array and process them, and a speaker system  14  arranged to generate sounds based on the processed audio signals. The microphone array  10  is located at the centre of a circle of 36 nominal source positions  16 . Sound sources  18  can be placed at any of these positions and the system is arranged to separate the sounds from each of the source positions  16 . Clearly in a practical system the sound source positions could be spaced apart in a variety of ways. 
     Referring to  FIG. 2 , the microphone array  10  comprises four microphones  120   a ,  120   b ,  120   c ,  120   d  placed at positions which correspond to the four non-adjacent corners of a cube of side length d, and therefore each equidistant from the other three. This geometry forms a tetrahedral microphone array. 
     Let us consider a plane wave arriving from the direction γ(ω,t) on the horizontal plane with respect to the center of the cube. If the pressure at the centre due to this plane wave is p o (ω, t), then the pressure signals p a , p b ,p c , p d  recorded by the four microphones  120   a ,  120   b ,  120   c ,  120   d  can be written as,
 
 p   a (ω, t )= p   o (ω, t ) e   jkd√{square root over (2)}/2 cos(π/4-γ(ω,t)) ,  (1)
 
 p   b (ω, t )= p   o (ω, t ) e   jkd√{square root over (2)}/2 sin(π/4-γ(ω,t)) ,  (2)
 
 p   c (ω, t )= p   o (ω, t ) e   −jkd√{square root over (2)}/2 cos(π/4-γ(ω,t)) ,  (3)
 
 p   d (ω, t )= p   o (ω, t ) e   −jkd√{square root over (2)}/2 sin(π/4-γ(ω,t)) ,  (4)
 
where k is the wave number related to the wavelength λ as k=2π/λ, j is the imaginary unit and d is the length of the one side of the cube. Using these four pressure signals, B-format signals, p W , p X  and p Y  can be obtained as:
 
 p   W =0.5( p   a   +p   b   +p   c   +p   d ),
 
 p   X   =p   a   +p   b   −p   c   −p   d  and
 
 p   Y   =p   a   −p   b   −p   c   +p   d .
 
     If, kd&lt;&lt;1, ie when the microphones are positioned close to each other in comparison to the wavelength, it can be shown by using the relations cos(kd cos γ)≈1, cos(kd sin γ)≈1, sin (kd cos γ)≈kd cos γ and sin (kd sin γ)≈kd sin γ that,
 
 p   W (ω, t )=2 p   o (ω, t ),  (5)
 
 p   X (ω, t )= j 2 p   o (ω, t ) kd  cos(γ(ω, t )),  (6)
 
 p   Y (ω, t )= j 2 p   o (ω, t ) kd  sin(γ(ω, t ))  (7)
 
     The acoustic particle velocity, ν(r,w,t), instantaneous intensity, and direction of the intensity vector, γ(ω,t) can be obtained from p x , p y , and p w . 
     Since the microphones  120   a ,  120   b ,  120   c ,  120   d  in the array are closely spaced, plane wave assumption can safely be made for incident waves and their directions can be calculated. If simultaneously active sound signals do not overlap directionally in short time-frequency windows, the directions of the intensity vectors correspond to those of the sound sources randomly shifted by major reflections. 
     It will be appreciated that the B-format signals, and the calculations described above, are based on the instantaneous difference in pressure between respective pairs of the microphones, and therefore the measured instantaneous pressure gradients in the respective directions between those pairs of microphones. 
     The exhaustive separation of the sources by decomposing the sound field into plane waves using intensity vector directions will now be described. This essentially comprises taking N possible directions, and identifying from which of those possible directions the sound is coming, which indicates the likely positions of the sources. 
     In a short time-frequency window, the pressure signal p W (ω,t) can be written as the sum of pressure waves arriving from all directions, independent of the number of sound sources. Then, a crude approximation of the plane wave s(μ,ω,t) arriving from direction μcan be obtained by spatial filtering p W (ω,t) as,
 
{tilde over ( s )}(μ,ω, t )= p   W (ω, t )ƒ(γ(ω, t );μ,κ),  (8)
 
where ƒ(γ(ω,t); μ,κ) is the directional filter defined by the von Mises function, which is the circular equivalent of the Gaussian function.
 
