Abstract:
The present invention relates to a microphone module comprising at least two directional microphones having different polar patterns and a single front sound inlet and a single rear sound inlet. The present invention provides a compact and space saving microphone module being less sensitive for mismatch or drift between the applied directional microphones and thereby very robust in directional performance. The microphone module of the present invention is, in particular, suitable in relation to hearing aid applications. The present invention further relates to an associated method for handling and processing signals from the at least two directional microphones.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of European Patent Application Serial No. 15154472.3, filed Feb. 10, 2015, and titled “Directional Microphone Module,” which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a microphone module comprising at least two directional microphones. In particular, the present invention relates to a microphone module which is less sensitive for mismatch or drift between the applied directional microphones and thereby very robust in directional performance. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various techniques to achieve directional hearing in a hearing aid have been suggested over the years. Examples of such techniques are as follows: 
         [0004]    Matched pair of two omni-directional microphones: Directional hearing in hearing aids may be achieved by the use of a matched pair of two omni-directional microphones. To generate a directional output signal the signals from the omni-directional microphones are subtracted. An electrical time delay may be applied to one of the signals to shift the notch angle of the polar pattern. It is a disadvantage of the matched pair that in case of a mismatch/drift the directivity degrades heavily, in particular in the low frequency ranges. Moreover, matching microphones as well as amplitude/phase correction in the hearing aid production are time consuming manual operations. 
         [0005]    Analogue directional microphone: Directional hearing in a hearing aid may also be achieved by the use of an analogue directional microphone. An analogue directional microphone is a microphone with one sound port in the front and one sound port in the rear volume. The advantage of an analogue directional microphone is that directionality cannot be degraded by drift or mismatch. However, the notch angle is at a fixed position and cannot be shifted by processing for beam forming purposes. 
         [0006]    WO 2012/139230 discloses PU microphone module consisting of one omni-directional microphone (P) and one analogue directional microphone (U). The microphone module has two ports. The directional microphone picks up the pressure difference between front and rear port. In one embodiment the omni-directional microphone picks up the pressure at the front port of the module. Another embodiment is that the omni-directional microphone picks up the average of the pressures at front and rear port. The advantage of the PU microphone module is that the directional output is robust to compensate for mismatch/drift because it makes use of an analogue directional microphone which has a stable notch at 90 degree. The closer the desired notch angle is to 90 degree the smaller the impact of mismatch/drift on directionality. However, for notch angles close to 180 degree mismatch/drift still have a significant impact on directionality. 
         [0007]    The so-called Jacobian module, cf. for example U.S. Pat. No. 8,254,609 comprises two directional microphones and one omni-directional microphone. The main advantage of the Jacobian principle is that a higher order directionality can be obtained. However, it is a disadvantage that the two directional microphones need to be matched very tightly. In case of mismatch/drift the directivity of the module degrades heavily. 
         [0008]    Finally, the Blumlein pair is a stereo recording technique (also known as M/S technique) that makes use of two directional microphones. One of the directional microphones has a cardioid polar pattern (notch at 180 degree) and the other one is a dipole (notch at 90 degree). The microphones are oriented in a 90 degree angle towards each other. It is disadvantage of the Blumlein pair that it is a rather bulky design that requires a significant amount of space. 
         [0009]    It may be seen as an object of embodiments of the present invention to provide a compact and space saving microphone module comprising a plurality of directional microphones, said microphone module being less sensitive for mismatch or drift between the applied directional microphones and thereby very robust in directional performance. 
       SUMMARY OF INVENTION 
       [0010]    The above-mentioned object is complied with by providing, in a first aspect, a microphone module comprising 
         [0011]    1) at least two directional microphones having different polar patterns, and 
         [0012]    2) a single front sound inlet and a single rear sound inlet. 
         [0013]    It is an advantage of the microphone module of the present invention that it, compared to known directional techniques used in hearing aids, is less sensitive to mismatch or drift between the applied microphones. 
         [0014]    The main difference between the microphone module of the present invention compared to the Blumlein pair is that the directional axes of the at least two directional microphones coincide while in the Blumlein pair they are oriented in an angle of 90 degree. Coinciding directional axes make it possible to share volumes/sound inlets, i.e. manufacturing a more compact hearing aid microphone module. 
