Abstract:
A hearing aid ( 21 ) is devised, comprising a first output converter ( 26 ), a second output converter ( 27 ), a first acoustic output transducer ( 34 ) and at least a second output transducer ( 35 ). The first output converter ( 26 ) and the first output transducer ( 34 ) are configured to reproduce the high frequencies of the processed signals, and the second output converter ( 27 ) and the second output transducer ( 35 ) are configured to reproduce the low frequencies of the processed signals. The output converters ( 26, 27 ) may preferably be embodied as direct digital drive output converters.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation-in-part of application No. PCT/DK2005/000538, filed on 23 Aug. 2005, in Denmark and published as WO-A1-2007022773, the contents of which are incorporated hereinto by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to hearing aids. More specific, it relates to hearing aids with more than one acoustic output transducer. The invention also relates to a processor for a hearing aid. 
         [0004]    Hearing aids essentially comprise a microphone for picking up acoustic sound waves and converting them into electrical signals, electronic circuitry for amplifying the electrical signals generated by the microphone, and an acoustic output transducer for reproducing the amplified electrical signals. The amplifier may favor certain frequency bands in the audio spectrum to other frequency bands according to a prescription in order to compensate for an individual hearing loss. 
         [0005]    In this application, the term “high frequencies” preferably refers to audio frequencies between 3 kHz and 15 kHz, and the term “low frequencies” preferably refers to audio frequencies between 20 Hz and 3 kHz. 
         [0006]    2. The Prior Art 
         [0007]    Hearing aids may be used to alleviate very different hearing impairments. Some examples of a hearing impairment are loss of a narrow band of frequencies, loss of the high frequencies, loss of low frequencies, or a more evenly distributed hearing loss across the entire audio spectrum. In cases where some residual hearing is present in the affected frequency range a hearing aid user may benefit from a hearing aid with means to process these frequencies. 
         [0008]    Present-day hearing aids have a limited high-frequency reproduction, usually capped at about 4-8 kHz, mainly due to limitations of the output transducer. For reasons in the mechanical interactions in the components, extension of the frequency range only comes against the cost of a reduced output power in the low frequency end, and a trade off needs to be found somewhere. Transducers for use in hearing aids are manufactured with focus on speech reproduction, and thus optimized for use in the 200 Hz-6 kHz frequency range, important for speech recognition. However, other sounds of interest, e.g. sounds originating from animals or machinery, are present in the 6 kHz-15 kHz range, too. Individuals with normal hearing are usually able to perceive sounds up to between 15 kHz and 20 kHz, and even persons with a profound hearing loss may still possess some ability to perceive sounds above and beyond 8 kHz, dependent on the individual nature of the hearing loss. 
         [0009]    Recent studies have shown that hearing-impaired young children still having residual hearing left in the 6 kHz-15 kHz range may benefit from the availability of this frequency range when learning to speak. In speech, the main part of the fricative sonic energy of the so-called morphemes /s/ and /z/, i.e. the speech sounds “s” and “z”, generally lies above 4 kHz, especially in the range of 4 kHz-8 kHz, and the ability to perceive and subsequently reproduce those sounds may be improved significantly if this frequency range is made available to hearing-impaired children under the circumstances mentioned earlier. A hearing aid having means to reproduce the frequency range from 200 Hz up to perhaps between 15 kHz and 20 kHz is thus desirable. 
         [0010]    Dual acoustic transducers embodied as composite units are known. For instance, the EJ transducer series from Knowles Electronics, Inc. are dual magnetic receiver types configured for use in hearing aid applications. Such receivers comprise two essentially identical transducer units sandwiched together to form a single unit for use in a hearing aid. During manufacture, great care is taken in order to ensure that the two transducer units eventually perform as identically as possible with respect to their electrical and mechanical characteristics. Dual acoustic transducers are mainly used in applications where high sound pressure levels are required, for instance in high-power hearing aid applications. 
