Abstract:
A hearing device and an associated method use an adaptive directional microphone for suppressing feedback. The hearing device includes an adaptation unit which sets the directional microphone so that a sound signal fed back from an earpiece of the hearing device to the directional microphone is attenuated. Acoustic feedback is advantageously attenuated or suppressed simply in an artifact-free manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2009 051 200.4, filed Oct. 29, 2009; the prior application is herewith incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention relates to a method and a hearing device for suppressing or compensating for acoustic feedback with the aid of the directional effect of an adaptive directional microphone. 
         [0003]    A frequent problem with hearing apparatuses is the feedback between the output and the input of the hearing apparatus, which is perceived as disruptive feedback whistling.  FIG. 1  shows the principle of acoustic feedback using the example of a hearing device  1 . The hearing device  1  includes a microphone  2 , which receives a useful acoustic signal  10 , converts it to an electrical microphone signal  11  and emits it to a signal processing unit  3 . In the signal processing unit  3 , the microphone signal  11  is, for example, processed and amplified and emitted as an earpiece signal  12  to an earpiece  4 . In the earpiece  4 , the electrical earpiece signal  12  is converted back to an acoustic output signal  13  and emitted to the eardrum  7  of a hearing device wearer. 
         [0004]    The problem is that some of the acoustic output signal  13  reaches the input of the hearing device  1  by way of an acoustic feedback path  14  and is overlaid there with the useful signal  10  and received by the microphone  2  as a sum signal. Disruptive feedback whistling results with a suitable phase relation and amplitude of the feedback output signal. Attenuation of the acoustic feedback is low in particular due to an open hearing device supply, thereby exacerbating the problem. 
         [0005]    Adaptive systems for suppressing feedback have been available for some time now to resolve that problem. With those systems, the acoustic feedback path  14  is simulated digitally. Simulation takes place, for example, through the use of an adaptive compensation filter  5 , which is fed by the signal  12  driving the earpieces. After filtering in the compensation filter  5 , a filtered compensation signal  15  is subtracted from the microphone signal  11 . Ideally, the effect of the acoustic feedback path  14  is eliminated as a result and an input signal  16  with feedback compensation results in the signal processing unit  3 . 
         [0006]    It is necessary to regulate or adjust the filter coefficient of the adaptive compensation filter  5  for effective feedback suppression. To that end an analysis unit  6  is used to evaluate the input signal  16  of the signal processing unit  3  and to check for possible feedback. Adjustment can produce artifacts, since additional signal components are generated if the compensation filters  5  are not optimally adaptive. Feedback whistling can also occur with a compensation filter  5  that is not optimally adapted. European Patent EP 1 033 063 B1, corresponding to U.S. Pat. Nos. 6,072,884; 6,097,824; 6,104,822; 6,219,427; 6,484,246; 6,498,858; and 6,831,986, discloses such a hearing device, wherein two adaptive compensation filters operating in parallel are used to improve feedback suppression. 
         [0007]    Directional microphone systems are among the methods now used for some years to suppress interference noise and are proven to improve the comprehensibility of speech in hearing situations where the useful signal and interference signals arrive from different directions. In modern hearing devices the directional effect is produced by the differential processing of two or more adjacent microphones with omnidirectional characteristics. 
         [0008]      FIG. 2  shows a simplified block diagram of a first order directional microphone system having two microphones  21 ,  22  at a distance of about 10 to 15 mm. An external delay of T 2  therefore results between the first and second microphones  21 ,  22  for sound signals coming from the front V which corresponds, for example, to the distance between the microphones  21 ,  22 . A signal R 2  of the second microphone  22  is delayed by a time T 1  in a delay unit  23 , inverted in an inverter  24  and added to a signal R 1  of the first microphone  21  in a first adder  25 . The sum produces a directional microphone signal RA which can be fed, for example, by way of signal processing to an earpiece. The directionally dependent sensitivity results substantially from a subtraction of the second microphone signal R 2  delayed by the time T 2  from the first signal R 1 . Sound signals from the front V are thus not attenuated after appropriate equalization while, for example, sound signals from the rear S are eliminated. 
