The diaphragm of a microphone is a thin plastic or metallic membrane that catches sound vibrations from the air. It is usually round, always thin, and hopefully very light in weight.
Some microphones have a diaphragm up to 25 mm (one inch) or more in diameter. Others have diaphragms as small as 3 mm (around one eighth of an inch). So what's the difference?
The answer is that a small diaphragm can be lighter and therefore more responsive to tiny sound vibrations. Microphones used for acoustic measurements, where total accuracy is vital, commonly have small diaphragms - around 6 mm. The other advantage of the small diaphragm in this respect is that in any microphone, when sound arrives at the diaphragm from the side, it will strike one side of the diaphragm before the other. If the diaphragm is large then this can cause cancellation of the vibrations. If the diaphragm is small, then the time delay can be so short that this effect is negligible.
So if small-diaphragm microphones (15 mm or smaller) are more accurate, why do we commonly use large-diaphragm (25mm or larger) microphones in the studio?
The answer is that generally they are perceived to have a 'warmer' sound. This is because all diaphragms, and in fact all objects, have a 'resonant frequency'. This is the frequency at which they tend to vibrate naturally. In the diaphragm of a microphone, energy at or around the resonant frequency will cause the diaphragm to vibrate very easily. Manufacturers do what they can to 'damp' this vibration, but its effects are still audible.
Although the large-diaphragm microphone is not entirely accurate, it turns out that its characteristic sound is actually rather pleasant, particularly for vocals. So large-diaphragm microphones are now almost universal in this application.
Another drawback of the large-diaphragm microphone is that although it may be subjectively very good for sounds arriving from directly in front, the time delay effect for sounds coming from any angle other than directly on-axis can make off-axis pickup rather unpleasant. It's something to listen out very carefully for.
As a general rule, large-diaphragm microphones can be subjectively very good for vocals and single instruments on-axis. For general room pickup, the more accurate small diaphragm type is preferred.
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