|Radio and Electronics (DED Philippinen, 66 p.)|
CARBON MICROPHON is a very old type but still in use when a cheap microphon is desired and fidelity is not so important. CARBON GRANULATES change their resistance, if they are pressed together by an external force. The inner hollow part of the microphon is filled with this type of carbon particles. At the front of the microphon is fixed a very thin sheet of metal which is here the diaphragm, and at the backside is fixed a second metalplate which stands here as an electrode to give contact to the carbon granulates.
When exposed to sound the diaphragm is moved by the air oscillations, and the pressure on the carbon granulates changes according to the frequency of the air-oscillations. Therefore the overall resistance of the carbon granulates changes according to the frequency of the sound.
Is working like a variable capacitor. The diaphragm is made from metal and stands for one plate of the capacitor. It is positioned very near to a second metalsheet with a lot of holes in it a few tens of millimeter inside of the microphone. This second metalplate stands for the second plate of the capacitor. If the diaphragm is hit by soundwaves it moves to and for, and by doing so, the distance between the tow plates changes. As well know from physics, the change of the distances lets also change the capacity of the capacitor. So the whole microphone stands for a capacitor which changes its capacity according to the sound waves hitting the diaphragm.
Here a coil is fixed to a diaphragm made from insulating material (like cardboard):
This coil is positioned free within the gaps of a strong permanent magnet.
If the diaphragm is moved by soundwaves, the coil is moving to and for as well.
This movement causes induction of a voltage in the coil and so this microphon is producing a voltage depending on the frequency of the sound waves.
Here is used the so-called PIEZO EFFECT. If a crystal is exerted to pressure there will appear a voltage across its edges. The force to press is produced again by diaphragms, now positioned in front and behind the crystal. If the diaphragms are moved to and for by air-pressures the microphone generates a low voltage which has the same frequency as the sound wave have it.