         [0015]    The at least two directional microphones of the microphone module of the present invention may include various types of microphones, such as analogue microphones. 
         [0016]    In addition to the at least two directional microphones the microphone module of the present invention further comprises the single front sound inlet and the single rear sound inlet in order to allow sound pressures to enter respective front and rear volumes of the directional microphones. 
         [0017]    The at least two directional microphones may share a common microphone module housing or cabinet. This sharing of a common microphone module housing or cabinet is advantageous in that it significantly simplifies the mechanical construction of the microphone module. By incorporating the directional microphones into a common microphone module housing or cabinet individual microphone housings or cabinets may be omitted. 
         [0018]    The front sound inlet may be acoustically connected to either a common front volume of the directional microphones or to separated front volumes of the directional microphones. The front sound inlet may be acoustically connected to the common front volume or the separated front volumes via one or more acoustical resistances. Such acoustical resistances ensure that appropriate acoustical delays may be introduced in a given acoustical path. As an example grids may be used as acoustical resistances. 
         [0019]    The rear sound inlet may be acoustically connected to either separated rear volumes of the directional microphones or to one of acoustically connected rear volumes of the directional microphones. The rear volumes may be acoustically connected via one or more acoustical resistances, such as grids. The rear sound inlet may be acoustically connected to the separated rear volumes or to one of the acoustically connected rear volumes via one or more acoustical resistances, such as grids. 
         [0020]    The at least two directional microphones of the microphone module may comprise a first directional microphone having an essential cardioid polar pattern, and a second directional microphone having an essential dipole shaped polar pattern. 
         [0021]    The dipole and cardioid shaped polar patterns are both well-defined polar patterns. It is thus well-established that the cardioid polar pattern has a notch at 180 degree, whereas the dipole polar pattern has a notch at 90 degree. 
         [0022]    Alternatively, the microphone module may comprise a first directional microphone having an essential cardioid polar pattern, and a second directional microphone having an essential hypercardioid shaped polar pattern. In combination therewith a third directional microphone having an essential dipole shaped polar pattern may optionally be provided. 
         [0023]    In a first embodiment the at least two directional microphones may share a common front volume. Moreover, the rear volumes of the directional microphones may be connected via one or more acoustical resistances, such as grids. Additionally, the acoustical connection between the rear sound inlet and one of the acoustically connected rear volumes may be implemented by means of one or more acoustical resistances, such as grids. Sharing volumes (front and/or rear) obviously saves space but it also leads to acoustic coupling of the membranes of the applied directional microphones. Acoustic coupling of membranes generally compromises the directional performance of the module. In a second embodiment the at least two directional microphones may have separated front and separated rear volumes. The acoustical connection between the rear sound inlet and the rear volumes may comprise one or more acoustical resistances, such as grids. Similarly, the acoustical connection between the front inlet and the separated front volumes may comprise one or more acoustical resistances, such as grids. If the applied directional microphones do not share common front/rear volumes the directional performance of the microphone module is not compromised by acoustic coupling of the membranes. 
         [0024]    In a third embodiment the at least two directional microphones may have separated front and acoustically connected rear volumes. The acoustical connection between the rear sound inlet and one of the rear volumes may comprise one or more acoustical resistances, such as grids. Similarly, the acoustical connection between the front inlet and the separated front volumes may comprise one or more acoustical resistances, such as grids. This embodiment is a compromise between saving space by sharing the rear volumes and reducing acoustic coupling of the membranes by separating the front volumes. 
         [0025]    In a fourth embodiment the at least two directional microphones share a common front volume and have separated rear volumes. The acoustical connection between the rear sound inlet and the separated rear volumes may comprise one or more acoustical resistances, such as grids. Also, the acoustical connection between the front inlet and the common front volume may comprise one or more acoustical resistances, such as grids. This embodiment is a compromise between saving space by sharing the front volumes and reducing acoustic coupling of the membranes by separating the rear volumes. 
         [0026]    The microphone module of the present invention may further comprise at least one omni-directional microphone having a separated rear volume. However, the at least two directional microphones and at least one omni-directional microphone may share a common front volume. 
         [0027]    In a second aspect the present invention relates to a hearing aid comprising a microphone module according to the first aspect of the present invention. 
