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Easy Chair
Fortiphone FM-5
Covert microphone

FM-5 was a miniature dynamic microphone, developed and manufactured from 1954 to approx. 1965 by hearing aid manufacturer Fortiphone in London (UK). The microphone was developed for their new generation of transistorized hearing aids, but was also used for covert listening devices.

Fortiphone was established in 1929 and had its headquarters at Langham House, 408 Regent Street 1 London (UK) [1]. Before WWII, Fortiphone sold hearing aids manufactured by Siemens in Germany. These devices contained valve-based amplifiers and were large and power-hungry.

During WWII, the Fortiphone factories continued to produce hearing aids, but were also adapted for the production of tank radio equipment [2]. Once the war was over, the roles were reversed and Siemens sold Fortiphone's miniaturized valve-based hearing aids from 1949 onwards.
Fortiphone FM5 microphone showing its sound port

This situation continued until 1951, when Siemens introduced its own Phonophor Alpha, and became Fortiphone's main competitor [3]. Fortiphone on the other and had meanwhile started the development of an even smaller device, based on the newly introduced transistor technology.

Several new parts were developed especially for the next generations of hearing aids, including the dynamic miniature FM-5 microphone shown in the image above. Although it may seem large by today's standards, with its 21 x 21 x 8 mm it was one of the smallest microphones of its time.

Apart from microphones, Fortiphone engineers also developed a range of miniature chokes and transformers that greatly reduced the size and weight of their transistorized pocket amplifiers. This technology was patented in the UK in 1954, and in 1955 also as US Patent 2,916,713 [4].
Fortiphone coils and transformers

The new improved and miniaturized components did not go unnoticed. In 1954, whilst doing super secret research on covert listening devices (bugs) for the US Central Intelligence Agency (CIA), under the codename Easy Chair, the Dutch Radar Laboratory (NRP) selected the small FM-5 microphone and the minature transformers and chokes, for the development of new equipment.

In 1955, the NRP managed to secure a contract with the CIA for development of a range of so-called Passive Elements (PEs), the first of which was the Easy Chair Mark I, introduced later that year. Being of a passive nature, the bug did not require a local power source, but was instead powered by a strong RF signal beamed at it by a powerful transmitter in a nearby listening post.

The image on the right shows three versions of the PEs used in the Easy Chair Mark I in 1955 and the Easy Chair Mark II in 1956. The trans­formers are clearly visible in the rightmost one.
Three versions of the PE

In the rightmost example above, the FM-5 microphone is integrated with the device. In 1958, whilst developing the Easy Chair Mark III, NRP engineers even managed to build the entire PE inside the antenna, which was possible because of the tiny Fortiphone transformers. For use in bugs, the Fortiphone FM-4 microphone was succeeded in the early 1960s by the Shure MC-11 and MC-30, and eventually by smaller and more sensitive ones from Knowles, like the BA-1501.

  1. After WWII, the address was 247 Regent Street, London (UK).

Fortiphone FM5 microphone showing its sound port
Fortiphone FM5 dynamic microphone
Fortiphone FM5 microphone compared to the size of a hand
Easy Chair Mark I with Fortiphone FM5 microphone
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Fortiphone FM5 microphone showing its sound port
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Fortiphone FM5 dynamic microphone
3 / 4
Fortiphone FM5 microphone compared to the size of a hand
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Easy Chair Mark I with Fortiphone FM5 microphone

  1. Grace's Guide, Fortiphone
    Retrieved April 2017.

  2. Grace Doe (née Seager), Working for the War Effort
    BBC, WW2 People's War, 30 September 2005.

  3. Siemens, Die Geschichte der Siemens-Hörsysteme, Ein Rückblick
    CC 1622 02144.5. Siemens AG, Febrary 2014. Page 15.

  4. Denis Lionel Johnston, US Patent 2,916,713
    Miniature Transformers and Chokes. Filed on behalf of Fortiphone Ltd.
    15 March 14955, priority date 17 March 1954.

  5. E. Dunkin & D. Johnston,
    Subminiature Transformers and their Application to Junction-Transistor Circuits
    IEEE. IRE Transactions on Components Parts 3(1):30. 44 May 1955.
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Crypto Museum. Created: Wednesday 05 April 2017. Last changed: Wednesday, 07 February 2018 - 08:30 CET.
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