         [0011]    U.S. Pat. No. 4,548,082 describes a hearing aid having two independently driven acoustic output transducers, denoted a woofer and a tweeter, respectively, for reproducing low-frequency and high frequency bands in the audible spectrum. The two acoustic output transducers are driven by a pair of sample-and-hold circuits, alternatingly sampling the output from a D/A converter for providing the acoustic output transducers with low-frequency and high-frequency sounds, respectively. The sample-and-hold circuits are controlled by a multiplexer providing the alternating signal feeds to the two acoustic output transducers. Optional anti-aliasing filters may be provided between the sample-and-hold circuits and the acoustic output transducers in order to filter out aliasing noises. 
         [0012]    Although this approach provides means for driving more than one output transducer in a hearing aid, it also has some serious shortcomings. Driving an acoustic output transducer through a sample-and-hold circuit is very likely to introduce noise, and various spurious and aliasing effects, degrading the quality of the output and needing compensation. 
       SUMMARY OF THE INVENTION 
       [0013]    The invention, in a first aspect, provides a hearing aid comprising a microphone, an input converter for receiving signals from the microphone, a signal processor, a first output converter, a second output converter, a first acoustic output transducer and a second acoustic output transducer, said signal process or being adapted for processing signals from the input converter in order to feed respective outputs to said first output converter and said second output converter, wherein said first output converter and said first output transducer are configured to reproduce the high frequencies of the processed signals, wherein said second output converter and said second output transducer are configured to reproduce the low frequencies of the processed signals, and wherein said signal processor has frequency selection means adapted to split the outputs according to a cross-over frequency tuned by programming. 
         [0014]    This gives the hearing aid the capability of reproducing a wider frequency range than a hearing aid having one output transducer, without the inherent problems of multiplexing the signals for the two output transducers in order to separate the frequency bands. 
         [0015]    According to an aspect of the invention, the first and the second acoustic output transducers are embodied as a single physical unit. The individual transducers making up the unit are configured differently in accordance with the frequency ranges they are intended to reproduce, respectively. The first output transducer is configured to reproduce the high frequencies, and the second output transducer is configured to reproduce the low frequencies. 
         [0016]    The configuration of the output transducers may be carried out at the design stage by adjusting selected dimensions of the individual output transducers, by adapting the physical features, dimensions or electrical parameters to suit the application, or by other suitable means known in the art. 
         [0017]    The invention, in a second aspect, provides a processor a processor for a hearing aid comprising an input converter for receiving signals from a microphone, a first output terminal, a second output terminal, means for processing signals from the input converter according to a prescription so as to produce a processed digital output signal, a first output converter configured for reproducing at a first output terminal a first frequency portion of the processed signal, a second output converter configured for reproducing at a second output terminal a second frequency portion of the processed signal, and frequency selection means for splitting the digital output signal into a first digital output signal suitable for driving the first output converter to reproduce the high frequency portion of the processed signal, and a second digital output signal suitable for driving the second output converter to reproduce the low frequency portion of the processed signal, said frequency selection means being adapted to split the processed outputs according to a cross-over frequency tuned by programming. 
         [0018]    Further features and embodiments will appear from the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will now be described in further detail with reference to the drawings, where 
           [0020]      FIG. 1  is a schematic showing a prior art hearing aid, 
           [0021]      FIG. 2  shows a prior art double-output transducer, 
           [0022]      FIG. 3  is a schematic showing a hearing aid according to the invention, 
           [0023]      FIG. 4  shows a double-output transducer for use with the invention, 
           [0024]      FIG. 5  is a schematic of a hearing aid according to the invention, 
           [0025]      FIG. 6  is an embodiment of a double-output transducer for use with the invention, 
           [0026]      FIG. 7  is an alternate embodiment of a double-output transducer for use in the invention, 
           [0027]      FIG. 8  is an alternate embodiment of a double-output transducer for use in the invention, and 
           [0028]      FIG. 9  is an embodiment of a separate double-output transducer configuration with common conduit for use in the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 1  is a schematic showing a prior art hearing aid  1  comprising a microphone  2 , an analog-to-digital converter (ADC)  3 , a digital signal processor (DSP)  4 , a multiplexer (MUX)  5 , a digital-to-analog converter (DAC)  6 , a first sample-and-hold block  10 , a second sample-and-hold block  11 , a first anti-aliasing filter block  12 , a second anti-aliasing filter block  13 , a first output transducer  14 , dedicated to reproducing high frequencies, and a second output transducer  16 , dedicated to reproducing the low frequencies, ref U.S. Pat. No. 4,548,082. 
