It is always a long way to go from the theoretical idea until the idea is proven to work in a prototype. A first prototype of a single housing design was tested in our lab 1978, see Figure 6. It consisted of an Oticon transducer now elastically suspended to a housing to which also a conventional BTE device was glued.

Figure 6: First prototype of a single housing BAHA.

Even if it worked, a number of new questions arrived; How to measure the output of the device i.e. measure the frequency response of the device? How to connect/disconnect the device to the implant on daily basis? How to adapt the frequency response of the transducer/ amplifier?

It was absolutely necessary to develop a method to objectively measure the mechanical output of the BAHA otherwise it was like grope for one’s way in the dark. The measuring device we needed must have a load impedance that have mechanical properties relevant for a “typical” patient having a titanium implant. Also, the measuring device should objectively measure the mechanical output of the BAHA which could be force, acceleration, velocity or displacement at the connection point. A conventional artificial mastoid that is available for use in conventional bone conduction devices could not be used since the impedance of skin covered skull and skin penetrated skull are quite different.

After a number of mechanical impedance measurement of patients, of different “skull” prototypes and model simulations a new design was suggested. In this new design the mechanical impedance of the human skull was not replicated but simulated the skull properties in a way that the force output was correctly measured. This device was capable of producing an electrical signal that is proportional to the force output from the BAHA transducer almost identical to that obtained if the device is attached to an implant on a patient. This new design is shown in Figure 7 (left) and is called Skull simulator. It was described by Håkansson and Carlsson 1989 and in a patent and this design is still used when manufacturing and repairing a BAHA. It is used together with an anechoic chamber and a measurement test station (Figure 7 middle) and it produces the frequency response chart (output force level at a specified input sound level). This equipment is also an invaluable tool in the research regarding the BAHA.

Figure 7: Skull simulator for objective force output measurements of the BAHA.

Using an early version of the Skull simulator it was found that the first single housing prototype (Figure 6) worked in the lab environment. Then the work started to develop a more realistic single housing prototype that could be used by the patients on daily bases.

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