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There has long been a need for a device that would allow the hard-of-hearing patient to use any telephone anytime and place without inconvenience or embarrassment, and I believe a new portable telephone amplifier called the "Phone-Aid" is the answer. It is a pocket-sized amplifier, weighing about 3 oz., which can be easily clipped to any telephone receiver in a moment. The case is molded from a very durable shock-resistant plastic, and its design is so compact and thin that it readily fits the receiver in a manner that does not disturb the normal relation of the mouth to the transmitter.
The amplification is accomplished by an ingenious printed circuit powered by two small batteries and three hermetically sealed transistors. This increases the acoustic output of the telephone by approximately 49 db., thereby providing sufficient amplification for the majority of patients who are obliged to wear hearing aids. With this
Hosmer MN. CLINICAL NOTES PORTABLE TELEPHONE AMPLIFIER. JAMA. 1956;160(4):285. doi:10.1001/jama.1956.02960390035010