         [0009]    Adaptive directional microphones are microphones which are able to adjust to different ambient situations during ongoing operation. The aim thereof is generally to receive and forward useful sound emitted from a useful sound source as efficiently as possible, while the interference sound from one or more interference sound sources is to be attenuated as effectively as possible in the output signal emitted by the adaptive directional microphone. A hearing device with an adaptive directional microphone is known from International Publication No. WO 00/19770 A1, corresponding to U.S. Pat. No. 6,751,325, wherein the directionally dependent amplification/attenuation can be varied according to the result of a signal analysis. 
       SUMMARY OF THE INVENTION 
       [0010]    It is accordingly an object of the invention to provide a hearing device and a method for suppressing feedback with a directional microphone, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, which are simple and provide feedback suppression with minor artifacts. 
         [0011]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for suppressing feedback in a hearing device. The method comprises providing the hearing device with an adaptive directional microphone and an earpiece and attenuating a sound signal fed back from the earpiece to the directional microphone by adaptation of the directional microphone. In other words the directional characteristic of the directional microphone features a notch in the direction of the feedback path. The invention has the advantage that acoustic feedback is attenuated simply in an artifact-free manner. 
         [0012]    In accordance with another mode of the invention, it is only possible to attenuate in predeterminable frequency bands. This means that undisrupted directional microphone operation can take place in the other frequency bands. 
         [0013]    In accordance with a further mode of the invention, attenuation can only take place in those frequency bands in which feedback occurs. This has the advantage that other frequency bands are not influenced. 
         [0014]    In accordance with an added mode of the invention, the frequency bands can also be determined by Open Loop Gain (OLG) measurement. 
         [0015]    In accordance with an additional mode of the invention, white noise can be emitted by the earpiece. The feedback signal components of the white noise received by the directional microphone are attenuated in that the directional microphone adjusts its directional characteristic by changing adaptation parameters. Adaptation is stopped when attenuation exceeds a predeterminable threshold value. The adaptation parameters thus determined are then stored. In other words they are “frozen” for further use. The threshold value may be a maximum attenuation. This has the advantage that the optimum settings of the adaptive directional microphone can be determined in a hearing device adjustment session. 
         [0016]    In accordance with yet another mode of the invention, the level of the frequency bands with feedback after feedback suppression can be adjusted to the level of adjacent frequency bands. This provides the sound pattern even if independent directional microphone operation takes place in frequency bands without feedback. 
         [0017]    In accordance with yet a further mode of the invention, the directional microphone can also be operated with the stored adaptation parameters. This insures operation with a low level of feedback. 
         [0018]    With the objects of the invention in view, there is also provided a hearing device, comprising an earpiece, an adaptive directional microphone for suppressing feedback and an adaptation unit setting the directional microphone for attenuating a sound signal fed back from the earpiece to the directional microphone. 
         [0019]    In accordance with another feature of the invention, only predeterminable frequency bands are attenuated. 
         [0020]    In accordance with a further feature of the invention, the hearing device can only attenuate in those frequency bands in which feedback occurs. 
         [0021]    In accordance with an added feature of the invention, the hearing device can also include the following units:
       a noise signal unit, which emits white noise by way of the earpiece,   a control unit to determine adaptation parameters of the adaptation unit, which controls the adaptation of the directional microphone by changing the adaptation parameters so that the feedback signal components of the white noise received by the directional microphone are attenuated and which stops the adaptation when attenuation exceeds a threshold value or a maximum attenuation, and   a storage unit to store the adaptation parameters thus determined.       
 
         [0025]    In accordance with an additional feature of the invention, the hearing device can also include a level adjustment unit which adjusts the level of the frequency bands with feedback after feedback suppression to the level of adjacent frequency bands without feedback. 
         [0026]    In accordance with a concomitant feature of the invention, the hearing device can include at least two omnidirectional microphones emitting microphone signals, which are connected electrically to one another in the adaptation unit to form a directional characteristic. 
