The first directional microphones responded to the difference in pressure between the front and back
of the diaphragm as the soundwave passed by. These were termed pressure-gradient microphones
and exhibited a figure-of-eight polar pattern. The diaphragm in these early microphones was a very
thin aluminum ribbon that was exposed on both sides; as the soundwave moved past it created a very
slight but nonetheless distinct difference in the air pressure on either side of the ribbon. This ribbon
was suspended in a magnetic field and thus generated a small electric current in direct response to its
movement. (ref. Figures 6A and 6B) The term velocity microphone also is commonly applied to ribbon
microphones because the current in the ribbon is directly proportional to the velocity of its motion in
the magnetic field.
Because these were dipoles, if a soundwave approached the diaphragm directly from either the front
or back, the ribbon would move equally. The only difference was the absolute polarity of the electrical
output: sounds arriving from the back produce polarity opposite to those arriving from the front. (ref.
Figure 7A and 7B)
