Abstract:
According to an embodiment, a transcutaneous active bone anchored hearing aid device is disclosed. The transcutaneous active bone anchored hearing aid device comprises an audio processor comprising means for being externally worn by a hearing aid user and an implantable part comprising transducer means for providing a structure-borne acoustic signal to the skull bone of the hearing aid user. The implantable part comprises a low frequency vibrator and a high frequency vibrator arranged next to each other.

Description:
FIELD 
       [0001]    The present disclosure generally relates to a transcutaneous active bone anchored hearing aid device. The present disclosure more particularly relates to a transcutaneous active bone anchored hearing aid device that comprises a dual electromechanical vibrator. 
       PRIOR ART 
       [0002]    Today&#39;s percutaneous transducers bone anchored systems has limited maximum force output. One of the limitations for the percutaneous systems relates to the use of the single transducer. In percutaneous bone anchored hearing system, it is custom to use a single transducer that which is optimized to work over the whole hearing frequency range (about 200 Hz-8 kHz). 
         [0003]    Further in a transcutaneous bone anchored system that applies a inductive link to transfer energy and signal through the skin (same as cochlear implants), it is an crucial limitation that the transducer as in the percutaneous case is associated to a significant loss of energy related to the signals transferred in the inductive link. The energy loss in such inductive link is typically around 10 dB. 
         [0004]    When going from percutaneous to a transcutaneous bone conductor system there are many challenges. The use of an implant comprising the vibrator under the skin requires the use of an inductive link, of the type that is used in cochlear implants, to transfer the signal and energy through the skin. 
         [0005]    The energy transfer is very inefficient and there is a risk of losing about 10 dBs of energy. Because of the energy loss a percutaneous system will always perform better than a transcutaneous system if the same type of vibrator is used (on the inside as the outside of the skin). 
         [0006]    EP 1617704 A2 discloses a moving armature receiver for a hearing aid. The moving armature receiver has at least two drive coils adapted to be driven by separate drive signal across different frequency ranges by using a frequency dividing network adapted to split an audio input signal into a first audio signal and a second audio signal of predetermined different frequency ranges. 
         [0007]    WO 9908476 A2 discloses an implantable hearing system comprising a plurality of electrical-to-mechanical transducers adapted to be placed in a middle ear. The hearing system comprises a signal driver for producing a first signal and a second signal. The hearing system moreover comprise a first and second electrical-to-mechanical transducers having respective first and second mechanical vibration frequency responses. These transducers are adapted to be coupled to an inner ear, thereby forming a combined output mechanical vibration comprising a superposition of the first and second mechanical vibration frequency responses. 
         [0008]    EP 1871141 A2 discloses a hearing aid having two physically separate receivers, one for outputting low frequency acoustic sounds and another for outputting high frequency acoustic sounds. The low frequency receiver&#39;s output port is connected to a tube in which the high frequency receiver is inserted. At the output of the high frequency receiver, the low frequency and high frequency acoustic sounds are combined to form an acoustic signal that is transmitted to the ear canal. 
         [0009]    WO 2008089914 A1 discloses a hearing aid with a microphone arrangement for receiving acoustic signals to be amplified. The hearing aid comprises at least two earpieces for emitting acoustic signals in different frequency ranges, and at least one signal connection for connecting the microphone arrangement to the earpieces. 
         [0010]    U.S. Pat. No. 6,072,885 A discloses a hearing aid system comprises an input transducer for converting acoustical information at an input to electrical signals at an output, an output transducer for converting electrical signals at an input to acoustical information at an output and a plurality of band-pass filters. The band-pass filters have an input connected to the output of the transducer. It is disclosed that the hearing aid system may comprise a plurality of Automatic gain control (AGC) circuits and that the band-pass filters and AGC circuits may be divided into two processing channels, one for low frequencies and one for high frequencies and may drive separate audio transducers, one configured for maximum efficiency at low frequencies and one configured for maximum efficiency at high frequencies. 