         [0028]    In a third and final aspect the present invention relates to a method for processing signals in a microphone module according to the first aspect, the method comprising the step of combining respective output signals from the at least two directional microphones. 
         [0029]    According to this method a robust directional signal may be generated by weighted summation of the output signals from the at least two directional microphones. To give an example for a microphone module having a cardioid and a dipole output signal: If the weights for a first and a second microphone are both +1 (no amplification) the notch angle of the resulting output signal will be at 120 degree. By amplifying the output signal of the first directional microphone (weight of the cardioid output higher than +1) the notch angle of the resulting output signal will be higher than 120 degree. By amplifying the output signal of the second directional microphone (weight of the dipole output higher than +1) the notch angle of the resulting output signal will be lower 120 degree. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The present invention will now be explained with reference to the accompanying figures where 
           [0031]      FIG. 1  shows a dipole polar pattern, a cardioid polar pattern and the sum of dipole and cardioid polar pattern (weights for first and second microphone are both +1), 
           [0032]      FIG. 2 a    and  FIG. 2 b    both show shows microphone modules having shared front volumes and connected rear volumes, 
           [0033]      FIG. 3  shows a microphone module having separated front and rear volumes, 
           [0034]      FIG. 4  shows a microphone module including an omni-directional microphone, 
           [0035]      FIG. 5 a    shows a microphone module with connected rear volumes and  FIG. 5 b    shows a microphone module with shared front volume, and 
           [0036]      FIG. 6  shows a microphone module with three directional microphones. 
       
    
    
       [0037]    While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0038]    In its broadest aspect the present invention relates to a microphone module comprising at least two directional microphones, such as two analogue directional microphones, i.e. a so-called UU microphone module. One of the directional microphones may have a dipole polar pattern (notch at 90 degree), whereas the other directional microphone may have a cardioid polar pattern (notch at 180 degree), cf.  FIG. 1 . The cardioid polar pattern can be created by adding acoustical resistance between a membrane and one of the sound inlets of the particular directional microphone. The acoustical resistance may be implemented by varying means, such as for example a grid. 
         [0039]    Alternatively, one of the directional microphones may have a cardioid polar pattern, whereas the other directional microphone may have a hypercardioid polar pattern. 
         [0040]    In the following the present invention will be described with reference to a microphone module having a directional microphone having a dipole polar pattern and another directional microphone having cardioid polar pattern. 
         [0041]    The directional microphones of the present invention share the same front and rear sound inlet or even the same front and rear volumes. Sharing sound inlets and/or volumes (front and/or rear) obviously saves space. Therefore the UU microphone module of the present invention may be considered as an effective way to make a robust directional microphone module that consumes as little space as possible. 
         [0042]    The advantage of the UU microphone module is that the directional output is even more robust to mismatch/drift compared to the PU microphone module. For comparison and as previously mentioned the PU microphone module comprises an omni-directional microphone (P) and a directional microphone (U) with a stable notch at 90 degree. The microphone module of the present invention has two stable notches: one at 90 and another one at 180 degree. 
         [0043]    Therefore, also for desired notch angles close to 180 degree the microphone module of the present invention is very stable. Moreover, notches at angles between 90 and 180 degrees are also more stable compared to the PU microphone module. 
         [0044]    Referring now to  FIG. 2 a    a first embodiment 200 comprising two directional microphones with associated membranes  201 ,  202  sharing the same front  203  and rear volumes  204 ,  205  are depicted. Membrane  201  generates the cardioid output, whereas membrane  202  generates the dipole output. The rear volumes  204 ,  205  are connected by means of an acoustical resistance  206 . The acoustical resistance may be implemented as a grid. A front sound inlet  207  and a rear sound inlet  208  are in acoustical connection with the front  203  and rear  205  volumes, respectively. The microphone module  200  has a common outer housing or cabinet  218  which is shared by the two directional microphones whereby individual microphone housings or cabinets can be omitted. 
         [0045]    Referring now to  FIG. 2 b    a second embodiment  209  comprising two directional microphones with associated membranes  210 ,  211  sharing the same front volume  212 . The connected rear volumes  213 ,  214  are depicted. Membrane  210  generates the cardioid output, whereas membrane  211  generates the hypercardioid output. The rear volumes  213 ,  214  are connected by means of an acoustical resistance  215 . The acoustical resistance may be implemented as a grid. A front sound inlet  216  and a rear sound inlet  217  including a grid are in acoustical connection with the front  212  and rear  214  volumes, respectively. Again, the two directional microphones share the same outer housing or cabinet  219 . 
