Most people know white noise, pink noise, and brown noise. Fewer know about grey noise. Which is a shame, because it might be the most interesting one.
Here's the problem with white noise.
White noise has equal energy at every frequency. Mathematically flat. But your ears don't hear frequencies equally. You're far more sensitive to sounds around 2–4 kHz (where speech lives) than to 60 Hz or 14 kHz. So white noise sounds harsh and hissy, even though it's technically balanced.
Pink noise compensates for this somewhat. It drops 3 dB per octave, which sounds warmer and more natural. But it's still not perceptually flat.
Grey noise takes a different approach entirely.
Instead of being calibrated to a measurement microphone (like white noise) or to a simple mathematical curve (like pink noise), grey noise is calibrated to how the human ear actually perceives loudness, using the ISO 226 equal-loudness contours.
The ISO 226 standard, published by the International Organization for Standardization, maps out the sound pressure levels at which different frequencies are perceived as equally loud by the average human ear. At low listening levels, you need significantly more energy at 60 Hz to perceive it as "the same loudness" as 1 kHz. The curve is not flat. It dips in the midrange where your ears are most sensitive and rises at the extremes.
Grey noise inverts this curve. It adds energy where your ears are less sensitive and removes it where they're most sensitive. The result: a noise that sounds equally loud across the entire frequency range.
For practical purposes, this means grey noise tends to feel:
- Less harsh than white noise, because the highs are tamed
- Less muddy than brown noise, because the lows aren't overwhelming
- More neutral than pink noise, because it's perceptually flat rather than just spectrally flat
It's the "Goldilocks" noise for people who find white noise too aggressive and brown noise too heavy.
Quilence includes a Grey Noise preset that's calibrated using ISO 226 equal-loudness data. Combined with the 10-band EQ, you can use it as a starting point and fine-tune from there. A touch more bass if you want warmth, a bit more treble for masking.
It's one of those features most noise apps don't bother with. But once you've heard perceptually flat noise, the others sound noticeably uneven.