         [0030]    Analog acoustic signals are picked up by the microphone  2  and converted into digital signals by the ADC  3 . The digital signals from the ADC  3  are then presented to the input of the DSP  4  for further processing and amplification according to a prescribed alleviation scheme in order to compensate for a detected hearing loss. The output signals from the DSP  4  are converted into analog signals by the DAC  6  and the analog output signals from the DAC  6  are then fed in parallel to the inputs of the first sample-and-hold block  10  and the second sample-and-hold block  11 . The sample-and-hold blocks  10 ,  11  are controlled by the MUX  5 , which in turn is controlled by the DSP  4 . 
         [0031]    The MUX  5  alternatingly opens one of the sample-and-hold blocks  10 ,  11  for passing signals from the DAC  6  in such a way that high frequencies are passed from the first sample-and-hold block  10  via the first anti-aliasing filter  12  to the first output transducer  14 , and low frequencies are passed from the second sample-and-hold block  11  via the second anti-aliasing filter  13  to the second output transducer  15 . The DSP  4  coordinates its output to the DAC  6  with its control signals to the MUX  5  in such a way that high-frequency signals are passed to the first output transducer  14  and low-frequency signals are passed to the second output transducer  15 . 
         [0032]    The prior art hearing aid  1  thus reproduces audio signals by alternatingly driving the first output transducer  14  and the second output transducer  15  carrying low-frequency audio signals and high-frequency signals, respectively. The alternation frequency with which the MUX  5  controls the first and second sample-and-hold blocks  10 ,  11  has to be above the highest audible frequency reproduced by the first output transducer  14  in order to be able to reproduce continuous signals. This means that the timing values of the MUX  5  have to meet very exact tolerances in order to prevent drop-outs or audible artifacts originating from the alternating switching process from reaching the output transducers  14 ,  15 . 
         [0033]      FIG. 2  shows a prior art acoustic output transducer unit  16  for a hearing aid comprising a sound outlet  17 , a first electroacoustic transducer  18  having a first set of electrical connecting terminals  28 , and a second electroacoustic transducer  19 , having a second set of electrical connecting terminals  29  (Knowles Electronics EJ). When connected to e.g. hearing aid circuitry (not shown), electrical signals entering the electrical connecting terminals  28 ,  29  are converted into corresponding acoustical signals in the electroacoustic transducers  18 ,  19 . The acoustical signals from the electroacoustic transducers  18 ,  19  are output from the sound outlet  17 . 
         [0034]    The electroacoustic transducers  18 ,  19  of the prior art output transducer  16  are essentially identical. When the same electrical signal is applied to the electrical connecting terminals  28 ,  29 , it may cause the membrane (not shown) of the first electroacoustic transducer  18  and the second electroacoustic transducer  19  to move in the same direction. The effective membrane area is thus doubled, resulting in an acoustic output transducer which is more power-efficient than a single electroacoustic transducer having a double-sized membrane. In order for the frequency response of the prior art output transducer  16  to be as smooth as possible, great care is taken during manufacture to render the electroacoustic transducers  18 ,  19  as similar as possible with regard to production parameters affecting the quality of the sound reproduction, as mentioned in the foregoing. 
