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Electret condenser microphone

SWM-28 was a high-performance professional miniature condenser microphone with built-in FET amplifier, made around 1971 by an unknown manufacturer for the US Central Intelligence Agency (CIA). It was intended for use in covert operations and was one of the first electret microphones.

The microphone is housed in a stainless steel enclosure that has a diameter of 7.5 mm and is 5 mm high. At the front is a perforated screen that is made of berillium-copper. The SWM-28 was available in two variants: one with a fixed cable and one with solder contacts at the rear.

The image on the right shows the first variant of the SWM-28. It has a one metre long grey teflon cable with a 4-pin female miniature Microtech plug at the end. A suitable socket was supplied with the kit. Also supplied with each new micro­phone, was an individual test report (see below).
SWM-28 electret condenser microphone

The first samples of the SWM-28 1 were delivered to the CIA in April 1971, hence the temporary name XSWM-28 on the initial documentation [A]. The microphones were found to be very suitable for use in combination with covert listening devices (bugs), especially in environments where the fidelity (i.e. the quality of the sound) was important for the intelligibility of the information.

  1. SWM is probably the abbreviation of Surveillance Wired Microphone.

SWM-28 microphone is storage box SWM-28 microphone with 1 metre teflon lead and connector SWM-28 microphone SWM-28 electret condenser microphone 4-pin female Microtech connector Model and S/N on the ring of the connector Socket Measured performance
1 / 8
SWM-28 microphone is storage box
2 / 8
SWM-28 microphone with 1 metre teflon lead and connector
3 / 8
SWM-28 microphone
4 / 8
SWM-28 electret condenser microphone
5 / 8
4-pin female Microtech connector
6 / 8
Model and S/N on the ring of the connector
7 / 8
8 / 8
Measured performance

Each SWM-28 microphone was supplied with an idividual test report that shows the frequency response of the device. Below is an example of such a test report, as supplied with the SWM-28 with serial number 18. The performance was measured with Brüel & Kjær test equipment.

Measured performance characteristics. Click for a better view.

  • Directivity
  • Frequency
    20 - 30,000 Hz (± 4dB)
  • Sensitivity
    0.32 mV/µbar, -70 dbV (re: 1V/µbar)
  • Noise level
    34 dB SPL
  • Max. SPL
    124 dB SPL (3% THD)
  • Vibration
    85 dB SPL
  • AC Hum susc.
    0.1 µV/gauss
  • Environment
    -10 to +60 °C, 0 - 95% RH
  • Impedance
    700 - 2500 ω (0.2 - 0.8V DC)
  • Supply
    1 - 20 V DC
  • Current
    200 µA (max.)
  • Connections
    (1) three solder pads, (2) Belden type 8643 3-conductor shielded cable
  • Dimensions
    0.29" diameter, 0.2" high
  • Weight
    0.9 grams (3 grams including cable)
  1. Technical Manual for XSWM-28 Electret Condenser Microphone (prototype)
    CIA. April 1971.
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Crypto Museum. Created: Sunday 26 March 2017. Last changed: Tuesday, 13 June 2017 - 11:01 CET.
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