     Spatial filtering involves, for each possible source direction or ‘look direction’ multiplying each frequency component by a factor which varies (as defined by the filter) with the difference between the look direction and the direction from which the frequency component is detected as coming. 
     For exhaustive separation, ie separation of the mixture between a total set of N possible source directions, N directional filters are used with look directions μ varied by 2π/N intervals. Then, the spatial filtering yields a row vector {tilde over (s)} of size N for each time-frequency component: 
     
       
         
           
             
               
                 
                   
                     
                       
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     The elements of this vector can be considered as the proportion of the frequency component that is detected as coming from each of the N possible source directions. 
     This method implies block-based processing, such as with the overlap-add technique. The recorded signals are windowed, ie divided into time periods or windows of equal length. and converted into frequency domain after which each sample is processed as in ( 9 ). These are then converted back into time-domain, windowed with a matching window function, overlapped and added to remove block effects. 
     Due to the 3D symmetry of the tetrahedral microphone array of  FIG. 2 , the pressure gradient along the z axis, p Z (ω,t) can also be calculated and used for estimating both the horizontal and the vertical directions of the intensity vectors. 
     The active intensity in 3D can be written as: 
     
       
         
           
             
               
                 
                   
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     The extension of the von Mises distribution to 3D case yields a Fisher distribution which is defined as 
                       f   ⁡     (     θ   ,     ϕ   ;   μ     ,   v   ,   κ     )       =       κ     4   ⁢   π   ⁢           ⁢   sinh   ⁢           ⁢   κ       ⁢     exp   ⁡     [     κ   ⁢     {       cos   ⁢           ⁢   ϕ   ⁢           ⁢   cos   ⁢           ⁢   v     +     sin   ⁢           ⁢   ϕ   ⁢           ⁢   sin   ⁢           ⁢   v   ⁢           ⁢     cos   ⁡     (     θ   -   μ     )           }       ]       ⁢   sin   ⁢           ⁢   ϕ       ,           (   13   )               
where 0&lt;θ&lt;2π and 0&lt;φ&lt;π are the horizontal and vertical spherical polar coordinates and κ is the concentration parameter. This distribution is also known as von Mises-Fisher distribution. For φ=π/2 (on the horizontal plane), this distribution reduces to the simple von Mises distribution.
 