         [0035]      FIG. 3  shows a hearing aid  21  according to the invention. The hearing aid  21  comprises a microphone  22 , an analog-to-digital converter (ADC)  23 , a digital signal processor (DSP)  24 , a first digital bit stream output stage (DBS)  26 , a second digital bit stream output stage (DBS)  27 , a first acoustic output transducer  34 , dedicated to the reproduction of high frequencies, and a second output transducer  35 , dedicated to the reproduction of low frequencies. 
         [0036]    Analog acoustic signals are picked up by the microphone  22  and converted into digital signals by the ADC  23 . The digital signals from the ADC  23  are then presented to the input of the DSP  24  for further processing and amplification according to a prescribed alleviation scheme in order to compensate for a detected hearing loss. The DSP  24  has means (not shown), essentially in the form of suitable software algorithms, for dividing the digital signals into high-frequency and low frequency digital signal parts, and means (not shown) for presenting the high frequency parts of the signals to a first output terminal and the low frequency parts of the signals to a second output terminal. 
         [0037]    The digital output signals from the first and second output terminals of the DSP  24  are converted into two serial digital bit streams by the first DBS  26  and the second DBS  27 . The bit stream from the first DBS  26 , originating from the first output terminal of the DSP  24  and thus, by definition, comprising the high frequencies of the signals, is used as the input signal for the first output transducer  34 , and the bit stream from the second DBS  27 , originating from the second output terminal of the DSP  24  and thus, by definition, comprising the low frequencies of the signals, is used as the input signal for the second output transducer  35 . 
         [0038]    The digital bit streams, having a basic frequency in the magnitude of 1 MHz, are capable of driving the output transducers  34 ,  35  directly as the driver coils (not shown) present in the output transducers  34 ,  35  filter away the drive frequency, limiting the acoustic output bandwidth in the output transducers  34 ,  35  to about 15-20 kHz. The output transducers thus make up part of the electrical output stage, essentially being driven as a class D digital output amplifier. This approach is very economical in terms of chip area demands and power consumption. Further details about the design of such output stages may be found in U.S. Pat. No. 5,878,146. A more advanced digital output stage, also suitable for use in combination with the invention, is the subject of an international application PCT/DK 2005/000077, filed on 4 Feb. 2005, and published as WO-A1 2005076664, counterpart of US-A1-20070036375. 
         [0039]    In use, the hearing aid  21  receives acoustic signals via the microphone  22  and converts them into digital signals with the aid of the ADC  23 . The digital signals from the ADC  23  are processed by the DSP  24 , amplified and compressed according to a prescription for alleviating a hearing loss, and separated into two independent digital output signals. The DSP  24  coordinates the digital output signals to the first and the second DBS  26 ,  27  in order for the analog output signals of the output transducers  34 ,  35  to be mutually coherent. 
         [0040]    The acoustic output transducers  34 ,  35  may be configured differently in order to most effectively cover the desired frequency spectrum distributed between them. The first output transducer  34  may be configured to favor frequencies above a selected crossover frequency and thus primarily reproduce the high frequencies of the output signal, and the second output transducer  35  may be configured to favor frequencies below a selected crossover frequency and primarily reproduce the low frequencies of the output signal. The crossover frequency is selected based on the acoustic characteristics of the output transducers  34 ,  35  and programmed into the DSP  24 . 
         [0041]    Programming operations to enter the selected cross-over frequency into the processor may take place during manufacturing of the electronics module of the hearing aid or later, e.g. during a hearing aid fitting session. 
         [0042]      FIG. 4  shows an acoustic output transducer unit  40  for a hearing aid according to the invention comprising a sound outlet  41 , a first electroacoustic transducer  42 , a second electroacoustic transducer  43 , a first set of electrical connecting terminals  44 , and a second set of electrical connecting terminals  45 . When connected to the hearing aid circuitry (not shown), electrical signals entering the electrical connecting terminals  44 ,  45  are converted into corresponding acoustical signals in the electroacoustic transducers  42 ,  43 . The acoustical signals from the electroacoustic transducers  42 ,  43  are output from the sound outlet  41 . 
