INDIVIDUAL NOISE FLOORS
Microphones and preamps each have their own noise floors.
When selecting a preamp, it’s important to know to what degree the preamp’s noise degrades the noise of the mic.
Different mic technologies use different terminology to describe noise.
Dynamic microphone data sheets rarely list noise as a specification because there is no active circuitry to generate noise; there is only a magnet and a coil. This category of mic is properly called electromagnetic or electrodynamic.
Output noise is very low, so low it’s not listed. However, some noise is generated, and this can be calculated by knowing the mic’s impedance.
Obtain the dynamic microphone impedance rating from the data sheet and use Table 2 to convert that into units of dBu, A-weighted. This noise is the white noise generated by the resistance of the wire used to create the coil, plus a correction factor of 5 dB for A weighting. (This is somewhat arbitrary, as true A weighting may decrease the level anywhere from 3-6 dB depending upon the nature of the noise, but agrees with Holman’s article and measured results).
Table 2: Output noise for dynamic mics (20 Hz - 20 kHz, 20 degrees C/68 degrees F). (click to enlarge)
The noise of the measuring standard 150 ohms (200 ohms for Europe) source resistor makes a good noise reference point. In Table 2, it equates to -136 dBu (A-weighted; -131 dBu when not). This means that you cannot have an operating mic stage, with a 150 ohm source, quieter than -136 dBu (A-weighted, 20°C/68°F, 20 kHz BW). Looking at Table 2 confirms that dynamic microphones, indeed, are quiet.
Use Table 3 to compare microphone output noise with preamplifier equivalent input noise (EIN). As an example, if your dynamic microphone’s output noise equals -136 dBu, and you are considering a preamplifier with a rated EIN of -136 dBu, then the difference between them is 0 dB.
Table 3: Output noise for condenser mics (dBu). (click to enlarge)
Table 3 illustrates that this preamp with this microphone will degrade the total noise by 3 dB. That is, the combination of mic and preamp adds 3 dB noise to the total. More on how this table works shortly.
Condenser, capacitor, or more properly, electrostatic microphone technology involves a polarizing voltage network and at least a buffer transistor built into the microphone housing, if not an entire preamp/biasing/transformer network - all of which contribute noise to the output. Electrostatic microphones are quite noisy compared to dynamic designs, but are very popular for other reasons.
Different manufacturers use different terminology on their electrostatic microphone specification sheets for noise: Self-Noise, Equivalent Noise SPL, Equivalent Noise Level, Noise Floor, and just plain Noise all describe the same specification. Microphone noise is referenced to the equivalent sound pressure level that would cause the same amount of output noise voltage and is normally A-weighted.