     For separation of sources in 3D, the directivity function is obtained by using this function, which then enables spatial filtering considering both the horizontal and vertical intensity vector directions. 
     Once the spatial filtering has been performed, sound received from one or more chosen directions can be selected and, for example, reproduced through the speaker system  14 . 
     Even though the microphones of the array are of the type which is referred to as omnidirectional, they are generally constructed in a way such that they can be considered to face in a particular direction. Typically each microphone has a sensing surface, generally being the surface of a diaphragm, which may be flat, and may be circular or square in shape, and therefore has a geometrical centre. This forms the centre point of the sensor, which is the point at which it is nominally measuring the pressure. A line from that centre point and perpendicular to the sensing surface can be considered as the central axis of the microphone and extends in the direction in which the microphone is facing. Referring to  FIG. 3 , in a known tetrahedral microphone array, the microphones are arranged such that each of them faces away from a common central point which is the centre of the tetrahedron. However, referring to  FIG. 4 , in this embodiment the four microphones  120   a ,  120   b ,  120   c ,  120   d  of the array are arranged in two pairs. In each pair, the two microphones making up the pair are arranged in the same orientation as each other, so that they both face in the same direction, as indicated by the arrows in  FIG. 4 , which is perpendicular to an imaginary line between their two centre points. The two pairs of microphones face in opposite directions, each pair facing generally away from the other pair. The positions of the microphones  120   a ,  120   b ,  120   c ,  120   d  in the array relative to each other is the same as in a tetrahedral array, with the centre point of each microphone being the same distance from the centre points of each of the other three. 
     Referring to  FIG. 5 , the microphone array  120  is constructed as a planar support member  130  having two opposite support surfaces  132 ,  134  on opposite sides. As the support member  130  is flat and regular, the two surfaces  132 ,  134  are parallel to each other. One pair of microphones  120   a ,  120   c  is mounted on one of the surfaces  132  and the other pair of microphones  120   b ,  120   d  is mounted on the other of the surfaces  134 . Each of the microphones  120   a ,  120   b ,  120   c ,  120   d  is a MEMS microphone and comprises a body  140  with a diaphragm  142  formed on it. The body has a rear surface which is adhered directly to the support member  130  and a front surface in which the diaphragm  142  is formed. The diaphragm  142  is circular and so has a centre point  143  at its geometrical centre as described above. The diaphragm also has rotational symmetry about a central axis  144  which extends through the centre point and perpendicular to the plane of the diaphragm. The central axis extends in the direction in which the microphone faces. The first pair of microphones  120   a ,  120   c  are mounted so that they both face in the same direction which is perpendicular to the plane of the support member  130 , and the second pair of microphones  120   b ,  120   d  are mounted so that they face in the opposite direction. The spacing between the diaphragm centres of the two microphones in the first pair is the same as the spacing between the diaphragm centres of the two microphones in the second pair, and is selected so that each of the microphone centre points is equidistant from the other three. 
     Referring to  FIG. 6 , a barrier  150  is provided between the two sensors  120   c ,  120   d  on one side of the support member  130 , and a further barrier  152  is provided between the two sensors  120   a ,  120   c  on the other side of the support member  130 . Each of the barriers is located symmetrically between the two sensors  120   a ,  120   b , or  120   c ,  120   d  that it is located between. The barriers  150 ,  152  are substantially flat and planar and therefore perpendicular to each other, and both perpendicular to the support member  130 . Although the sensors are not shown with significant height in  FIG. 6 , the centres of the sensors  120   c ,  120   d  are a certain height above the surface of the support member  130 . The barriers  150 ,  152  extend upwards from the support member  130  to a height that is above the centres of the sensors  120   c ,  120   d , or  120   a ,  120   b.    
     Referring to  FIG. 7 , in a further embodiment, the barrier  150  is orientated so as not to be perpendicular to the line between the two centre points of the sensors  120   a ,  120   c  that it separates. 
     Referring to  FIG. 8 , in a further embodiment, each of the barriers  150 ,  152  of  FIG. 6  is replaced by a pair of barriers  160 ,  162 . The two barriers  160 ,  162  on each side are parallel to each other, but in offset planes, and perpendicular to the two barriers on the other side of the support member. There is a gap  163  between each pair of barriers, but this does not leave open a straight line path between the centre points of the two sensors  164   a ,  164   c  that they separate. 
