For persons with impaired hearing, the hearing aid devices which are most commonly used today are those based on the principle that the sound is amplified and fed into the au- ditory meatus and stimulates the eardrum from the outside.

In order to prevent acoustic feedback problems in these devices, the auditory meatus is almost completely plugged by a hearing plug or by the hearing aid device itself.

This causes the user a feeling of pressure, discomfort, and sometimes even eczema. In some cases it even causes the user problems like running ears due to chronic ear in- flammations or infections in the auditory canal.

However, there are other types of sound transmitting hea- ring aids on the market, i e bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibra- tor. The hearing aid device is connected to an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works irrespecti- ve of a disease in the middle ear or not. The bone ancho- ring principle means that the skin is penetrated which ma- kes the vibratory transmission very efficient.

This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of im-

paired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in US Patent No. 4,498, 461 and it is also referred to the BAHAO bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.

Other types of bone conducting hearing aids are described in US Patent 4,904, 233 and in Swedish patent application 0002071-9.

A common feature for the hearing aid devices which have been described so far is that some type of vibratory gene- rating means, vibrators, are required. Different types of vibrators are well known in the art. There are a number of known vibrator principles today. In traditional as well as in bone anchored hearing aid devices it is normally used a vibrator principle which was described already by Bell in 1876. There is a detailed description of this principle applied on a bone anchored, bone conducting hearing aid device in"On Direct Bone Conduction Hearing Devices", Technical Report No. 195, Department of Applied Electro- nics, Chalmers University of Technology, 1990. Other vi- brators of this type are described in Swedish patents 0002072-7 and 0002073-5.

In order to improve the sound quality and reduce the risk for acoustic feed back problems in the hearing aid it is necessary to damp the resonance frequency of the vibrator, i e the resonance frequency which is generated by the mass-spring system, which consists of the counter-acting mass (including coil, magnet etc), and the inner spring in the vibrator. In conventional bone conductors there is no need for any internal damping of this frequency as the skin between the vibrator and the bone has a damping func- tion in itself. When the vibrator is connected directly to

the bone, however, a significant resonance peak is genera- ted in the frequency response characteristics which gives a poor sound quality and feed-back problems.

The above-mentioned problems with the bone anchored hea- ring aids can be solved by providing the vibrator spring with some kind of mechanical damping. Then the original design of the spring has to be changed significantly, for instance it is changed into a sandwich structure in the form of a damping material applied between thin plates.

In this context it is referred to Swedish Patent No.

85. 02426-3 in which it is illustrated a vibrator compri- sing a vibrator plate and a coil which is wound around a bobbin base having a core and two side walls. It also com- prises means for damping the resonance frequency of the vibrator in the form of a spring provided with a layer of a damping material or a built-in damping material.

Also other types of mechanical damping means have been proposed, for instance ferro-fluid damping as described in Swedish patent application 0102206-0. In this case the gap between the vibrator plate and the bobin base, or some ot- her spacing in the vibrator in which a relative movement between two surfaces is generated during the vibratory function, is at least partially filled with a fluid or a gel. The purpose of this fluid or gel is to provide the main part of the damping of the resonance frequency of the vibrator. Preferably the fluid or gel comprises ferro-mag- netic particles, a so-called ferro-fluid, in order to keep the fluid in place and increase the magnetic conductivity in the magnetic circuit.

It has turned out that these types of vibrators with me- chanical damping means in the form of a damping spring or a damping fluid not always give an optimal function of the hearing aid. The damping spring is a mechanically compli- cated and exposed part in the hearing aid and the ferro

fluid damping is also a rather complicated technical solu- tion.

A vibrator spring with an integrated damping in the form of a damping material or mass has also a number of disad- vantages. In the first place, the damping material not on- ly has a damping function but it also gives the spring a more uncontrolled stiffness. This is a serious disadvanta- ge as the spring stiffness is a sensitive parameter in this type of vibrator. If the spring is too weak there is a risk for collaps, on the other hand if the spring is too rigid it has a negative effect on the overall performance.

Furthermore, the damping material has a stiffness which depends on the temperature which means that the performan- ce is seriously effected by temperature changes. If the temperature is too low, the vibrator is significantly wea- ker.

Also, the damping mass has a frequency dependent stiffness which means that the spring becomes more stiff at audio frequencies. This is quite in contrast to what you want as it gives an unnecessarily high resonance frequency compa- red to the case with no damping mass at all, which means that approximately twice as high weight has to be used in order to obtain the same resonance frequency. This is of course not acceptable in the case of small, compact devi- ces.

Secondly, when the damping material is growing older it has a negative effect on the vibratory performance due to the fact that the resonance frequency increases with the increased stiffness of the damping mass. From a manufactu- ring point of view the introduction of a damping mass is not what you want. Even the reliability of the vibrator is seriously effected by the damping mass as there is a ten- dency that the damping mass will be creeping away after the manufacturing process which means a risk for collapse of the vibrator.

A further disadvantage with the mechanical damping is the fact that the degree of efficiency is decreased. In order to dampen the resonance peak a valuable amount of battery power is consumed just in the form of heat generation in the damping mass.

