April 04, 2012, by Rick Chinn
The Altec Lansing 639A/B microphone made its debut in the early 1940s, and was originally sold by Western Electric (WECO) under the same model number.
When the U.S. government forced the breakup of WECO in 1947, Altec spun off to continue manufacturing the WECO sound reinforcement and related products, and they continued to make/offer the 639 for many years afterward.
The microphone earned the nickname of “birdcage” because of its size and body design.
It’s an early unidirectional microphone, and what made it unique was use of two microphone elements: ribbon and dynamic.
The ribbon offered a figure-8 pattern; the dynamic, being a pressure microphone, provided an omnidirectional pattern.
By internally blending the two microphone element outputs, the omni cancelled the rear lobe of the figure-8, creating a cardioid pattern.
Both models included a pattern selector switch. The “A” model offered three settings – ribbon, dynamic and cardioid, while the “B” model supplied three additional settings to modify the cardioid characteristic.
Design credit for this 639 goes to William R. Harry and Robert N. Marshall of Bell Laboratories. They received patent number 2,227,580 in early 1941.
Model 639 Condensed Specs
Sensitivity: -84 dB re 1v/dyne/cm2
Power Output Level: -56 dBm at 10 dynes/cm2
Frequency Range: Uniform from 40 Hz -10 kHz
Impedance: 40 ohms (average value, intended for use with equipment having a rated source impedance of 25 ohms to 50 ohms)
Dimensions: 7 inches by 4.4 inches by 3.4 inches
Weight: 3.25 pounds
References: A History of High Quality Studio Microphones. AES Journal, December 1976; Vintage Broadcast Microphones web site; and Dorrough Electronics web site.