         [0046]    Referring now to  FIG. 3  a second embodiment of the present invention is depicted. According to this embodiment in the microphone module  300  the two directional microphones have separate front volumes  301 ,  302  and separate rear volumes  303 ,  304 . Membrane  305  generates the cardioid output, whereas membrane  306  generates the dipole output. A shared front sound inlet  307  is acoustically connected to the front volumes  301 ,  302  whereas a shared rear sound inlet  308 ,  309  is acoustically connected to respective rear volumes  303 ,  304 . Part of the shared rear sound inlet  308  comprises an acoustical resistance in the form of a grid in the acoustical connection to the rear volume  303 . Similar to  FIG. 2  the two directional microphones share the same outer housing or cabinet  310 . 
         [0047]    The first and second embodiments of the present invention have no useful omni-directional output because subtracting a cardioid and a dipole output from each other will lead to an omni-directional output having a poor sensitivity, in particular in the low frequency range. Therefore, an omni-directional microphone (P) may optionally be added as illustrated in  FIG. 4 . 
         [0048]      FIG. 4  shows how an omni-directional microphone comprising membrane  401  may be incorporated into the UU microphone module  400  of the present invention. In  FIG. 4  the membrane  401  generates the omni-directional output, membrane  402  generates the cardioid output, whereas membrane  403  generates the dipole output. The three microphones share the same front volume  404 , said front volume being acoustically connected to front sound inlet  405 . In  FIG. 4  the three microphones share the same outer housing or cabinet  412 . 
         [0049]    A rear sound inlet  406  is acoustically connected to the rear volume  409  which is connected to rear volume  408  by an acoustical resistance  410  optionally in the form of a grid. Rear volumes  408 ,  409  are physically separated from rear volume  407  by a rigid wall  411 . 
         [0050]    Alternatively, it is also possible to use a separate omni-directional microphone to have more freedom in designing the hearing aid. 
         [0051]      FIG. 5 a    shows an embodiment  500  having connected rear volumes  502 ,  503  and separated front volumes  501 ,  504 . The front volumes  501 ,  504  are acoustically separated by a rigid wall  509 . The rear volumes  502 ,  503  are connected by an acoustical resistance in the form of a grid  508 . The front sound inlet is divided in two inlets  505 ,  506 . Sound inlet  505  is acoustically connected to front volume  501 , whereas sound inlet  506  is acoustically connected to front volume  504 . A rear sound inlet  507  is acoustically connected to rear volume  503 . The front volumes  501 ,  504  are separated from the rear volumes  502 ,  503  by membranes  510  and  511 . The microphones of  FIG. 5 a    share the same outer housing or cabinet  522 . 
         [0052]    In  FIG. 5 b    an embodiment  512  having a shared front volume  513  and separated rear volumes  514 ,  515  is depicted. The rear volumes  514 ,  515  are acoustically separated by a rigid wall  521 . The rear sound inlet is divided in two inlets  516 ,  517 . Sound inlet  517  is acoustically connected to rear volume  515 , whereas inlet  516  is acoustically connected to rear volume  514  via acoustical resistance means, such as a grid. A front sound inlet  518  is acoustically connected to front volume  513 . The front volume  513  is separated from the rear volumes  514 ,  515  by membranes  519  and  520 . Similar to  FIG. 5 a   , the microphones of  FIG. 5 b    share the same outer housing or cabinet  523 . 
         [0053]      FIG. 6  shows a microphone module  600  including three directional microphones having a shared front volume  601  and three connected rear volumes  602 ,  603  and  604 . The rear volumes are connected by acoustical resistances  605 ,  606  in the form of grids. A front sound inlet  607  is acoustically connected to the front volume  601 , whereas a rear sound inlet  608  is acoustically connected to the rear volume  604 . The grids  605 ,  606  and the rear volumes  602 ,  603 ,  604  are mutually configured in a manner so that the membranes  609 ,  610 ,  611  generate cardioid, hypercardioid and dipole polar patterns, respectively. The microphones of  FIG. 6  share the same outer housing or cabinet  612 .