         [0043]    The first electroacoustic transducer  42  is configured to reproduce the upper part of the audio spectrum and the second electroacoustic transducer  43  is configured to reproduce the lower part of the audio spectrum. The first electroacoustic transducer and the second electroacoustic transducer are mechanically integrated into one unit, so as to facilitate handling of parts and assembly of the hearing aid. 
         [0044]      FIG. 5  is a schematic of a hearing aid  21  comprising a microphone  22 , an electronics module  20 , and an output transducer unit  40 . The electronics module comprises an input amplifier  25 , an A/D converter  23 , a digital signal processor  24 , a first digital bit stream output stage (DBS)  26 , a second digital bit stream output stage (DBS)  27 , and means  33  for selecting a cross-over frequency. The digital signal processor  24  comprises a controller  30 , a high-pass filter (HPF)  31 , and a low-pass filter (LPF)  32 . 
         [0045]    The output transducer unit  40  comprises an outer shell  52 , a first set of inputs  44 , a second set of inputs  45 , a first transducer  42  comprising a first transducer coil  47  and a first transducer membrane  49 , a second transducer  43  comprising a second transducer coil  46  and a second transducer membrane  48 , a separating wall  50  of the shell  52  separating the first transducer  42  from the second transducer  43 , and a common sound outlet  41 . 
         [0046]    The microphone  22  of the hearing aid  21  picks up sound signals of the entire useable frequency range from about 20 Hz to approximately 15 kHz and converts the sound signals into electrical signals which are presented to the input of the input amplifier  25 . The amplified electrical signals from the input amplifier  25  are converted into digital signals in the analog-to-digital (A/D) converter  23  for further processing by the DSP  24 . 
         [0047]    The digital signals from the A/D converter  23  are presented to the controller  30  of the DSP  24 . The controller  30  performs amplification, compression and conditioning of the digital signals according to a prescription scheme in order to alleviate a hearing loss. The controller  30  of the DSP  24  presents the resulting digital output signals to the HPF  31  and the LPF  32 . The output of the HPF  31  is presented to the first DBS  26 , and the output of the LPF  32  is presented to the second DBS  27 . The cross-over frequency selection means  33  are connected to the HPF  31  and the LPF  32  for selecting a cross-over frequency from a plurality of available cross-over frequencies determining at which frequency the cut-off frequencies for the HPF  31  and the LPF  32  is to be set. 
         [0048]    The output signals from the first DBS  26  are fed to the first transducer coil  47  of the first output transducer  42  via the first set of input terminals  44 , and the output signals from the second DBS  27  are fed to the second transducer coil  46  of the second output transducer  43  via the second set of input terminals  45 . The first transducer coil  47  drives the first transducer membrane  49 , converting the electrical output signals from the first DBS  26  into acoustical signals for the sound outlet  41 . In a similar manner, the second transducer coil  46  drives the second transducer membrane  48 , converting the electrical output signals from the second DBS  27  into acoustical signals for the sound outlet  41 . 
         [0049]    The signal path comprising the HPF  31  of the DSP  24 , the first DBS  26 , the first output transducer  42  and the sound outlet  41 , is essentially configured to reproduce the frequencies above the selected cross-over frequency, and the signal path comprising the LPF  31  of the DSP  24 , the second DBS  27 , the second output transducer  43  and the sound outlet  41 , is essentially configured to reproduce the frequencies below the selected cross-over frequency. The first transducer membrane  49  and the second transducer membrane  48  are mechanically separated by the separating wall  50  in order to ensure independency and efficiency in reproducing the separate frequency bands. 
         [0050]    The entire reproduced acoustical sound spectrum output from the sound outlet  41  thus comprises a high band and a low band of frequencies separated by the cross-over frequency and combined at the sound outlet  41 . This enables the first output transducer  42  and the second output transducer  43  to be optimized for reproducing the separate parts of the acoustical sound spectrum. 