         [0011]    None of these documents provide a solution that solves the problem of inefficient energy transfer associated with the use of transcutaneous systems. 
         [0012]    Thus, there is need for a transcutaneous active bone anchored hearing aid device that is more effective than the prior art transcutaneous active bone anchored hearing aid devices. 
         [0013]    Accordingly, it is also an object of the present disclosure to provide a transcutaneous active bone anchored hearing aid device that is more effective than the prior art transcutaneous active bone anchored hearing aid devices. 
       SUMMARY 
       [0014]    The object of the present disclosure can be achieved by a transcutaneous active bone anchored hearing aid device as defined in claim  1  and by a method as defined in claim  15 . Preferred embodiments are defined in the dependent sub claims and explained in the following description and illustrated in the accompanying drawings. 
         [0015]    The transcutaneous active bone anchored hearing aid device according to the disclosure is a transcutaneous active bone anchored hearing aid device comprising:
       an audio processor comprising means for being externally worn by a hearing aid user,   an implantable part comprising transducer means for providing a structure-borne acoustic signal to the skull bone of the hearing aid user,       
 
         [0018]    The implantable part comprises a low frequency vibrator and a high frequency vibrator arranged next to each other. 
         [0019]    Hereby it is possible to provide a transcutaneous active bone anchored hearing aid device that is more effective than the prior art transcutaneous active bone anchored hearing aid devices. 
         [0020]    By having a transcutaneous active bone anchored hearing aid device according to the disclosure the maximum force output has a sufficient magnitude over the whole hearing frequency range (from about 200 Hz to about 8 kHz). 
         [0021]    The transcutaneous active bone anchored hearing aid device according to the disclosure comprises an audio processor comprising means for being externally worn by a hearing aid user. The audio processor may be an audio processor of any suitable type and size. It is preferred that the audio processor is as small as possible as long as it is capable of being attached to the skin of the user of the hearing aid device by means of magnetic attraction and at the same time is capable of transmitting a signal through electromagnetic induction between an externally worn audio processor and the implantable part. 
         [0022]    The implantable part comprises transducer means for providing a structure-borne acoustic signal to the skull bone of the hearing aid user. The structure-borne acoustic signal is a signal that can be transmitted into the bone, into the skull, and to the cochlea (preferably both  cochleae ) bypassing the outer and middle ear. 
         [0023]    The implantable part comprises a low frequency vibrator and a high frequency vibrator arranged next to each other. The low frequency vibrator and the high frequency vibrator may be any suitable type of electromechanical vibrator. However, it is preferred that the electromechanical vibrators have a small area and a small thickness. 
         [0024]    It may be useful that the transcutaneous active bone anchored hearing aid device comprises a vibrator housing, and that the low frequency vibrator and a high frequency vibrator are arranged in the vibrator housing. 
         [0025]    The use of a vibrator housing makes it possible to attach both the low frequency vibrator and the high frequency vibrator in the same depth and thus having the same distance from ipsilateral cochlea, which is the one of the patient&#39;s two cochlear organs closest to the vibrator housing. In this way it is possible to achieve equal conditions for both the low frequency vibrator and a high frequency vibrator regarding transmission of mechanical vibrations through the bone structure. 
         [0026]    It may be beneficial that the hearing aid device comprises a magnet housing comprising a magnet and a coil, and that the magnet is adapted to provide a magnetic field sufficiently large to keep the audio processor attached to the skin of the hearing aid user, when the implantable part has been implanted into the tissue between the skin and the skull bone of the hearing aid user. 
         [0027]    Hereby it is possible to use the link between the externally worn audio processor and the implanted part to transfer signals (energy) through the skin in an effective manner throughout the hearing frequency range (200 Hz-8 kHz). 
         [0028]    It may be useful that the audio processor comprises an external magnet for attaching the audio processor to the skin by means of magnetic attraction between the external magnet of the audio processor and the magnet of the magnet housing. 
