| Themes > Science > Physics > Solid State Physics > Properties of Solids > Dynamic Properties > The Microphone |
In order to record and measure acoustic pressure waves, we convert sound energy into electrical energy by carrying out two operations at the same time. First of all the sound waves strike a surface in the microphone which is called a diaphragm, causing it to move to and fro. The second operation is that the movement of the diaphragm changes some property in an electrical circuit. For instance it can cause one plate of a capacitor to move. Alternatively it may be part of an electro-magnetic system, or it may cause changes of resistance. In each case we use these changes to control the current flowing in a circuit. The Carbon Microphone Let us first look at the case where sound waves cause a change in resistance. Carbon is one of the materials used to make resistors. Let us take a lot of Carbon particles and bring them together so that they are only in light contact with each other. If we now measure the resistance from one end to the other we shall find that it is high. However if we apply pressure to the Carbon, the resistance will be much lower. If we connect the Carbon in series with a battery, the current through it will be low when there is no pressure applied, and as we apply pressure the current will get bigger. Carbon gives big changes in resistance when subjected to small changes in pressure, and this means it is very useful in the manufacture of microphones. Sound waves passing through holes in the mouthpiece of a Carbon microphone cause the diaphragm to vibrate. A button attached to the back of the diaphragm is in contact with one side of the Carbon particles, so that a changing pressure is exerted on them making their resistance change. The changing resistance causes a corresponding change in current flowing through the Carbon. Thus we have an electrical signal which varies in sympathy with the original sound wave. The Moving Coil Microphone In this kind of microphone, the diaphragm is attached to a coil of wire and when it vibrates it makes this coil move to and fro in the field of a strong permanent magnet. When a wire moves in a magnetic field, a voltage is induced in it. So, as the coil moves in time with the incoming sound waves, the voltage induced varies in the same way. The Crystal Microphone There are certain crystals, quartz is one of them, which have a property known as the Piezo-Electric Effect. This effect means that when pressure is applied to one of these crystals it generates a voltage. The crystal is mounted in such a way that when the diaphragm moves it causes pressure on one side of the crystal. The voltages which are generated as a result of this pressure change in time with the vibrations of the diaphragm, and so in time with the incoming sound waves. In all these kinds of microphones, and others too, such as condenser microphones (in which capacitance varies as the diaphragm moves), changes in air pressure are converted to analogous changes in some measurable electrical quantity. |
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