MIcrophone Design II Reading

4. Large vs Small Diaphragm

Before choosing between a large and a small diaphragm microphone it is important to know the difference in features between them.

Large Diaphragm

Any microphone with a diaphragm larger than (and potentially including) 3/4" is considered to be a Large Diaphragm microphone. In general, Large Diaphragm microphones tend to have a "big" sound that engineers find especially pleasing where a little more character might be advantageous.

Small Diaphragm

Most professionals and manufacturers agree that any diaphragm smaller than 5/8" would be considered a Small Diaphragm. Generally speaking, Small Diaphragm microphones tend to do a good job of capturing high frequency content and transients. They will tend to have a bit more "air" to their sound and often have less coloration than large diaphragm microphones. Most of this is due to the reduced mass of the smaller diaphragm, which allows it to more closely follow any air disturbances it is subjected to.

Differences

A common myth is that large diaphragm mics capture more low frequencies than small diaphragm mics. Sometimes their coloration, due to being less precise in reproducing high frequencies, may make it sound like this is the case. A properly designed small diaphragm mic is more likely to be accurate throughout a wide range of frequencies, whereas the coloration of a large diaphragm mic can tend to enhance certain desirable characteristics in a sound, which sometimes amounts to more apparent bass or low end.

A small diaphragm has a higher self noise due to the fact that the small diaphragm is less compliant and therefore more sensitive to the bombardment of air molecules that causes some of the self noise of a microphone. And since the large diaphragm is softer than the small, it is easier to move and therefore more sensitive – even at very low levels. This means that the small diaphragm, because it’s stiff, can handle a higher sound pressure without clipping or distortion, but is less sensitive and needs more amplification, which also adds a little noise.

When reproducing very high frequencies, large diaphragm microphones have a more limited range than the small diaphragms. This is caused by three factors:

• A large diaphragm tends to break up and will no longer act as a true piston. This phenomenon is also recognized in loudspeaker technology and is the reason why loudspeakers are manufactured with different sizes of diaphragms to handle different frequencies.
  

• The weight of the diaphragm will attenuate the displacement of the diaphragm for higher frequencies.

• The diffraction around the edges of the microphone capsule will limit the microphone's capability to handle very high frequencies.