1. Hearing loops
We now know enough information to compute the magnetic flux density for the field coil unit.
Hearing loops consist of a loop of wire around the perimeter of a room which is fed the audio from a public address audio system as a current (instead of a voltage). This, the magnetic flux inside the loop is the audio signal.
Hearing aids have a "T-coil" feature which picks up this magnetic field and sends the audio signal to the hearing aid’s speaker. They were originally created to pick up the audio from the voice coil of a telephone’s earpiece to give a higher quality pickup of a telephone system instead of using a microphone which picks up all ambient sound.