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Recording Sound
Drop a pebble into a still pond. There is a little splash and rings of ripples spread out in all directions. Have you ever paused to think just what is happening? The water hit by the stone does not actually move out across the pond. If it did, it would carry with it all the leaves floating on the surface, and this obviously does not happen. What does spread out is something which makes the water level rise slightly. This something is a pressure wave.
Just as the stone disturbs the water and produces spreading ripples or pressure waves, so sound disturbs the air and produces similar pressure waves. Our ears detect these pressure fluctuations and convert them into nerve impulses. A microphone does a very similar job, converting the fluctuations of air pressure into fluctuations of voltage.
The voltage variations produced by a microphone are usually very small. We must therefore amplify them considerably to obtain sufficient power to operate, say, a loudspeaker. The loudspeaker then converts the variations of voltage or electrical pressure into variations of air pressure. These should constitute an exact replica of the original pressure waves except that they will be of greater or lower intensity according to the degree of amplification used.
Now suppose that in some way we make a detailed record of how the electrical pressure from the microphone varies from instant to instant. Then we need not use the pressure variations immediately to operate a loudspeaker. Instead, we can at any time use our detailed record to construct a sequence of pressure variations exactly similar to those we recorded. When these are applied to a loudspeaker it again reproduces the original sounds.
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