     In one implementation, one or both of the barriers  160  and  162  may be constructed from electronic components, for example capacitors such as electrolytic capacitors. DC-blocking capacitors are required to couple electronic signals from MEMS or electret microphone capsules and advantageously should be located close to the drive voltage pin (Vdd) of the capsule. This makes use of deliberately locating the capacitors around the microphone capsule as shown in  FIG. 8 , or in a different location, to form the barrier producing the acoustic effect described above. 
     Referring to  FIG. 9 , in a further embodiment the flat barrier  150  of  FIG. 7  is replaced by a barrier  170  having two mutually perpendicular surfaces  170   a ,  170   b  facing each of the sensors  174   a ,  174   c . The surfaces  170   a ,  170   b  are again all perpendicular to the surfaces of the support member on which sensors  174   a ,  174   c  are mounted. Therefore each sensor  174   a ,  174   c  is located in a cavity defined by three mutually perpendicular surfaces. 
     Referring to  FIGS. 10 and 11 , in a further embodiment of the invention, a mobile phone  200  includes a sound reception system comprising a processor (not shown) on the phone, a deformable button  201 , and a microphone array comprising four microphones  202   a ,  202   b  mounted on a support member  204 , as described above with reference to  FIG. 5 , and as shown in more detail in  FIGS. 11, 12   a  and  12   b . The phone also comprises a casing or housing  206 , having front and rear walls  208 ,  210  which are parallel to each other, and parallel to the support member  204 , which is in the form of a printed circuit board (PCB) with the microphones  202   a ,  202   b , the processor not shown, and other electronic components mounted on it. The microphone array  202  is located within the housing  206  between the front and rear walls  208 ,  210 , with the push button  201  extending over it and a bezel  203  around it. The barriers  212 ,  214  in this case are formed as part of the housing  206 , and project inwards from the front and rear walls  208 ,  210  respectively with their inner ends contacting the support member  204 . Each the front and rear walls has two apertures  216 ,  217 ,  218 ,  219  through it, one on each side of the barrier  212 ,  214 . The apertures  216 ,  218  in the front wall  208  each have side walls that are perpendicular to the parallel surfaces of the front wall  208 , and are in the form of elongate curved slots that form diagonally opposite quarters of a rounded square. As shown in  FIG. 11 a   , the two apertures  217 ,  219  in the rear wall  210  are the same size and shape, and located so that, viewed from the front as in  FIG. 11 , they form the remaining two quarters of the same rounded square. Each of the apertures is closest to a respective one of the microphones  202   a ,  202   b ,  202   c ,  202   d , and the apertures are each the same distance from the respective microphones. The exact shape and location of the apertures can vary, and the depth of the air gap between each microphone and its respective opening, to attain the necessary microphone directivity of each microphone and thereby overall microphone array performance. In some embodiments, the fact that the four apertures are the same shape as each other and symmetrically arranged with respect to the microphone array helps to improve the performance of the system. 
     As can be seen in  FIGS. 12 a  and 12 b   , the centre point  220  of the microphone array, about which the four microphones are symmetrically arranged and from which they are equidistant, is located in the middle of the support member  204  and in the centre plane of the barriers  212 ,  214 . One line  222 , passing through that centre point, and through the centre points  223   a ,  223   d  of two of the microphones  202   a ,  202   d , also extends through two of the apertures  216 ,  217 , and another line  224 , passing through that centre point, and through the centre points  223   b ,  223   c  of two of the other two microphones  202   b ,  202   c , also extends through the other two of the apertures  218 ,  219 . Here, passing through the aperture only requires that, for each aperture, the line passes through the volume bounded by the side walls of the aperture and the planes of the parallel surfaces of the wall  208 ,  210  of the housing. 
     The sensitivity of each of the microphones  202   a ,  202   b ,  202   c ,  202   d  is shown in  FIGS. 12 a  and 12 b    as the dotted lines  230   a ,  230   b ,  230   c ,  230   d . Specifically the distance of the line in any direction from the sensing centre point of the sensor  223   a ,  223   b ,  223   c ,  223   d  indicates the sensitivity of the microphone in that direction, resulting from the support and the barrier and other objects present. The sensitivity in a direction here is the inverse of the attenuation of sound coming from that direction. Therefore it can be seen that the attenuation in the (‘preferred’) direction away from the array centre point, in this case through the aperture  216 ,  217 ,  218 ,  219 , is significantly less that the attenuation in the perpendicular direction. It is preferable for the attenuation to be at least 1 dB lower in the preferred direction than in any perpendicular direction, and more preferable for it to be at least 2 dB lower, or even 3 dB lower. 
