The first prototype was machined in aluminum, see Figure 8, and we used the, at that time, very popular Widex A 6 as processor and a naked Oticon transducer elastically suspended to the driving side of the housing. This design was successfully tested in our lab in 1979 on one of the first three patients.

Figure 8: First single housing BAHA tested on a patient.

Before a series of this design could be manufactured we had to develop an efficient coupling to be used to connect and disconnect the BAHA. Looking back this was probably the most challenging task that will be described in the next paragraph.
 
The coupling

There are more demands on a coupling that you might not think of in the first place. We had to learn it the hard way. The most important demands are that the coupling must:

be possible to connect/ disconnect on daily bases (also for those with impaired manual dexterity),

maintain the high mechanical impedance otherwise feedback problems will occur,

have a connection force exceeding the dynamic forces otherwise it will introduce distortion,

have an overload release function to protect the implant external excess forces,
have a low profile i.e. small size of protruding part,

withstand debris from skin, sweat, cosmetic products, hair etc, and

be possible to manufacture in a reproducible way and to a reasonably cost.

This was a real challenge to accomplish and the basic principles tested were various versions of bayonet couplings, magnetic couplings and snap couplings. Some examples of prototypes that were tested in these extensive trials over many years are shown in Figure 9.

Figure 9: Examples of different couplings tested: external snap ( 1 ), internal snap ( 2 ), magnetic ( 3 ), bayonet ( 4 ).

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