How far does a microphone diaphragm move with a really loud sound source?
The answer is much less than you may think.
Some microphone users worry about the diaphragm "wearing out" because of use. In reality, a mic diaphragm moves so little that wearing out is not a cause for concern. Let's see how far a microphone diaphragm actually moves when exposed to a loud sound.
120 db SPL is the "threshold of pain" for the human ear. For the sound source, we will use 140 dB SPL. 140 dB SPL is a weather warning siren at 50 feet; a level that would quickly damage one's hearing.
Using this dangerously loud acoustic signal, the diaphragm of a typical professional quality microphone will move a total of 8 microns. A human hair is about 80 microns thick, so the diaphragm moves about 1/10 the diameter of a human hair.
8 microns = .000008 meter
8 microns = .0008 millimeter
8 microns = .00003 inch
8 microns = 1/30,000 of an inch
Source: Microphone Engineering Handbook; ISBN 0-7506-1199-5; copyright 1994