     Referring to  FIGS. 13 a  to 13 f   , in various other embodiments, the basic arrangement of the system is as shown in  FIGS. 12 a  and 12 b   , but the shape of the apertures is different. In the embodiment of  FIG. 13 a   , the apertures are a simple oval shape, and offset slightly from the equivalent of lines  222  and  224 . In the embodiment of  FIG. 13 b   , the apertures are again of a simple oval shape, but again located on the equivalents of the lines  222 ,  224 . In the embodiment of  FIG. 13 c   , the apertures are rectangular and, though located on the equivalent of the lines  222 ,  224 , are not orientated so as to be aligned with them. In the embodiments of  FIGS. 13 d, e  and  f   , the apertures are of various different shapes and all orientated so as to be aligned with the equivalent of the lines  222 ,  224 . 
     Referring to  FIG. 14 , a microphone array forming part of a system of a further embodiment of the invention comprises four microphones  520   a ,  520   b ,  520   c ,  520   d , again spaced so that the centre point of each of them is equidistant from the centre points of the three others. However, in this case a group of three of the microphones  520   a ,  520   b ,  520   c  all face in the same direction, and the one remaining microphone  520   d  faces in the opposite direction. The group of three microphones  520   a ,  520   b ,  520   c , lie in a common plane, and the direction in which they face is perpendicular to that plane, and generally away from the other, fourth, microphone  520   d . The fourth microphone  520   d  faces away from the common plane of the other three  520   a ,  520   b ,  520   c.    
     Referring to  FIG. 15 , the array of  FIG. 14  is constructed in a similar manner to that of  FIG. 5 , except that three of the microphones  520   a ,  520   b ,  520   c  are mounted on one surface of the support member  530 , and the other microphone  520   d  is mounted on the opposite surface of the support member  530 . The array is mounted on a phone housing  540  having front and rear walls  542 ,  544 , and an aperture  546   a ,  546   b ,  546   c ,  546   d  is provided in the housing  540  for each microphone. As in other embodiments, each of the apertures is closest to, and aligned with, a respective on of the microphones. In this case, this requires three apertures in the rear wall  544 , each equidistant from the other two, and further apart than the microphones themselves, and one in the front wall  542  directly above the microphone  546   d . A barrier  550  is provided between each pair of adjacent microphones in the group of three that are on the rear side of the array. As can best be seen in  FIG. 16 , each of these three barriers is arranged perpendicular to the line between the sensing centre points of the two microphones that it separates, and equidistant from those two centre points. 
     Referring to  FIG. 17 , which shows a side projection of an array similar to that of  FIGS. 12 a  and 12 b   , the four microphones  620   a ,  620   b ,  620   c ,  620   d  are mounted on opposite sides of the support member  630 , and the two microphones on each side of the support member are separated by a barrier  650 , and apertures  646   a ,  646   b ,  646   c ,  646   d  are provided in the housing, one for each microphone. The dotted lines show the directionality of the microphones. 
     Referring to  FIG. 18 , in a further embodiment, the barriers  750  are formed by a solid block of material in which the microphones  720   a ,  720   b ,  720   c ,  720   d  are embedded, and the apertures are formed as channels  746   a ,  746   b ,  746   c ,  746   d  formed within the block of material. In this case, the channels are of equal length to each other, and each extend, from the microphone centre point, along the line equivalent to the lines  222 ,  224  through the array centre point. 
     The embodiment of  FIG. 19  is similar to that of  FIG. 17 , but the walls of the phone housing to not extend over the array of microphones  820   a ,  820   b ,  820   c ,  820   d , so there are not separate apertures for each of the microphones. However, the barriers  850  are present to provide the symmetry of response. 
     In the embodiment of  FIG. 20 , as in  FIG. 18 , the microphones  920   a ,  920   b ,  920   c ,  920   d  are again embedded in a block of material which forms the barriers  50 , and in this case there are two channels extending outwards through the block form each microphone centre point, one  946   a  in the direction in which the microphone is facing, and one  946   b  perpendicular to that. 
