Basic Audio for Video

by Mark Tomlonson
Published on Jun 22, 2001 12:00 AM



Every electrical circuit has three physical components - resistance, capacitance and inductance - that determine how much current will flow in the circuit. Changing any one of these three will change the amount of current. Micophones work by transforming the changing air pressure of sound into a changing electrical current. We can divide microphones into three groups, based on what physical component of the circuit they change.

Microphones that change the resistance of a circuit are found in museums more than anywhere else, so we won't discuss that group here.

Microphones that change the capacitance of a circuit are called condenser microphones. "Condenser" is the old term for what is now called "capacitor". Condenser mics require a power supply to work. This is because the changes in the electrical circuit that a condenser mic makes are so small that an amplifier must be placed in the microphone housing itself, or within a few feet of it. Some condenser mics are powered by batteries, or from "phantom" power. It's called "phantom" because it has no effect on other types of microphones (except one - see below).

Condenser mics give a higher degree of fidelity than any other type. Certain models are also capable of offering a continuous change in pattern from omni directional to cardiod just by turning a knob. Some models can change pattern by changing the front element of the microphone. These systems have heads in all five microphone patterns, as well as some with special features. For example, a long thin gooseneck between the microphone element and the case/amplifier housing, a type popular in televison and other places where the microphone needs to be as unobtrusive as possible. Condensers are the most common type found in a studio environment because of their fidelity and flexibility. They also tend to be found in studio environments because they are generally more fragile than other types (except one - see below) and need the protection from rain and humidity that a studio offers. However, they perform so well that certain models have been designed with field work in mind.

Microphones that change the inductance of a circuit are called dynamic microphones. Dynamics are further divided into two groups: ribbon microphones and coil microphones.

Ribbon microphones have no diaphram like other types, but suspend a small piece of foil between two magnets. There is no physical connection to the foil, the suspension is strictly by magnetic force. Changing air pressure moves the foil back and forth, creating the changes in inductance. Ribbon microphones are extremely fragile. A sharp gust of air can blow the foil out from between the magnets. They are very susceptible to the effects of moisture and humidity. Plugging a ribbon microphone into phantom power will destroy the magnets that suspend the ribbon. For all of these reasons, ribbon microphones are rarely used in field work, and infrequently in the studio. But they do have a uniquely transparent yet warm sound that keeps them in demand for certain applications.

Coil microphones, usually just called "dynamic microphones", are by far the most rugged type. I have witnessed a factory representative drive a nail into a piece of wood with one and have it still work afterwards. (I would not suggest trying this yourself, however!) Dynamic microphones are available in all patterns. Some dynamics can switch patterns, but not on the continuous basis of some condensers. Dynamics also do not have the type of interchangable head system found on some condensers. The fidelity of a dynamic microphone can be very, very good, with a top-quality dynamic outperforming a cheap condenser. However, as group, dynamics are not as good as condensers in this regard. But their ruggedness makes them the most popular for video news and rough location work. They are also cheaper to manufacture than other types, which also contributes to their popularity.