The “Birdcage” Microphone

I’d like to cover one more of the mainstays of the so called “Golden Age” microphone, the Western Electric model 639 dual element mic.

As the 1930s were drawing to a close Bell Labs developed an excellent microphone design that was to come into direct competition with RCA’s model 77 series that we covered previously. Up to this point Western Electric, the manufacturing arm of Bell Telephone, had only built omnidirectional or nondirectional mics. They needed to come up with something new.

The Bell Lab engineers came up with a mic that was two mics in one! They paired an “improved” bidirectional ribbon element with a nondirectional dynamic unit similar to their very excellent “Eight Ball” mic. They called this mic a “multi mic” and dubbed it the model 639; it was given the nickname “the birdcage” mic.  By combining the units two outputs the result was the uni or cardioid pattern. This, of course, was the exact same result as RCA obtained with their dual ribbon design without going against RCA’s patents; with a cardioid pattern (heart-shaped) or unidirectional microphone design that didn’t impinge on RCA patents for the dual ribbon unidirectional design of the RCA 77. In fact, up to this point, Western Electric hadn’t made a ribbon microphone but they did invent the dynamic mic design.

The original idea from Bell Labs was to develop a microphone that would be useful in PA sound systems to prevent feedback under poor acoustical conditions. One sound system that had bad acoustics was the US Congressional chamber. Various mics had been tried to no avail. Western Electric did a test in the chamber with the 639 and the sound greatly improved.   The 639 mics were used for decades in the Congressional Hall. This mic became a favorite in the broadcast and motion picture industries as well.

The original model 639-A had 3 basic patterns: C-Cardioid (unidirectional, D-Dynamic (nondirectional) and R-Ribbon (bidirectional).    Later the 639-B came out with three additional variations on the cardioid pattern giving varying degrees of front to back pickup.

Here’s an audio demonstration of the pick up patterns of the model 639 microphone:

When the United Nations building was first opened in New York Western Electric supplied all the microphones used in the General Assembly hall. There were 3 model 639s on the main desk up front and many model 633 Saltshakers used at each nations delegation position in the hall.

As we mentioned in our earlier articles on Western Electric mics in 1949 manufacture of these units was transferred to Altec-Lansing Corp.  Altec continued to make the 633 Saltshaker and the 639 Multi mic clear up into the late ’60s. These mics were very popular during this period and were mainstays in the broadcasting and recording industries for decades. Even today they are still sought after for their excellent quality even though they haven’t been manufactured for some 50 years!

The 639 “Birdcage” was a favorite with announcers and disc jockeys; they loved the deep solid lows and the crisp high frequency pick up of their voices.

The 639 was used on overhead mic booms in the early days of TV as well as in film and recording studios for multi-track recording.

So there you have it, a brief story of one of the great American made high quality microphones of the “Golden Age,” the Western Electric/Altec model 639 “Birdcage”.