North Suburban HAMMOND ORGAN Society

The one thing that all microphones have in common is some type of thin, flexible diaphragm. This is the surface upon which sound waves impinge, and when they do, they make this diaphragm vibrate in a manner that essentially copies the shape of the sound wave in the air. The major differences between the various types of microphones mainly concern the structure of the diaphragm and how the diaphragm vibrations become an audio AC signal that likewise faithfully copies the shape of the diaphragm vibration. The first microphone that we'll look at is the carbon microphone, called that because of its operating principle.

hand-held carbon microphone

Figure 3. A modern, hand-held carbon mic, a push-to-talk mic for conversation only.

In the carbon microphone there is a small capsule or chamber that is filled with carbon granules. In operation, a DC voltage is applied to the carbon granules and causes a current to flow through them. If the granules are loosely packed, the electrical resistance of the pack is higher than it it will be if the granules are packed tightly together. In the carbon microphone, the diaphragm vibrates in response to soundwaves. This vibration slightly alters the packing of the carbon granules, and thus varies the electrical resistance of the entire pack of carbon granules contained in the capsule. This action results in a variation of the current flow through the granule pack which then becomes the electrical output signal of the microphone. You can subsequently amplify, record or even drive a loudspeaker with this signal.

disassembled carbon microphone element

Figure 4. A carbon microphone element taken apart. To the left is the diaphragm that receives the soundwaves. At the right you will notice that some of the carbon powder has spilled from the central region, a risk you take when you open one of these devices.

In figure four we have a view of a disassembled carbon microphone element, downloaded from Wikipedia from the Internet. This picture shows some of the carbon granules that have spilled out of the central region where in normal usage the button at the center of the diaphragm would change the tightness of their packing when it vibrates in response to a sound wave.

The carbon microphone has the advantage of outputting a very strong signal, which in some applications does not even need to be amplified. Unfortunately the carbon microphone is also a very low fidelity device. It is very useful for ordinary speech where a frequency range of 60 to 5000 Hz is more than adequate and good fidelity is not a requisite, but it is absolutely useless for making good quality recordings of music.

It also has a very uneven frequency response which we'll look at later, but this essentially says to us that it adds what is known as "coloration" to whatever sounds it picks up. Coloration in this context means that the microphone's uneven frequency response alters at least to some degree the quality and characteristics of the sounds that it receives.

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