Short answer: Decibels measure sound level; sones measure how loud a sound seems. One sone is the loudness of a 1 kHz tone at 40 dB SPL, which is 40 phons. Each doubling of sones means twice as loud and adds 10 phons, so 2 sones ≈ 50 phons and 4 sones ≈ 60 phons. Bathroom fans under about 1 sone are generally considered quiet.
If you have shopped for a bathroom fan or cooker hood in the US, you have probably seen noise given in sones rather than decibels. The two units answer different questions. This guide explains what a sone is, how it links to decibels through the phon, gives a sones-to-phons chart, explains why fans use sones and shows why there is no exact conversion from sones to dB(A).
What Is a Sone?
The sone is a unit of perceived loudness. It is defined so that 1 sone is the loudness of a 1,000 Hz tone at 40 dB SPL, and the scale is linear in perception: 2 sones sounds twice as loud as 1 sone, 4 sones twice as loud again. That makes sones easy to compare. A 3-sone fan sounds about three times as loud as a 1-sone fan, whereas the equivalent statement in decibels needs a grasp of the logarithmic scale.
Phons: the Link Between Sones and Decibels
The phon is the step between the two. A sound has a loudness level of N phons if it sounds as loud as a 1 kHz tone at N dB SPL. The ear is not equally sensitive at all frequencies, so phons and decibels match only at 1 kHz. At 100 Hz, a tone needs roughly 20 dB more sound pressure to sound as loud as a 40 dB tone at 1 kHz, while between about 2 and 5 kHz the ear is most sensitive. These equal-loudness contours are standardised in ISO 226. A-weighting was originally based on the 40-phon contour, which is why dB(A) roughly tracks loudness at moderate levels (see dB vs dBA and dB vs Hz).
Sones to Phons Formula and Chart
Above about 40 phons, loudness doubles for every 10 phon increase:
phon = 40 + 10 · log2(sone) and sone = 2(phon − 40)/10
| Sones | Phons | Loudness compared with 1 sone |
|---|---|---|
| 0.5 | 30 | Half as loud (the rule is approximate below 40 phons) |
| 1 | 40 | Reference: a 1 kHz tone at 40 dB |
| 2 | 50 | Twice as loud; a 1 kHz tone at 50 dB |
| 4 | 60 | 4 times as loud; a 1 kHz tone at 60 dB |
| 8 | 70 | 8 times as loud |
| 16 | 80 | 16 times as loud |
| 32 | 90 | 32 times as loud |
| 64 | 100 | 64 times as loud |
Notice the pattern: 10 phons more means double the sones, which matches the everyday rule that +10 dB sounds about twice as loud. The sones ↔ dB tab of our decibel calculator converts in both directions.
Sones vs Decibels: Key Differences
| Decibels (dB) | Sones | |
|---|---|---|
| Measures | Physical sound level | Perceived loudness |
| Scale | Logarithmic | Linear in perception |
| Twice as loud | About +10 dB | ×2 |
| Frequency | Unweighted, or weighted with A or C | Built in: accounts for the ear’s sensitivity |
| Measured with | Sound level meter | Calculated from a measured spectrum |
| Typical use | Almost everything: regulations, hearing safety, audio | Fan and appliance ratings, psychoacoustics |
Decibels come in several varieties, from dB SPL to dB(A) and dB(C); the decibel units guide and dBA vs dBC explain them. For the basics of the unit itself, start with what a decibel is.
Why Fans and Range Hoods Are Rated in Sones
In the US, bathroom fans and range hoods are commonly tested and certified through the Home Ventilating Institute (HVI), which publishes sone ratings. A linear scale suits shoppers: a 0.5-sone fan is half as loud as a 1-sone fan. As a general guide, bathroom fans under about 1 sone are considered quiet, fans around 1–2 sones are noticeable but unobtrusive, and range hoods at high speed are often several sones. Ratings come from standard laboratory conditions, so a fan fitted with long or narrow ducting can be louder at home.
In Europe, range hood energy labels state noise in decibels as A-weighted sound power, which is not the same as the sound pressure a meter reads in the room (see sound pressure level). If you are choosing a fan for a bedroom or nursery, our baby room noise guide and typical room noise levels show how quiet those rooms should be.
Can You Convert Sones to Decibels?
Only approximately. For a 1 kHz tone the conversion is exact: 1 sone = 40 phons = 40 dB SPL. Real sounds such as fan noise spread across many frequencies, and loudness depends on the whole spectrum. Standard methods in ISO 532 (including Zwicker’s method) calculate sones from a measured spectrum. Because broadband noise spreads over many hearing bands, it tends to sound louder than a pure tone of the same dB, so a 1-sone fan often reads noticeably below 40 dB(A) on a meter. Treat any single sones-to-dB(A) figure as a rough guide.
How to Hear and Measure Loudness Yourself
Play a 1 kHz tone with the tone generator, then lower it by 10 dB: it should sound about half as loud, which is one sone step. The volume level comparator lets you hear 3, 6 and 10 dB differences side by side. To check a fan at home, measure it with our sound decibel meter about 1–1.5 m below it, then use the frequency analyzer to see whether a low hum or a high hiss dominates. The decibel chart shows how the reading compares with everyday sounds.
Frequently Asked Questions
How many decibels is 1 sone?
For a 1 kHz tone, 40 dB SPL (40 phons). For fan noise there is no exact figure; a 1-sone fan usually reads below 40 dB(A).
What is a good sone rating for a bathroom fan?
Under about 1 sone is considered quiet; around 1–2 sones is typical for a good fan.
Is 3 sones loud?
It is clearly audible: about 56 phons and roughly three times as loud as a 1-sone fan. It is common for basic fans and range hoods on a low setting.
Is a lower sone rating better?
Yes. Lower sones means quieter. Halving the sones halves the perceived loudness.
Are phons and decibels the same?
Only at 1 kHz. At other frequencies a sound needs a different dB level to reach the same number of phons.
Addito is an audio analysis and sound measurement writer specializing in decibel testing, noise level analysis, sound frequency measurement, and audio engineering tools for creators, audio professionals, students, and everyday users. He creates practical content focused on sound measurement, hearing safety, microphone testing, and acoustic analysis tools.