Grey noise is random noise shaped by an equal-loudness curve, so it sounds about equally loud at every pitch. White noise has equal power at every frequency, but the ear does not hear every frequency equally. Grey noise corrects for that. It is one of the noise colours, and the odd one out, because it is defined by how we hear rather than by a fixed slope.
What is grey noise?
Grey noise is random noise whose spectrum follows an equal-loudness contour. Wikipedia describes it as containing all frequencies at equal perceived loudness, as opposed to white noise, which contains all frequencies with equal energy. One common choice of curve is an inverted A-weighting curve.
Think of it as white noise with a correction applied for the ear. Where hearing is weak, grey noise adds power. Where hearing is strong, it holds back. The goal is a sound that doesn't lean toward any one pitch.
There's a catch. The same page notes that equal-loudness curves vary with the listener and with the playback level, so there is no one true grey noise. Different grey noises follow different curves.
Why does white noise not sound flat?
White noise is flat on paper, but not in your ear. Hearing is uneven.
- The ear covers about 20 Hz to 20,000 Hz, according to a review on the NIH's PMC archive.
- It is most sensitive between 1500 and 4000 Hz, the range that carries normal speech.
- Sensitivity falls away toward both ends. The same review reports middle-ear gain close to zero at 7 kHz and above.
- Low frequencies need much more power to sound as loud. Below about 100 Hz, the equal-loudness contours rise steeply, by as much as 24 dB per octave.
| Part of the range | What the ear does | What grey noise does |
|---|---|---|
| Low (below about 100 Hz) | Needs much more power to sound as loud | Turns it up |
| Middle (about 1.5 to 4 kHz) | Most sensitive | Holds it back |
| High (7 kHz and above) | Gain falls toward zero | Turns it up |
The table is qualitative on purpose. The exact amounts depend on the curve you pick and the volume you play at.
Where does the uneven hearing come from?
Part of it is the shape of the ear itself. The same NIH-hosted review says the outer ear amplifies roughly 2 to 5 kHz, with a maximum gain of about 20 dB. The middle ear adds gain of about 20 dB between 250 and 500 Hz, peaks near 1 kHz, then falls by about 8.6 dB per octave toward the top of the range. Across the whole system, the mid-band gets around 25 dB of help.
That's why a flat signal doesn't sound flat. The middle of the spectrum gets boosted by the ear before the sound reaches the brain, and the edges do not. A grey noise recipe is, in effect, the reverse of that boost.
It also explains why the ear's own sensitivity changes with age. The upper limit of hearing declines over the years, so a curve built for an average young listener will not match everyone. Grey noise is an approximation of a typical ear, not a fit for yours.
What is an equal-loudness contour?
An equal-loudness contour is a line showing how much power each frequency needs to sound as loud as the others. Harvey Fletcher and Wilden Munson measured the first set in 1933. Robinson and Dadson re-measured them in 1956, and ISO 226:2003 is the current standard revision, according to the equal-loudness contour article.
Loudness level is measured in phons. Two tones share the same level in phons if an average young listener without significant hearing impairment hears them as equally loud.
The contours also change with level. A curve measured at a quiet level is not the same shape as one measured at a loud level. That's why grey noise has no single definition.
How is grey noise different from white and pink noise?
White noise has the same power at every frequency. It is flat per hertz, and because the ear is most sensitive in the middle of the range, it sounds bright and hissy. For the full comparison of the flat, tilted and rounded colours, see the difference between white, pink and brown noise.
Pink noise has equal energy per octave instead of per hertz. That tilts it toward the low end, and Wikipedia notes it is closer to "equally loud at all frequencies" than white noise is. It is a simple stand-in for grey noise, not a match. We compare the two in pink noise vs white noise.
Grey noise skips the shortcut. It's shaped to a loudness curve directly, so it has no single slope in decibels per octave to quote.
Try it
The easiest test is a swap: Play white noise at a comfortable, low volume, then switch to grey at the same setting. Listen for whether any one pitch stands out. White noise tends to lean toward the hiss, and grey should sound more even.
On the Hush Machine dial, grey sits between white and pink. The noise is made live on your phone, so nothing loops and nothing repeats, and Hush Machine is coming to iPhone. You can read about it on the grey noise sound page.
Bottom line
Grey noise is white noise corrected for how people hear, shaped by an equal-loudness curve so it sounds even across pitches. There is no single standard version, because the curves change with the listener and the volume. Browse the rest of the noise colours to hear how the others compare.
Questions
- Is grey noise the same as gray noise?
- Yes. Grey is the British spelling and gray the American one. Both name random noise shaped by an equal-loudness curve.
- Is there one true grey noise?
- No. Equal-loudness curves vary with the listener and the playback level, so a grey noise is built for a chosen curve, such as an inverted A-weighting curve.
- Is grey noise louder than white noise?
- Not by definition. Loudness depends on the volume you set. What changes is the balance: grey noise is shaped so no pitch stands out as louder than another, while white noise only has equal power at every frequency.
- Can pink noise stand in for grey noise?
- Roughly. Pink noise has equal energy per octave, so it sounds closer to equally loud across the range than white noise does. It is not shaped to a loudness curve, though.