         [0027]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0028]    Although the invention is illustrated and described herein as embodied in a hearing device and a method for suppressing feedback with a directional microphone, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0029]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0030]      FIG. 1  is a schematic and block diagram of a hearing device with feedback suppression according to the prior art; 
           [0031]      FIG. 2  is a schematic and block diagram of a microphone configuration with directional characteristics according to the prior art; 
           [0032]      FIG. 3  is a flow diagram of a method for suppressing feedback with an adaptive directional microphone, according to the present invention; 
           [0033]      FIG. 4  is a schematic and block diagram of a hearing device with feedback suppression through the use of a directional microphone according to the present invention; and 
           [0034]      FIG. 5  is a graph showing measurement curves of frequency-dependent feedback in a hearing device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 3  thereof, there is seen a flow diagram of a method for feedback suppression in hearing devices with the aid of an adaptive directional microphone. In a first step  100 , white noise WR is emitted from an earpiece of a hearing device worn by a hearing device wearer in a measuring booth with the best possible soundproofing. An acoustic feedback path between the earpiece of the hearing device and at least one microphone of the directional microphone causes signal components of the white noise to be received by the microphone. In a step  101 , adaptation parameters AP of the adaptive directional microphone are changed so that the feedback signal components are attenuated. 
         [0036]    In a step  102 , adaptation is stopped, when the signal components of the feedback white noise have been attenuated by a predeterminable threshold value. The threshold value may be a maximum attenuation. In a step  103 , the adaptation parameters AP thus determined are stored. Attenuation preferably only takes place in those frequency bands FB, in which feedback occurs. These are determined by using an OLG measurement in step  104 . In a step  105 , the hearing device is operated with the adaptation parameters AP thus determined and the frequency bands FB thus determined. 
         [0037]      FIG. 4  shows a block diagram of a hearing device  30  with a directional microphone  33  to suppress acoustic feedback. Sound signals are received by two omnidirectional microphones  31 , converted to electrical signals and fed to an adaptation unit  32 . The microphone signals of the two microphones  31  are connected electrically to one another in the adaptation unit  32 . It is possible to change the directional characteristic of the directional microphone  33  formed by the microphones  31  by changing adaptation parameters in the adaptation unit  32 . 
         [0038]    A directional microphone signal leaves the adaptation unit  32  and is fed to a level adjustment unit  34 . It is possible to adjust the signal levels of different frequency bands to one another as required with the aid of the level adjustment unit  34 . The signal thus adjusted is fed to a signal processing unit  36  and leaves the same amplified and prepared. The signal processed by the signal processing unit  36  is fed to an earpiece  37  and emitted thereby as a sound signal. Part of the sound signal is fed back by way of an acoustic feedback path  38  to the microphones  31  and can cause feedback whistling. Therefore, a control unit  39  is used to change the adaption parameters of the adaptation unit  32  so that the feedback signal components are attenuated. In other words, the notch of the directional characteristic of the directional microphone  33  points in the direction of the feedback path. 
         [0039]    The adaptation parameters are preferably determined in a hearing device adjustment session. To this end white noise is emitted from the earpiece  37  with the aid of a noise signal unit  41  in the hearing device  30 . Some of this white noise is fed back acoustically and received by the microphones  31 . The adaptation parameters of the adaptation unit  32  are now changed until the feedback noise signal is attenuated by a predeterminable threshold value. The adaptation parameters thus determined are stored in a storage unit  40  and used during hearing device operation. 
         [0040]    Advantageously, only those frequency bands in which feedback occurs are attenuated. The critical frequency bands are determined, for example, through the use of OLG measurement. 
         [0041]    In order to avoid sound differences for signals from the rear in non-attenuated frequency bands during directional microphone operation, the level adjustment unit  34  aligns the levels of the attenuated frequency bands with the levels of adjacent frequency bands. 
         [0042]      FIG. 5  shows a diagram of a signal level R in dB as a function of frequency f in kHz for a sound signal fed back into an omnidirectional microphone and for a sound signal fed back into a directional microphone with a 70° notch. 
         [0043]    It is clear from the two level curves OM and RM of the omnidirectional microphone and the directional microphone that feedback occurs at a frequency f of around 3 kHz, with the feedback in the directional microphone being around 25 dB lower than in the omnidirectional microphone due to the selected 70° directional effect.