A Cavity Resonator as a Hearing Aid?
The fact that a cavity resonator will respond strongly to a single frequency reportedly led Helmholtz to experiment with it as a hearing aid.
The loss of sensitivity to high frequencies with age (presbycusis) leads to difficulty in understanding speech for older persons. Low frequencies tend to mask the frequencies in the normal maximum sensitivity range (2000-5000 Hz) that you depend upon for clear understanding. If you boost a frequency in that range while discriminating against the direct sounds, you might be able to understand better. The Helmholtz resonator acts as a band-pass filter at the resonant frequency.
This is a set of brass Helmholtz resonators tuned to a scale of frequencies. This set is part of the sound demonstration equipment at the University of Utah in Salt Lake City.
Today's miniature hearing aids are quite a contrast to the Helmholtz resonator type devices which had to be held to the ear.