Noise Floor

Anything below the orange floor drowns in hiss; the blue-to-clip gap is your dynamic range.

What it is

The constant low-level hiss and hum your gear makes even with no signal playing.

Key facts

• Noise floor = the residual signal level present with NO intended audio, measured in dBu, dBV or dBFS.
• Dynamic range = peak level minus noise floor. Bigger gap = cleaner, more headroom for quiet detail.
• Thermal (Johnson) noise of a ~150 Ohm source at room temp over 20 Hz to 20 kHz = about -132 dBu. This is the theoretical electronic floor (a 1 kOhm resistor is noisier, around -123 dBu).
• 16-bit audio = 96 dB max dynamic range (6.02 dB per bit x 16). 24-bit = 144 dB. Formula: dynamic range = 6.02 x bits + 1.76 dB.
• Each extra bit of resolution lowers the digital noise floor by 6 dB.
• 0 dBFS = digital clipping ceiling. You want signal high and noise floor low, far below 0.
• Pro line level = +4 dBu (1.228 V RMS). Consumer = -10 dBV (0.316 V). The +4 standard sits ~12 dB hotter, riding further above the floor.
• Doubling a signal's voltage = +6 dB. Doubling acoustic distance = -6 dB (inverse square law).
• Half the power = -3 dB. Smallest level change a human reliably hears = about 1 dB; ~3 dB sounds 'a bit louder'; ~10 dB sounds 'twice as loud'.
• Quiet concert hall ambient noise floor sits around 25 to 30 dB SPL; gear hiss should stay below the acoustic noise so it is masked.

How it works

1. Every resistor, transistor, op-amp, cable and converter emits tiny random thermal and shot noise.
2. These add up to a constant residual level with nothing plugged in or playing.
3. Adding gain raises both your signal AND the noise floor together.
4. Signals quieter than the floor get buried in the hiss and are lost.
5. Cascading gain stages or cheap preamps stack each stage's noise, raising the floor.
6. Good gain staging keeps the wanted signal far above the floor so noise stays inaudible.

Real examples

• Turn a mic channel's gain to max with no one talking - that rising hiss is the noise floor.
• A cheap DI box adds audible hum to an acoustic guitar's quiet fingerpicking.
• A laptop on the same dirty power as the desk injects a 50 Hz mains buzz into the PA.
• Soft spoken-word at an awards night vanishes under amp hiss when gain staging is sloppy.
• Recording at -40 dBFS then normalising later drags the noise floor up loud and obvious.

How it helps in live sound

• Set gain so peaks hit around -18 to -12 dBFS on a digital desk: signal well above the floor, room before clip.
• Use line level (+4 dBu) and short balanced XLR runs to keep the floor down.
• Kill unused open channels or pull their faders - every live channel adds its noise.
• Use a high-pass filter ~80 to 100 Hz to bin low rumble and mains hum on vocals.
• Star-ground / single power source the rig to stop 50 Hz earth-loop hum (Australia = 50 Hz mains).
• For quiet talkers, a hotter clean preamp beats fader push - boosting later just lifts the hiss too.