New kind of headphone receiver
R. L. Wallace · The Journal of the Acoustical Society of America · 1975
This talk describes the design of a special-purpose headphone receiver which uses the Western Electric 640AA condenser microphone as the motor element. The acoustic circuit between the 640AA and the ear canal is so chosen that the frequency response at the eardrum is essentially flat from below 20 Hz to about 25 000 Hz.