Microphone
The microphone is the input transducer par excellence of the hearing-aid: through a very thin metal foil placed in an electrical field it picks up the sound waves from the surrounding environment and transforms them (hence the name transducer) into corresponding variations in electrical voltage, subsequently sending them to the hearing-aid’s amplification section.
Telephone coil
Next to the microphone is the telephone coil: an electrical winding that picks up the electromagnetic field generated by the telephone. The user can choose to shift a special switch on the appliance to the position T, when talking on the phone, in order to cut out the the microphone and the sound signals coming from the outside. Some hearing-aids can keep the microphone and coil connected in parallel, thus remaining in contact with the surrounding environment, by turning the switch to MT. Certain modern telephones bar the use of the telephone coil, for the simple reason that the receivers placed in the telephone are of the piezoelectric type, i.e. they do not generate electromagnetic fields picked up by the coil
Audio input
Together with the microphone and the telephone coil there is a third input to the hearing-aid: the audio input. Not a transducer, but simply a small “electric socket” for connecting a series of supplementary devices, and creating a direct connection, hence free of interference and distortion.
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