         [0029]    Hereby it is possible to attach an externally worn audio processor to the skin of the hearing aid user by means of a magnet within the audio processor. The magnet of the externally worn audio processor is referred to as the “external” magnet, as it is externally with respect to the user of the device. 
         [0030]    It may be beneficial that the low frequency vibrator and the high frequency vibrator comprise a basically circular body member comprising a coil. 
         [0031]    By using such a construction it is possible to provide a body member of minimum size, so that the extension of the low frequency vibrator and the high frequency vibrator can be minimized. 
         [0032]    This may be useful since the low frequency vibrator and the high frequency vibrator have to be implanted under the skin. 
         [0033]    It may be useful that the implantable part comprises a demodulator extending between the magnet housing and the vibrators. 
         [0034]    Hereby it is possible to provide a reliable and implantable part having such a flat structure that it can be implanted under the skin of the hearing aid user. 
         [0035]    It may be beneficial that the implantable part comprises a side housing extending between the magnet housing and the vibrator housing and that the demodulator is arranged in the side housing. 
         [0036]    Hereby the demodulator can be protected from the tissue surrounding the implanted part. 
         [0037]    It may be useful that the low frequency vibrator and the high frequency vibrator are driven in parallel. 
         [0038]    It may be useful that a capacitor is arranged in series with the high frequency vibrator. 
         [0039]    Hereby it is possible to cut off the current consumption of the high frequency vibrator in the low frequency range. 
         [0040]    It may be beneficial that the resonance frequency of the high frequency vibrator is within the range 1000-4000 Hz, preferably within the range 1500-3500 Hz, such as 2000-3000 Hz. 
         [0041]    Hereby it is possible to provide a transcutaneous active bone anchored hearing aid device that is optimized to work over the whole hearing frequency range (about 200 Hz-8 kHz) and capable of providing a vibrator force of sufficient magnitude throughout the entire hearing frequency range. 
         [0042]    It may be useful that the area of the magnet housing is significantly larger than the area of the vibrator housing. 
         [0043]    Hereby it is possible to provide a firm attachment of the audio processor to the skin of the hearing aid user and at the same time minimise the size of the magnet housing of the implanted part. 
         [0044]    It may be useful that the audio processor comprises a dual-microphone array and noise reducing means. 
         [0045]    Hereby it is possible to provide the hearing aid user with the most optimal sound experience. 
         [0046]    It may be beneficial that the implantable part comprises an attachment magnet centrally arranged in a basically cylindrical magnet housing. 
         [0047]    Such construction makes it easy to arrange a circular coil and a magnet in the magnet housing. Furthermore, the cylindrical shape of the magnet housing makes it possible to provide an easy and user-friendly attachment of the audio processor to the skin. 
         [0048]    It may be useful that the attachment magnet is cylindrical or disk-shaped (having a circular cross-section) and fits into the cylindrical magnet housing. 
         [0049]    It may be useful that the magnet is surrounded by a circular coil. The coil is concentrically arranged as well in the magnet housing as with respect to the magnet. 
         [0050]    Hereby it is possible to provide a very compact implantable part. 
         [0051]    It may be useful that the low frequency vibrator and the high frequency vibrator are arranged in such a distance to the bone structure that they are capable of transferring mechanical vibrations to the user&#39;s inner ears through the bone structure of the head of the hearing aid user. 
         [0052]    Hereby the transfer of mechanical vibrations can be performed in the most suitable way. 
         [0053]    It is preferred that the low frequency vibrator and the high frequency vibrator are arranged in a very small distance to the bone structure. 
         [0054]    It may be useful that the frequency vibrator and the high frequency vibrator are in mechanical contact with the bone structure. 
         [0055]    The objects of the disclosure can be achieved by a method for implanting an implantable part of a hearing aid device, which method comprises the step of attaching a magnet under the skin of a hearing aid user, which method moreover comprises the step of inserting transducer means for providing a structure-borne acoustic signal to the skull bone of the hearing aid user, where the method comprises the step of implanting a low frequency vibrator and a high frequency vibrator arranged next to each other and that the low frequency vibrator and the high frequency vibrator are arranged in such a short distance to the bone structure that they are capable of transferring mechanical vibrations to the user&#39;s inner ears through the bone structure of the head of the hearing aid user. 
