Electro-Voice 664 Dynamic Model--1958-64
The EV-664 was primarily a communications
microphone with highly directional characteristics,
comprising of three sound canceling entrances
into cavities at different fixed distances behind the
diaphragm, thus providing a front to back spacing
that varied in distance (from the diaphragm)
inversely with the frequency. The 664
demonstrated a highly directional cardioid pattern.
The cavities provided the necessary amplitude and
phase conditions to result in a pattern that was
constant over a wide frequency range.