Friis L., Ohlrich M.

There are many aesthetics and structural design requirements to modern hearing aids. These have resulted in designs where the loudspeaker and microphones are placed very close to one another. As a consequence of this, problems with vibro-acoustic transmission from the loudspeaker to the microphones often arise. Vibrations and sound pressure are picked up by the microphones and an unwanted electrical/vibro-acoustical loop is formed. The phenomenon is called feedback, and at certain critical gain levels in the hearing aid this causes an uncomfortable howling sound. This feedback problem is investigated by combined electrical and vibro-acoustical measurements on a hearing aid. These measurements show a frequency dependent relationship between electrical input voltage to the loudspeaker and the voltage generated on the microphone terminals when the loudspeaker and microphones are electrically disconnected. This transfer function is shown to be strongly related to the microphone vibration caused by vibration and sound pressure from the loudspeaker. Moreover, the vibratory part of the vibro-acoustic transmission from the loudspeaker to the microphones is examined through a simple mathematical vibration model of the hearing aid. This simple model includes the main parts of the hearing aid such as the loudspeaker (telephone), hook, shells, battery and connections. By employing mobility synthesis, these components are modeled as lumped masses, springs and beam components, which are connected to one another. Results from the vibration model reveal a complicated pattern of resonances governed by the elastic components and their interaction with one another. Especially above 1000 Hz distinct resonances occur. These resonances, which are primarily caused by component modes related to the shells and the hook, produce strong microphone vibrations.