         [0056]    Hereby it is possible to provide a transcutaneous active bone anchored hearing aid device that is more effective than the prior art transcutaneous active bone anchored hearing aid devices. 
         [0057]    In the present context, a “hearing aid device” refers to a device, such as e.g. a hearing aid, a listening device or an active ear-protection device, which is adapted to improve, augment and/or protect the hearing capability of a user by receiving acoustic signals from the user&#39;s surroundings, generating corresponding audio signals, possibly modifying the audio signals and providing the possibly modified audio signals as audible signals to at least one of the user&#39;s ears. 
         [0058]    A “hearing aid device” further refers to a device such as an earphone or a headset adapted to receive audio signals electronically, possibly modifying the audio signals and providing the possibly modified audio signals as audible signals to at least one of the user&#39;s ears. Such audible acoustic signals are transferred as mechanical vibrations to the user&#39;s inner ears through the bone structure of the user&#39;s head and/or through parts of the middle ear. 
         [0059]    A hearing aid device may be configured to be worn as a partly implanted unit. A hearing aid device may comprise a single unit or several units communicating electronically with each other. 
         [0060]    More generally, a hearing aid device comprises an input transducer for receiving an acoustic signal from a user&#39;s surroundings and providing a corresponding input audio signal and/or a receiver for electronically receiving an input audio signal, a signal processing circuit for processing the input audio signal and an output means for providing an audible signal to the user in dependence on the processed audio signal. 
         [0061]    Some hearing aid devices may comprise multiple input transducers, e.g. for providing direction-dependent audio signal processing. In some hearing aid devices, the receiver may be a wireless receiver. In some hearing aid devices, the receiver may be e.g. an input amplifier for receiving a wired signal. 
         [0062]    In some aid hearing devices, an amplifier may constitute the signal processing circuit. In the hearing aid devices according to the disclosure, the output means comprises an output transducer formed as a vibrator for providing a structure-borne or liquid-borne acoustic signal. 
         [0063]    The hearing aid device according to the disclosure comprises a vibrator member that is adapted to provide a structure-borne acoustic signal transcutaneously to the skull bone. 
         [0064]    The hearing aid device according to the disclosure may be a “hearing system” referring to a system comprising one or two hearing aid devices. A “binaural hearing system” refers to a system comprising one or two hearing aid devices that is being adapted to cooperatively provide audible signals to both of the user&#39;s ears. 
         [0065]    The hearing aid device according to the disclosure may be a “hearing system” or binaural hearing system comprising “auxiliary devices”, which communicate with the hearing aid devices and affect and/or benefit from the function of the hearing aid devices. Auxiliary devices may be e.g. remote controls, remote microphones, audio gateway devices, mobile phones, public-address systems, car audio systems or music players. 
         [0066]    Hearing aid devices, hearing systems or binaural hearing systems may e.g. be used for compensating for a hearing-impaired person&#39;s loss of hearing capability, augmenting or protecting a normal-hearing person&#39;s hearing capability and/or conveying electronic audio signals to a person. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0067]    The disclosure will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative of the present disclosure. In the accompanying drawings: 
           [0068]      FIG. 1  A) shows an implant according to the disclosure; 
           [0069]      FIG. 1  B) shows an audio processor of a hearing aid according to the disclosure; 
           [0070]      FIG. 2  A) shows a schematically diagram of a dual vibrator according to the disclosure; 
           [0071]      FIG. 2  B) is a graph illustrating the vibrator force as function of frequency of a high frequency vibrator with and without a capacitor; 
           [0072]      FIG. 2  C) is a graph illustrating two current curves of a high frequency vibrator with and without a capacitor; 
           [0073]      FIG. 3  A) is a graph illustrating a single vibrator, a low frequency vibrator, a high frequency vibrator and a dual vibrator force curves as function of frequency; 
           [0074]      FIG. 3  B) is a graph illustrating three current curves of a single vibrator, a low frequency vibrator and a high frequency vibrator; 
           [0075]      FIG. 3  C) is a graph illustrating the current curve of a dual vibrator according to the disclosure and 
           [0076]      FIG. 4  shows a schematically cross-sectional view of the head of a hearing aid user wearing a hearing aid device according to the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0077]    Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present disclosure, a schematically view of the implantable part  2  of a hearing aid device according to the disclosure is illustrated in  FIG. 1  A). 
