An unusual 1936 hearing aid from RCA Sonotone, which later became a major brand in HAs. This unit wasn’t a freestanding amplifier. Practical HAs had to wait for submini tubes in 1947 and especially transistors in 1954.
This was just an earphone or a BONE CONDUCTION transducer to connect to the earphone output of a radio. The transducer is familiar in audiometric settings, but I’ve never seen one in this context!
Lorgnettes were still used with eyeglasses, so RCA tried to extend the fashionable connotation to earglasses.
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Found more detailed pictures in an RCA catalog for dealers.
Note the sliding volume control on a neat little module in the middle of the cord. Slider pots were not common until the ’60s.
Also note the one-paragraph textbook on audiology, which was enough before the profession was credentialized. It’s still enough for most purposes.
Polistra's Mill 