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1954: Phonophor Epsilon. Lighter than a tennis ball, as small as a matchbox

Shortly after the first pocket hearing aids were introduced, Siemens added a new model to its product range, one that was even smaller and only weighed about one-fourth as much: The Phonophor Epsilon weighed only about 50 grams (less than two ounces), including the batteries, and was the size of a matchbox. This was made possible by a discovery that had been made just a short time before, and one that has gone on to become a fixture of our everyday lives today: transistor technology. Along with reducing the size and weight of hearing aids, the shift from subminiature tubes to transistors brought many other advantages. These new developments helped push the Epsilon, which was designed especially for moderate hearing loss, to become the top-selling Siemens hearing aid in Germany and abroad within just a short time.

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Phonophor Epsilon, 1959

Subminiature tubes represented an important step towards ever-smaller hearing aids. But not many years would go by before they were replaced by a revolutionary new technology: the transistor. Developed starting in the 1920s by many different researchers, most of them working independently, transistors were ready for series production in 1954. Used as amplifiers, transistors offer advantages similar to those of subminiature tubes while outperforming them in many respects.

Siemens developed the Phonophor Epsilon, a fully transistor-based hearing aid that was distinct from others of its kind in various ways, starting with outward appearance. The Epsilon was much smaller than devices that used subminiature tubes. Its light weight was immediately apparent when held in the hand; it was lighter than a tennis ball and weighed noticeably less than earlier hearing aids. This leap was made possible primarily because transistors’ low power needs. Older hearing aids had to devote about half their size to a battery, but the Phonophor Epsilon managed with just a button cell.

Transistors not only made the Phonophor Epsilon compact and energy-efficient; they also further enhanced sound quality, especially in the upper frequency range. The newly developed microphone also contributed to this. Unlike older crystal microphones, the new model from Siemens was also Phonophor Epsilon, 1959 Size comparison of subminiature tube and transistor, 1955 Advertising catalogue, 1955 based on a transistor, one that absorbed the sound and converted it electromagnetically. The transistors brought with them a practically unlimited lifespan: They were impact-resistant, and they did not have any cathodes that could age or filaments that could burn out. The Epsilon even stood up better to extreme temperature fluctuations – from high temperatures in the summer to bitter cold in the winter – than older pocket hearing aids. For areas with especially hot climates, Siemens developed an even more rugged version: the Phonophor Epsilon Tropic, which delivered the same excellent performance at temperatures of up to 50 degrees Celsius (122 degrees Fahrenheit).

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