     The skilled man will of course appreciate that barriers in some arrangements might produce a Helmholtz cavity, with undesirable results. However the skilled man will equally be able to avoid this occurring in a practical system. 
     While two barriers are shown in the examples describe, in some cases, where more limited directionality is required, one barrier may be sufficient. 
     Referring to  FIGS. 21 a  and 21 b   , in an experiment a microphone array arranged as in  FIG. 8  was used, in a system as described with reference to  FIG. 1 , firstly without any barriers between the microphones, and then with the barriers  161 ,  162  formed from plastic 4 mm in height above the PCB. The results for the setup without the barriers are shown in  FIG. 21 a   , and it can be seen that the system was unable to distinguish clearly directions around 90° and 270°, ie close to the plane of the PCB. With the barriers inserted, the results were as shown in  FIG. 21 b   , and it can be seen that the ability of the system to accurately distinguish between sounds from angles around 90° and 270° is greatly improved. As can be seen, in this case the error in the measured angle, i.e. the angle between the measured direction and the true direction of the source, is nowhere more than 30°. This 
     Referring to  FIGS. 22 a  and 22 b   , in a microphone array according to a further embodiment of the invention, a substrate  1030  has two opposite parallel surfaces, with two microphones  1020   a ,  1020   b  mounted on one and two further microphones  1020   c ,  1020   d  mounted on the other. A first barrier  1050  is provided between the first two microphones  1020   a ,  1020   b , and a second barrier  1052  is provided between the second two microphones  1020   c ,  1020   d . In this embodiment the two barriers are parallel to each other. As described above, and as is the case in the embodiments described above, the barriers  1052 ,  1054  are arranged so that, for any given frequency of sound, they have the same attenuation effect as the substrate  1030 . This means that, if the sound is travelling across the face of the substrate  1030 , the pressure difference p 1 −p 2  between the two microphones  1020   a ,  1020   b  on one side of the substrate, is the same as the pressure difference p 1 −p 3  between two of the microphones  1020   a ,  1020   c , on opposite sides of the substrate when the sound is travelling perpendicular to the plane of the substrate, i.e. through the substrate. In terms of pressure ratios, this means that p 2 /p 1 =p 3 /p 1  for the transverse and perpendicular cases of  FIGS. 22 a  and 22 b    respectively. 
     In the arrangement of  FIGS. 22 a  and 22 b   , it is the combination of equal spacing of the microphones and the equal attenuation of sound in the two perpendicular directions parallel and perpendicular to the plane of the substrate that makes the array respond in a symmetrical manner to sounds coming from all directions. However, rather than using a barrier to increase the attenuation in the direction parallel to the substrate, it is also possible to use the spacing of the microphones to achieve this. For example, referring to  FIGS. 23 a  and 23 b   , in a further embodiment of the invention, the substrate  1130  has two opposite parallel surfaces, with two microphones  1120   a ,  1120   b  mounted on one and two further microphones  1120   c ,  1120   d  mounted on the other. In this case the distance between the two microphones  1120   a ,  1120   b  on one side of the substrate is the same as the distance between the two microphones  1120   c ,  1120   d  on the other side, and when seen in front view, i.e. looking perpendicular to the plane of the substrate, as in  FIG. 23 a   , the four microphones are seen to form a square. The four microphones are therefore at four non-adjacent corners of a rectangular cuboid with two square faces parallel to the surfaces of the substrate  130 . The spacing L 31  between the microphones in the direction perpendicular to the plane of the substrate, which is the length of the shortest sides of the rectangular cuboid, is less than the shortest spacing L 21  between the microphones in two directions parallel to the plane of the substrate, which is the length of the longer sides of the rectangular cuboid. 
     Where the substrate is a truly planar sheet material, directions perpendicular to, and parallel to, the plane of the substrate will be clearly defined. For less regular support members, in order to clearly define the directions parallel to, and perpendicular to, the plane of the substrate, or support member, that plane can be considered to be a geometrical plane  1121  which is parallel to the line between the sensing centre points of the two sensors  1120   a ,  1120   b  on one side of the substrate, and parallel to the line between the sensing centre points of the two sensors  1120   c ,  1120   d  on the opposite side of the substrate, and equidistant from those two lines. 
     For the purpose of explanation it can be assumed that the sound pressure varies with distance as it travels between two microphones m 1  and m 2  according to the formula:
 
