One of the questions that comes up when designing a microphone circuit is what the voltage levels should be. What’s “normal” mic level? What are the limits?

Mic levels vary a lot for a given sound source. The sensitivity of a mic is usually given in millivolts per pascal. A pascal is a unit of pressure of one newton per square meter or about 0.00001 atmosphere. A sound pressure of one pascal is 94dBA or about as loud as riding a subway car. See  http://www.noisehelp.com/noise-level-chart.html

Regard a pascal as on the loud side, equivalent to close miking a voice or instrument in the studio, or an orchestra at forte level heard from the front of the hall. Let’s consult

http://recordinghacks.com and see what various mics would be putting out. At the quiet end, we’d have passive ribbons from AEA and Royer with around 1 to 3mV/Pa or around -60 to -50dBV at 94dBA SPL. For these, we need a preamp with 60+dB of very quiet gain. Next we have dynamics like the Shure SM57 in the 2mV/Pa range, but these are commonly used up close. In the middle we find small diaphragm condensers at around 10mV/Pa and large diaphragm studio condensers like the AKG C-414 and Neumann u-87 at around 20mV/Pa. At the loud end, there are condensers with sensitivities of 40mV/Pa or more. At that sensitivity, a pad is mandatory for loud sources, but S/N ratio will be very good for quiet sounds because the preamp gain can be turned down.

So what is “mic level”? It can range from a microvolt from a ribbon mic 6 feet from an acoustic guitar to over a volt from a condenser close miking drums. In a studio, typical average levels are a millivolt to 50 millivolts. FET mics with transformers typically start to compress at the upper end of that range, and clip in the 100--200mV area. Transformer-less mics often clip at more than a volt.

A typical modern transformer-less mic input on a digital interface has an input gain trim range of 20 to 60dB feeding a converter typically designed for 2V maximum, which means it can handle up to 200mV before digital clipping. Some are better, but for the home studio, this is what you can count on. The background noise level of a decent condenser mic is around a microvolt. Mic level therefore covers a range roughly of a microvolt to 100mV, or 100dB. When measuring mic preamps and mic guts, figure a normal signal level of 1 to 10mV, but test at the top and bottom of the range.