Introduced in 1938, the Western Electric 639A was a multipattern microphone design that offered switch-selectable pickup patterns. Nicknamed the “birdcage” microphone, under its large protective shell and windscreen were two separate mic capsules — dynamic and ribbon. Selecting the dynamic element alone yielded an onmidirectional response; the ribbon capsule offered a figure-8 pattern; and equally combining the output of the two resulted in a directional (yet wide 120°) cardioid response. 

With its flexibility and a (then) wide 40 Hz to 10k Hz frequency response, the microphone was quickly adopted by broadcast, recording and P.A. users and remained in production for years. A later 639B variant featured a choice of six directivity patterns, and beginning in 1941, both the 639A and 639B models were sold and marketed under the Altec brand.