         [0078]    The implantable part  2  comprises two electromechanical vibrators  6 ,  8 . The implantable part  2  comprises a low frequency vibrator  6  and a high frequency vibrator  8  arranged next to each other in a vibrator housing  66 . Both the low frequency vibrator  6  and the high frequency vibrator  8  comprise a basically circular body member of similar size. 
         [0079]    The implantable part  2  comprises an attachment magnet  10  centrally arranged in a basically cylindrical magnet housing  68 . The magnet  10  is surrounded by a circular coil  12  concentrically arranged in the magnet housing  68 . The circular coil  12  is concentrically arranged with respect to the magnet  10 . 
         [0080]    Throughout the description “magnet” is used to designate a body with either permanent or temporary magnetic properties, such that such a body may be attracted to another body comprising magnetic properties. The magnets referred to may be single body magnets with similar magnetic properties throughout the entire body, or they may comprise assemblies of magnets or magnetically attractable units having dissimilar magnetic properties. 
         [0081]    A demodulator  14  is arranged in a side housing  70  extending between the magnet housing  68  and the vibrator housing  66 . The demodulator  14  is connected to the vibrator housing  66 . The area A 2  of the magnet housing  68  is significantly larger than the area A 1  of the vibrator housing  66 . The area A 2  of the magnet housing  68  is more than twice as large as the area A 1  of the vibrator housing  66 . 
         [0082]    The implantable part  2  is configured to be placed surgically under the skin next to the skull bone of the hearing impaired person. The magnet  10  is configured to function as an attachment means for attachment of an outer part—the audio processor that is described in the following. 
         [0083]      FIG. 1  B) illustrates a schematically perspective view of an audio processor  4  of a hearing aid device according to the disclosure. The audio processor  4  is the “outside part” of the hearing aid device and may comprise a dual microphone solution configured to reduce interference originating from behind and from the sides of the listener. 
         [0084]    This may be done by applying a directional mode. Hereby the hearing aid offers the user increased comfort and enhanced listening ability in noisy situations so that the user can understand conversation in the immediate vicinity more clearly and distinctly. 
         [0085]    The audio processor  4  is adapted to be externally worn by the hearing impaired user. 
         [0086]    The audio processor  4  comprises an external magnet  11  for attachment to the attachment magnet  10  of the implantable part  2 . In use, the audio processor  4  is held in place by the magnetic attraction between the magnet  10  of the implantable part  2  and the external magnet  11  of the audio processor  4 . 
         [0087]    The audio processer  4  comprises at least one microphone (e.g. one microphone array) that picks up sounds from the surroundings of the user of the hearing aid. The audio processor  4  converts these sounds into electrical signals that are transmitted through the skin to the implantable part  2  via an inductive link. 
         [0088]    Referring again to  FIG. 1  the vibrators  6 ,  8  are electromechanical vibrators each having a magnetic circuit and a coil. The impedance Z of the coil is given by the following equation [1]: 
         [0000]        Z=R   DC   +jωL=|Z|e   j0   [1]
 
         [0089]    Where R DC  is the DC-resistance of the coil, L is the inductance of the coil and θ is the phase difference between voltage and current, where e is Euler&#39;s number (approximately 2.71828), and where j is the imaginary unit (complex numbers). 