         [0051]    In one embodiment, the first output transducer  42  is optimized to reproduce frequencies above, say, 2.7 kHz with a roll off of frequencies below 2.7 kHz, while the second output transducer  43  is optimized to reproduce frequencies below 2.7 kHz with a roll off of frequencies above 2.7 kHz, while a cross-over frequency of 2.7 kHz is programmed into the cross-over frequency selection means  33 . Such optimizations may be achieved by adjusting the physical dimensions and materials and other relevant parameters of the individual transducers  42 ,  43  during design and manufacture of the transducer unit  40 . The benefits of the optimizations are an improved capability of the transducer unit  40  to reproduce frequencies above 5-6 kHz without adversely affecting reproduction of frequencies below 2-3 kHz significantly. 
         [0052]      FIG. 6  shows an embodiment of a double-transducer arrangement  40  for use with the invention. It comprises a first transducer  42  having a first set of input terminals  44 , a second output transducer  43  having a second set of input terminals  45 , and a common sound outlet  41 . The first transducer  42  is attached to the second transducer  43  on one of its long sides in such a way that the first transducer  42  and the second transducer  43  may share the common sound outlet  41 . The first transducer  42  is somewhat shorter in length in comparison with the second transducer  43  in order to facilitate reproduction of higher frequencies, and the first set of input terminals  44  of the first output transducer  42  are thus placed further into the double-transducer arrangement  40  than the second set of terminals  45  of the second transducer  43 . 
         [0053]      FIG. 7  shows an alternate embodiment of a double-transducer arrangement  40  for use with the invention. It comprises a first transducer  42  having a first set of input terminals  44 , a second output transducer  43  having a second set of input terminals  45 , and a common sound outlet  41 . The first transducer  42  is attached to the second transducer  43  on one of its long sides in such a way that the first transducer  42  and the second transducer  43  may share the common sound outlet  41 . The first transducer  42  is somewhat narrower than the second transducer  43  in order to facilitate reproduction of higher frequencies, and the first set of input terminals  44  of the first output transducer  42  are thus aligned with the second set of terminals  45  of the second transducer  43 . 
         [0054]      FIG. 8  shows an alternate embodiment of a double-transducer arrangement  40  for use with the invention. It comprises a first transducer  42  having a first set of input terminals  44 , a second output transducer  43  having a second set of input terminals  45 , and a common sound outlet  41 . The first transducer  42  is attached to the second transducer  43  on one of its short sides in such a way that the first transducer  42  and the second transducer  43  may share the common sound outlet  41 . The first transducer  42  is somewhat shorter in length in comparison with the second transducer  43  in order to facilitate reproduction of higher frequencies, and the first set of input terminals  44  of the first transducer  42  are thus placed opposite the second set of input terminals  45  of the second transducer  43 . 
         [0055]      FIG. 9  shows an alternate embodiment of a double-transducer arrangement  40  for use with the invention. It comprises a first transducer  42  having a first set of input terminals  44  and a first sound outlet  52 , a second output transducer  43  having a second set of input terminals  45  and a second sound outlet  53 , and an essentially Y-shaped conduit element  60  comprising a first conduit  54  for connecting matingly to the first sound outlet  52  of the first transducer  42 , a second conduit  55  for connecting matingly to the second outlet  53  of the second transducer  43 , the first conduit  54  and the second conduit  55  merging to form a common conduit  56  making up a common sound outlet of the double-transducer arrangement  40  of  FIG. 9 . 
         [0056]    In the embodiment shown in  FIG. 9 , the transducers  42 ,  43  may be more liberally disposed in the hearing aid in comparison with the embodiments shown in  FIGS. 6 ,  7  and  8 . This may be an advantage in certain situations where the space available in the hearing aid shell is limited. The first conduit  54  and the second conduit  55  of the conduit element  60  may also be adapted specifically to the characteristics of the transducers  42 ,  43  so as to further optimize sound reproduction from the double-transducer arrangement  40 .