It is an object of the present invention to provide a vi- brator device which has a less number of mechanically sen- sitive parts and which eliminates the above-mentioned dis- advantages. According to the invention the vibrator in it- self has no integrated, mechanical damping, instead the damping is made electronically, so that the signal proces- sing is used for removing the frequency peak. The inven- tion is characterised in that the signal processing circu- it comprises analog or digital electronic filtering means having a frequency response which is adapted to attenuate the signal from the hearing aid microphone at the resonan- ce frequency of the vibrator.

According to a preferred embodiment the filtering means are pre-programmed with a predetermined filter setting.

According to a further preferred embodiment the filter setting is adapted to each individual hearing aid appara- tus in order to eliminate individual variations between different vibrators.

In the following the invention will be described more in detail with reference to the accompanying drawings in which figure 1 schematically illustrates a bone-anchored hearing aid apparatus, figure 2 illustrate the frequency response of the hearing aid apparatus, figure 3 illustrates the frequency response of the filte-

ring means, figure 4 illustrates an electric circuit diagram with ana- log signal processing comprising filtering means according to the invention, and figure 5 illustrates a corresponding circuit diagram with digital signal processing.

Figure 1 illustrates the general principle for a bone-an- chored hearing aid apparatus. The hearing aid apparatus is anchored directly into the skull bone, preferably into the mastoid bone 1 behind the outer ear, by means of a titani- um fixture 2, for instance a titanium fixture of the type which is described in SE 002627-8. The figure shows the two main parts of the hearing aid apparatus, i e the bone anchored part and the hearing aid part 3 which is connec- ted to the bone anchored part by means of a coupling ar- rangement, such as a bayonet coupling or any other type of coupling 4 based on mechanically spring member parts. The bone-anchored hearing aid part comprises, in addition to the titanium fixture, a spacer element or skin penetrating member 5 which is connected to the bone anchored titanium fixture by means of a spacer screw. The fixture is prefe- rably made of titanium as titanium has the ability to be integrated into the surrounding bone tissue, so-called os- seointegration. The hearing aid apparatus picks up the sound via a microphone 7. The signal from the microphone is amplified and filtered in the electronic unit (signal processing means) 8 which unit is powered by means of a battery 9. The amplified signal is supplied to a vibrator 10 in which the electrical signal is converted into vibra- tions which are transferred to the skull bone via said ti- tanium fixture 2. Apart from the fact that the vibrator has no mechanical damping, the hearing aid part with its electronic components should be known per se and the indi- vidual components are therefore not described in any de- tail here.

As mentioned in the introductory portion of our descrip- tion it is necessary to damp the resonance frequency of the vibrator, i e the frequency which is generated in the mass-spring system which comprises the counter-acting mass (including coil, magnet etc) and the inner spring in the vibrator 10, in order to reduce the risk for feed-back problems and poor sound quality due to a deteriorated fre- quency response in the hearing aid apparatus. When the vi- brator is connected directly to the bone, without any in- termediate skin 11 as illustrated in figure 1, it is gene- rated a significant, undesired resonance peak fr in the frequency response characteristics of the hearing aid ap- paratus, which corresponds to the resonance frequency of the vibrator, see figure 2. Such a resonance peak gives rise to sound quality problems as well as feed back pro- blems and it is previously known to damp such a peak me- chanically as described above and with the disadvantages introduced by these mechanically damping arrangements. The desired frequency response characteristics has been indi- cated by dotted lines in the figure.

According to the invention analog or digital filtering me- ans having a frequency response which is adapted to dampen the signal level from the hearing aid microphone just at the resonance frequency fr of the vibrator are arranged in the signal processing chain of the hearing aid apparatus.

Figure 3 illustrates the frequency response of an electro- nic filter arranged to reduce the amplification as much as the desired dampening of the resonance frequency. When such a filter is included into the signal processing chain a desired frequency response characteristics for the hea- ring aid is obtained.

In figure 4 it is illustrated an electric circuit diagram with analog signal processing means and in which the sig- nal processing chain between the microphone 7 and the vi-

brator 10 includes an analog filter 12 according to the invention. Preferably, the filter 12 is included in the same physical unit 13 as the signal processing circuits in the form of an amplifier 14 and an output amplifier 15.

This is the most common form of signal processing arrange- ment of a hearing aid apparatus today. But it is also pre- viously known to use separate units for for example the pre-amplifier and the output amplifier. The filter is in- cluded in the chain before the output amplifier in order to reduce losses in the circuit. The filter in itself can be any type of band suppression filter, a filter type which has already been used in other hearing aid applica- tions, for instance it could be of the type Gennum GA3216.

According to the invention the filter is pre-set to the dedsired frequency, i e the resonance frequency of the vi- brator. Then there are two possibilities: Either each hea- ring aid apparatus is pre-programmed with a standard, pre- determined filter setting, or alternatively, each indivi- dual apparatus is measured and the filter setting is adap- ted to each such apparatus. This latter method eliminates possible variations between individual vibrators.

In figure 5 it is illustrated a digital example in which a digital filter 12'is included in the signal processing chain between the microphone 7 and the vibrator 10. In this case it is included an A/D converter 16 and signal processing means 17 with the electronic filter and a D/A converter 18 including an output amplifier. So even in this case the filter circuit is included in the signal processing chain before the output amplifier. Digital band suppression filters are also known per se and will not be described in any detail here. Like the analog filter exam- pie, the digital filter setting could be pre-programmed or it could be adapted to each individual vibrator.

The invention is not limited to the embodiments which have been illustrated here but can be varied within the scope of the accompanying patent claims.