         [0090]    The resistance R DC  proportionates to the wire length l and the inverse of the wire cross section area A and the resistivity ρ Cu  of the wire material (in this case copper, Cu). Accordingly, the resistance R DC  is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     
                       D 
                        
                       
                           
                       
                        
                       C 
                     
                   
                   = 
                   
                     
                       l 
                       · 
                       
                         ρ 
                         Cu 
                       
                     
                     A 
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
         [0091]    The inductance L is proportional to the square of the number, n, of turns in the coil. This can be expressed in the following way: 
         [0000]        L∝n   2   [3]
 
         [0092]    The impedance, Z, of each of the vibrators  6 ,  8  is related to the resonance frequency of the electromechanical vibratory system. Each of the vibrators  6 ,  8  are assumed to behave like a harmonic oscillator. When displaced from its equilibrium position, the system experiences a restoring force, F, proportional to the displacement. 
         [0093]    When optimising each of the vibrators  6 ,  8  the counterweight mass, m, the vibrator spring constant, k, the number of turns, n, in the coil and the cross-sectional area, A, may be varied: 
         [0094]    When optimising a single vibrator  6 ,  8  the resonance frequency is typically selected to be approximately 800-900 Hz. The number of turns, n, in the coil, the wire length, l, of the coil and the cross section wire area of the coil to match maximum current of the battery. If a given battery has an upper limit of 20 mA for example, the parameters may be chosen so that at maximum voltage output for the hearing aid driver integrated circuit ( 16  see  FIG. 2  A) the vibrator  6 ,  8  should not consume more than 20 mA. This gives the vibrator  6 ,  8  both an acceptable performance in the low frequency range and in the high frequency range. 
         [0095]    The vibrator efficiency depends on the product between n, of turns in the coil and I, the current. 
         [0000]        n·I   [4]
 
         [0096]    Accordingly, to get good performance in the low frequency range the number of turns has to be high. However, a high number of turns, n, result in a high impedance and thus a poor performance in the high frequency range. 
         [0097]    On the other hand, a coil having few turns will have good performance in the high frequency range, but low performance in the low frequency range. 
         [0098]    Accordingly, it is difficult to optimize one single vibrator  6 ,  8  to perform well in the low frequency range and in the high frequency range at the same time. Consequently, there it is a major improvement to apply two vibrators  6 ,  8 , where one of the vibrators  6  is optimized for the low frequency range and where the other vibrator  8  is optimized for HF. 
         [0099]    In the percutaneous applications in which the vibrator is arranged on the outside of the skin, the use of two vibrators  6 ,  8  would be very challenging because of the extension of the vibrators  6 ,  8 . If the vibrators  6 ,  8  are placed on the top of each other the thickness (height) will be two large. On the other hand, the area of the vibrators  6 ,  8  will be too large when placed next to each other. Besides, none of the solutions will be cosmetically appealing. 
         [0100]    On the other hand, if the two vibrators  6 ,  8  are implanted and placed next to each other, they can be hidden under the skin provided that the thickness (height) of the vibrators  6 ,  8  can be kept small. 
         [0101]      FIG. 2  A) illustrates a schematically diagram of a dual vibrator according to the disclosure. The dual vibrator comprises a low frequency vibrator  6  and a high frequency vibrator  8 . Both vibrators  6 ,  8  are electrically connected to a driver integrated circuit  16 . Since the low frequency vibrator  6  and the high frequency vibrator  8  are driven in parallel, it is of great importance to cut off the current consumption of the high frequency vibrator in the low frequency range. This is done by arranging a capacitor  18  in series with the high frequency vibrator  8 . 
         [0102]    Hereby it is achieved that the current is effectively cut off for the low frequencies. The capacitor  18  in series with the high frequency vibrator  8  creates a LC circuit with a resonant frequency ω given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     ω 
                     = 
                     
                       1 
                       
                         LC 
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
         [0000]    where L is the inductance and C is the is the capacitance. 
