First prototypes of the BAHA

The first three patients were fitted in 1977 with the same conventional bone conductor (Oticon) with a snap connector glued to the housing as is shown in Figure 4.

Figure 4: First prototype was based on a conventional Oticon bone conductor and a conventional behind the ear hearing aid.

In Figure 4 (left) it is shown that no skin reduction was used at that time, just a punched hole through the soft tissues. A conventional Behind The Ear (BTE) hearing aid was used to drive the transducer which was connected via a cable as shown in Figure 4 (right, from the thesis by Håkansson2). This first prototype system, using mainly conventional components, showed that the skin penetration principle worked but there were severe problems with the coupling and with feedback.

It was realized that if this new system should be successful compared with a conventional bone conduction hearing aid it must be a single housing design without cables between transducer and the BTE. This was a real challenge since the vibrating transducer should be incorporated into the same housing as the microphone and thus severe feedback was anticipated.

By doing extensive mechanical impedance measurements; of the skull, of the transducer and of the coupling it was clear that a specially designed suspension system suspending the housing to the part of the system showing the lowest vibration levels could significantly minimize these anticipated feedback problems. It was found that the lowest vibration levels were at the implant side and not on the transducer reaction mass side, and hence the housing (in which the microphone is incorporated) should be elastically suspended to the implant side which is illustrated in Figure 5 (left). This idea was first presented in the proceedings of a conference in 1981. This is still the same principle used in today’s BAHA design as illustrated in Figure 5 (below).

Figure 5: Principle design of a single housing BAHA.

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