 p 2/ p 1=exp(−α L   21 −β 21 ) and  p 3/ p 1=exp(−α L   31 −β 31 )
 
where:
     α is the natural rate of sound pressure decline in free space with no impediments;   L 21  is the physical separation between the two microphones m 1  and m 2  in the direction of the pressure wave travel;   β 21  is the pressure wave attenuation presented by the barrier placed between m 1  and m 2 .   

     This is an approximation which is valid over short distances. 
     For the embodiments where the microphones are all equidistant from each other at the corners of a cube,
 
 L   21   =L   31   =L , and so
 
 p 2/ p 1=exp(−α L   21 −β 21 )=exp(−α L−β   21 ), and
 
 p 3/ p 1=exp(−α L   31 −β 31 )=exp(−α L−β   31 )
 
where L is the length of the side of the cube.
 
     Then to achieve p 2 /p 1 =p 3 /p 1 , i.e. to get equal attenuation for both transverse and perpendicular sound directions, it is necessary to achieve β 21 =β 31 . This can be achieved by matching the barrier to the substrate in the embodiments described above. 
     However, the same effect can be achieved by varying L 21  and L 31  i.e. be altering the spacing of the microphones. 
     In the case shown in  FIGS. 23 a  and 23 b   , L 21  and L 31  are chosen such that:
 
 p 2/ p 1 trans   =p 3/ p 1 perp  
 
     Where p 2 /p 1   trans  is the ratio of the pressures at microphones on the same side of the substrate when the sound is travelling transverse to, or parallel to, the surface of the substrate, and 
     p 3 /p 1   perp  is the ratio of the pressures at microphones on opposite sides of the substrate when the sound is travelling perpendicular to the surface of the substrate, and therefore
 
exp(−α L   21 −β 21 )=exp(−α L   31 −β 31 ).
 
     This means that the rotational symmetry of the response of the microphone array can be improved by physically separating the microphones such that the combined effect of the separation distance between them, and any physical barriers between them, is the same for sound travelling perpendicular and parallel to the plane of the substrate. 
     Where no barriers are provided between the microphones on the same side of the substrate, and only the spacing of the microphones can be adjusted, the symmetry is achieved by making
 
α L   21   =αL   31 +β 31  
 
     Referring to  FIGS. 23 a  and 23 b   , it will be appreciated that the microphone spacing of that embodiment is well suited to use in a mobile telecommunications device. Such devices are usually relatively thin, having two large parallel surfaces forming the front and back of the device with a screen on the front surface. The microphone array of  FIGS. 23 a  and 23 b    can be housed within such a telecommunications device with the substrate parallel to, and between, the front and back surfaces, so that two of the microphones  1120   a ,  1120   b  are under the front surface, between the front surface and the substrate, and the other two microphones  1120   c ,  1120   d  are under the rear surface, between the rear surface and the substrate. As with the embodiments described above, the substrate, or support member, can comprise the PCB carrying the other electronic components of the device. In this arrangement the spacing between the front two microphones  1120   a ,  1120   b  can be at least 100% greater than the spacing between the front and rear 
     It will be appreciated that each of the different physical arrangements of sensors described above can be used with any of the processing methods described above with reference to equations (1) to (13). 
     The systems described above are arranged for source separation, i.e. to identify the components of a sound mixture coming from each of a plurality of sources in different locations. However it will be appreciated that in other embodiments the systems can be arranged to identify components of sound from just a single direction. This can be useful in directional listening devices.