         [0103]    The LC circuit is capable of storing electrical energy oscillating at its resonant frequency ω. This “resonance effect” takes place when the magnitude of the inductive and capacitive reactances are equal. 
         [0104]    This “resonance effect” can be applied to boost the curve in-between the low frequency vibrator resonance and the high frequency vibrator resonance i.e. smoothing out the dip in output curve (see the curve  30  in  FIG. 2  B). 
         [0105]      FIG. 2  B) shows a graph  20  illustrating the vibrator force  26  as function of frequency  24 . The vibrator force  26  is measured in dB μN. A first frequency  38  corresponding to 900 Hz and a second frequency  40  corresponding to 2500 Hz are indicated with vertical dotted lines. 
         [0106]    The graph  20  contains two curves  30 ,  32 . The first curve  30  depicts the vibrator force  26  versus frequency  24  for the capacitor  18  placed in series with the high frequency vibrator  8 . The second curve  32  depicts the vibrator force  26  versus frequency  24  for a high frequency vibrator  8  with no capacitor  18  in series with the high frequency vibrator  8 . 
         [0107]      FIG. 2  C) shows a graph  22  illustrating two current curves  34 ,  36 , where the current  28  (e.g. measured in units of mA) is depicted as function of frequency  24 . The curve  34  depicts the current curve of a high frequency vibrator  8  with a capacitor  18  (the capacitor  18  is placed in series with the high frequency vibrator  8 ). The curve  36  depicts the current curve of a high frequency vibrator  8  without a capacitor. 
         [0108]    A first frequency  38  corresponding to 900 Hz and a second frequency  40  corresponding to 2500 Hz are indicated with vertical dotted lines. 
         [0109]    In the low frequency range the curve  34  takes close-to-zero values. Hereby, it is possible to cut off the current consumption  28  of the high frequency vibrator  8  in the low frequency range below the frequency  38 . 
         [0110]      FIG. 3  A) shows a graph  42  illustrating different vibrator force curves  44 ,  46 ,  48 ,  50  as function of frequency  24 . The vibrator force  26  is measured in dB μN and three frequency areas  52 ,  52 ′,  52 ″ are indicated. Moreover the frequencies corresponding to 600 Hz, 900 Hz and 2.5 kHz are indicated with vertical dotted lines. 
         [0111]    The curve  48  illustrates the vibrator force  26  versus frequency  24  for a (prior art) single vibrator solution. It can be seen that the vibrator force  26  is low both in the first frequency area  52  and in the third frequency area  52 ″. 
         [0112]    The curve  50  illustrates the vibrator force  26  of a low frequency vibrator of a hearing aid according to the disclosure. The vibrator force  26  is depicted versus frequency  24 . It can be seen that the vibrator force  26  is significantly higher than the curve  48  in the first frequency area  52 , but very low in the third frequency area  52 ″. 
         [0113]    The curve  46  illustrates the vibrator force  26  of a high frequency vibrator of a hearing aid according to the disclosure. The vibrator force  26  is depicted versus frequency  24 . It can be seen that the vibrator force  26  is lower than both of the curves  48 ,  50 , in the first frequency area  52 , however, in the third and high frequency area  52 ″ is the vibrator force  26  significantly larger than both of the curves  48 ,  50 . 
         [0114]    The curve  44  illustrates the vibrator force  26  of a dual vibrator hearing aid according to the disclosure. The vibrator force  26  is depicted versus frequency  24  and it can be seen that a large vibrator force  26  is achieved in both the first frequency area  52 , the second frequency area  52 ′ and in the third frequency area  52 ″. 
         [0115]    Therefore, the hearing aid according to the disclosure is capable of transferring signals through the skin in an efficient manner. Accordingly, a reliable and operable hearing aid can be achieved. 
         [0116]    The first frequency area  52  includes frequencies up to 600 Hz and represents the low frequency area—an area in which the vibrators are not in phase. 
