Noise Floor
Definition
The noise floor is the level of the residual noise that a system produces when no signal is present. It comes from thermal noise in resistors and semiconductors, power-supply noise and electromagnetic interference. For microphone preamplifiers, it is specified as equivalent input noise (EIN). The thermal noise of a 150 Ω source over a 20 kHz bandwidth at room temperature is about −130.8 dBu, so even a noiseless amplifier cannot do better with that source, and very good real preamplifiers are specified around −128 dBu. The noise floor sets the bottom of the dynamic range and limits how much gain can be applied before the noise becomes audible.
In the field
A microphone with a sensitivity of −55 dBV/Pa delivers about −52.8 dBu at 94 dB SPL, which is about 75 dB above the −128 dBu equivalent input noise of a good preamplifier. With a quiet source such as a softly played acoustic guitar, the level at the microphone is much lower and the preamplifier hiss becomes audible sooner, so low-noise preamplifiers and sensitive microphones are preferred for quiet sources.
Related lessons
Sources
- Johnson–Nyquist noise (Wikipedia) : tension de bruit thermique √(4kTRB) · en.wikipedia.org
- Ballou, Handbook for Sound Engineers : bruit ramené à l'entrée (EIN) des préamplis micro
- Line level (Wikipedia), conversion dBV/dBu : 0 dBV ≈ +2,2 dBu · en.wikipedia.org
- Ballou, 4e éd. 2008, p. 545 : 94 dB SPL = 1 Pa, référence de sensibilité micro
- AudioTechnology, « Microphones: Noise 2 », bruit thermique d'une source de 150 Ω à 20 °C sur 20 kHz : environ −130,9 dBu · audiotechnology.com