Audio
Noise Floor
01 — Definition
The level of residual background noise in a system when no signal is passing. It comes from thermal noise in resistors and transistors, power supply noise and electromagnetic interference. Equivalent Input Noise (EIN) for mic preamps runs into a physical limit: a 150 Ω source with an ideal noiseless amplifier gives −130.8 dBu, and very good real preamps land around −128 dBu. The noise floor sets the bottom of the dynamic range, and how much gain you can apply before it becomes audible.
02 — In the field
A preamp with −128 dBu EIN and a microphone of −55 dBV sensitivity leaves you roughly 73 dB of headroom before the noise floor. On a quiet acoustic guitar you will hear the preamp hiss at moderate gain: choose a low-noise preamp for quiet sources.