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“The 640A/AA condenser mic came about from Western Electric’s need for a reliable calibration mic and the conclusion that the 394 condenser was too big and bulky to do the job. In 1932, a miniature condenser microphone system with a one-inch diaphragm called the D96436 was successfully tested and re-designated the 640A. In 1942, Bell Labs’ testing of the 640A revealed it to be susceptible to changes in ambient temperature, pressure and relative humidity. They changed the capsule diaphragm material from aluminum alloy to highly corrosion-resisting steel and changed other internal components to lessen this effect. They re-designated this capsule as the 640AA. The 640AA was the mainstay calibration mic of anyone involved in serious audio work, including such entities as RCA, Altec and, of course, the National Bureau of Standards. Western Electric eventually decided that with the small demand for calibration mics, it would be better to buy them as needed rather than making them. They turned over the manufacturing of calibration mics to Brüel & Kjær, which constructed the 4160 to meet Western Electric needs. Once the 4160 was in production, 640AA capsules ceased to be made.”
Commentary by Jim Webb in Twelve Microphones That Made History.
Presented here with the permission of the author.
The following twelve images are provided via the courtesy of Mr. Tré Perry.
The following ten images are provided via the courtesy of Dr. Warren S. Goff.
Obviously this mic has been modified.