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 just a short time before that has gone on to become a fixture of our everyday lives — transistor technology. Along with reducing the size and weight of hearing aids, the shift from sub-miniature tubes to transistors brought many other advantages. These new developments helped push the Epsilon, which was designed especially for moderate hearing loss, into becoming the top-selling Siemens hearing aid in Germany and abroad within a short time.

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Phonophor Epsilon, 1959

Sub-miniature tubes represented an important step towards ever-smaller hearing aids. But few years would go by before they were replaced by a revolutionary new technology: the transistor. Development began on the transistor in the 1920s by many different researchers, most of them working independently. Transistors were ready for series production in 1954. Used as amplifiers, transistors offered advantages similar to those of sub-miniature tubes, while outperforming them in many respects.

Siemens developed the Phonophor Epsilon, a fully transistor-based hearing aid distinct from others of its kind, starting with outward appearance. The Epsilon was much smaller than devices that used sub-miniature tubes. Its light weight was immediately apparent when held — it was lighter than a tennis ball and weighed noticeably less than earlier hearing aids. This leap was made possible primarily because of 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.

A newly-developed microphone contributed to this. Unlike older crystal microphones, the new model from Siemens was also based on a transistor, one that absorbed 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 to extreme temperature fluctuations — from high temperatures in the summer to bitter cold in the winter — better 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).