Because microphone elements are very small, the electric signal generated is very small in comparison with line level devices such as instruments and mixing consoles. Because of this microphones require preamplifiers to boost its output signal to line level. A preamp is usually found in the mixer or recorder the microphone is plugged into.
Note: Impedance converters inside condenser microphones should not be confused with a preamp, a preamp is still required for a condenser microphone.
A microphone’s output level is always related to a specific input sound pressure level and frequency, usually at 1000Hz. This is an indicator of the input sensitivity of the microphone, and a more sensitive microphone will have higher output at a given sound pressure level.
To make this easy, let’s say you have a microphone with in input sensitivity of 1, and an output of 2, and another microphone with an input sensitivity of 2, and an output of 4 (these numbers don’t relate to anything, they are just used for example). So the more sensitive the microphone is, the higher output it will have (most of the time).
For technical terms, I will refer you to the Yamaha Sound Reinforcement Handbook, Page 126:
Two reference SPL levels are commonly used for microphone ouput level specifications. These are 74 dB SPL
(which is the level of an average speaking voice at a distance of 3 feet) and 94 dB SPL (corresponding to a relatively loud speaking voice at 1 foot). These levels may also be expressed as:
74 dB SPL == 1 microbar or 1 dyne/cm/
94 dB SPL == 10 microbar or 10 dyne/cmf
Microbars and dynes-per-squarecentimeter are both units of pressure. The microphone output signal level is given in dB, with either of two different references: dBV (dB re 1 volt) and dBm (dB re 1 milliwatt). The first
is a voltage reference and the second is a power reference, so the two units are not directly comparable without
knowing the specific load impedance (see Sections 13 and 14). A typical microphone sensitivity specification, then, might read:
-74 dBm re ImW/microbar
Translated, this means that the microphone will deliver a signal at 1 microbar whose power is 74 dB below
one milliwatt. To determine the signal voltage that this power level corresponds to, we need to know the load
A more useful form of sensitivity specification is:
Output level of -47 dBVat
This specification needs no translation, and allows direct and simple calculation of the output signal voltage at
various sound pressure levels.
-By John Redick