(3) Shotgun Microphones. These special purpose type microphones, sometimes referred to
as "machine-gun" microphones, are highly directional and designed to pick up sounds over relatively
great distances. The shotgun microphone is a rugged, omnidirectional microphone with tubes that range
from 2 inches to 5 feet in length. These tubes make the omnidirectional microphone highly directional.
The long barreled shotgun microphone, aimed at a distant sound source, follows that sound source much
as a gun follows a moving target. Unfortunately, this microphone is quite heavy and must rest on a
special pedestal which permits simultaneous tilting and panning. Despite its highly directional feature,
the audio pickup is adequate, at best, and is generally used when quality is the least important factor in
the sound transmission.
(4) Parabolic Reflector Microphones. The parabolic reflector is one of the earliest devices
developed for picking up sound over relatively great distances. Instead of the long barrel of the shotgun
microphone, a parabolic disc catches and focuses the distant sound waves and directs them into an
ordinary microphone which is placed in the focal point of the parabolic disc. Depending on the size and
material of the disc, only certain frequencies can be picked up. Since high and low frequencies react
quite differently, the overall quality is not satisfactory. So far, parabolic pickups have been used for
broadcasting crowd reactions, band music originating across the stadium, and other sounds that provide
atmosphere rather than specific information. Of course, there is ample room for experimentation in long
distance sound pickup.
(5) Contact Microphones. A device used to pick up sound vibrations from a solid material
such as an electrical guitar is called a contact microphone. For the changing of acoustical energy to
electrical energy any microphone could be used, but a contact microphone is supported only by the
surface, thus eliminating the problem of microphone placement.
(6) Variable Polar Pattern. These are microphones in which the operator changes the polar
pattern manually from omnidirectional to bidirectional, to unidirectional (cardoid), or to a combination
of all three. Microphones of this type, offering adjustable pickup patterns, are termed polydirectional.
An example is RCA 77DX velocity microphone.
(7) Differential Microphone. Noise-cancelling microphones, basically designed for use in
automobiles, aircraft, boats, tanks, public address systems, and industrial plants are used to advantage
where the ambient noise level is greater than 100 db. They discriminate against all sounds originating
more than 1/4 inch from the front of the microphone. This microphone is of the dynamic type and the
pickup pattern is unidirectional.
(8) Impedance. The output of a microphone will either be high impedance working directly
into the circuitry of the input amplifier stages, or low impedance using a 30, 150, or 250 ohm
built for various applications have a tapped output transformer with a screwdriver adjusted switch to
select either high or low impedance outputs.