         [0117]    The second frequency area  52 ′ extends from 600 Hz to 2.5 kHz. The vibrators are in phase in this frequency area  52 ′. 
         [0118]    The third and high frequency area  52 ″ extends above 2.5 kHz. The vibrators are not in phase in the third frequency area  52 ″. 
         [0119]      FIG. 3  B) shows a graph  54  illustrating three current curves  56 ,  58 ,  60 . The graph  54  depicts current  28  versus frequency  24 . The first curve  56  shows the current curve of a (prior art) single vibrator. 
         [0120]    The second curve  58  shows the current curve of a low frequency vibrator according to the disclosure. The third curve  60  shows the current curve of a high frequency vibrator according to the disclosure. 
         [0121]    The first frequency area  52 , the second frequency area  52 ′ and the third frequency area  52 ″ are shown in  FIG. 3  A) are also shown in  FIG. 3  B). 
         [0122]    Moreover, the like in  FIG. 3  A), the frequencies corresponding to 600 Hz, 900 Hz and 2.5 kHz are indicated with vertical dotted lines. 
         [0123]      FIG. 3  C) shows a graph  62  illustrating the current curve  64  (current  28  versus frequency  24 ) of a dual vibrator according to the disclosure. The graph  62  contains the frequencies 600 Hz, 900 Hz and 2.5 kHz indicated with vertical dotted lines in the same way as in  FIG. 3  A) and in  FIG. 3  B). When compared to  FIG. 3  B), it can be seen that the current curve  64  shown in  FIG. 3  C) varies much less than the current curve  56  of a (prior art) single vibrator. 
         [0124]      FIG. 4  illustrates a schematically cross-sectional view of the head  76  of a hearing aid user  74  wearing a hearing aid device  80  according to the disclosure. The hearing aid device  80  comprises an audio processer  4  that is attached to the skin  82  above the ear  78  of the hearing aid user  74 . 
         [0125]    The hearing aid device  80  comprises an implantable part  2  consisting of a housing  68  having a magnet  10  that is not visible but can be seen in  FIG. 1  A). The audio processer  4  comprises an external magnet  11  that is attracted to the magnet  10  within the magnet housing  68  of the implantable part  2  of the hearing aid device  80 . Accordingly, the audio processer  4  is detachably attached to the skin  82  by means of magnetic attraction between the magnets within the magnet housing  68  and within the audio processer  4 . The implantable part  2  is implanted in the tissue between the skin  82  and the skull bone  72 . 
         [0126]    The implantable part  2  of the hearing aid device  80  comprises a vibrator housing  66  with a low frequency vibrator  6  and a high frequency vibrator  8  arranged to provide a structure-borne acoustic signal transcutaneously to the skull bone  72 . A modulator (shown in  FIG. 1  A) is arranged in a side housing  70  extending between the magnet housing  68  and the vibrator housing  66 . The modulator is connected to the vibrator housing  66 . 
         [0127]    The hearing aid device  80  according to the disclosure provides an alternative to the prior art hearing aid devices—an alternative that is cosmetically appealing and is reliable. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           2  Implantable part 
           4  Audio processor 
           6  Low frequency vibrator 
           8  High frequency vibrator 
           10  Magnet 
           11  External magnet 
           12  Coil 
           14  Demodulator 
           16  Driver integrated circuit 
           18  Capacitor 
           20 ,  22  Graph 
           24  Frequency 
           26  Force [dB μN] 
           28  Current [mA] 
           30 ,  32 ,  34 ,  36  Curve 
           38  First frequency 
           40  Second frequency 
           42  Graph 
           44 ,  46 ,  48 ,  50  Curve 
           52 ,  52 ′,  52 ″ Frequency area 
           54  Graph 
           56 ,  58 ,  60  Curve 
           62  Graph 
           64  Curve 
           66 ,  68 ,  70  Housing 
           72  Bone 
           74  Hearing aid user 
           76  Head 
           78  Ear 
           80  Hearing aid device 
           82  Skin