Most appliance noise is rated in decibels. Bathroom exhaust fans and range hoods are the exception. They are rated in sones, a unit most people have never met, and the difference matters when you compare products. This page explains what a sone is, using the certifying body's own definitions.
What a sone is
The Home Ventilating Institute, HVI, certifies the airflow and sound ratings of residential ventilation products. It defines the sone as an internationally recognized unit of loudness, and describes the scale as linear, like inches.
In technical terms, HVI says, one sone is equal in loudness to a pure tone of 1,000 cycles per second at 40 decibels above the listener's threshold of hearing. In everyday terms, it is equivalent to the sound of a quiet refrigerator.
HVI also gives a rough ladder for bathroom fans:
- 0.5 sone: the sound of rustling leaves.
- 1.0 sone: the sound of a refrigerator.
- 3.0 sones: typical office noise.
- 4.0 sones: a television at normal volume.
Why sones are easier to compare than decibels
Sones scale the way you hear. HVI puts it in one line: double the sone value is double the loudness. A fan rated at 3 sones makes half the sound of one rated at 6.
Decibels do not behave like that. They follow a logarithmic scale, and it takes an increase of about 10 decibels to sound twice as loud, according to the National Park Service. A shopper looking at 50 and 53 dBA has to know that. A shopper looking at 1.0 and 2.0 sones does not. The second fan is twice as loud, and that is all there is to it.
HVI describes the relationship this way: sones translate laboratory decibel readings into numbers that correspond to the way people sense loudness. The decibel readings come first, in the lab. The sone rating is what is worked out from them. Our guide to dBA covers the decibel side.
Why we never convert sones to decibels
You will find charts online that offer an exact decibel equivalent for each sone value. We do not use them, for three reasons.
First, HVI publishes no such table. The body that defines and certifies the ratings does not offer a conversion.
Second, the translation runs one way. HVI says sones are worked out from laboratory decibel readings so that they correspond to the way people sense loudness. It gives no method for turning a sone rating back into a single decibel figure, and no maker we read gives one either.
Third, we do not need to. Every certified fan and hood is rated in sones, so they can be compared with each other directly. The only reason to convert is to compare a range hood with a dishwasher, and that comparison tells you little.
The catch: which sone rating are you reading?
A sone rating is only as useful as the conditions behind it. Two details catch buyers out.
Bathroom fans: the test pressure. The rating on the box is taken at a low static pressure of 0.1 inch of water, which assumes a short, easy duct run. Real ducts with length and elbows push back harder, and a fan working against more resistance is louder. Some makers also publish a rating at 0.25 inch. In our bathroom exhaust fan guide, one fan that is rated under 0.3 sone on the box is rated at 0.9 sone at the higher pressure. We rank on the tougher figure wherever a maker publishes it.
Range hoods: the speed. The headline sone figure on a range hood is usually taken at its slowest speed. ENERGY STAR measures hood noise at what it calls working speed, the first speed at or above 100 CFM, which gives every maker an official low-speed figure to print. At full speed, the quiet-marketed hoods in our range hood guide were certified at 5 to 13 sones. Find the rating for the speed you will cook at.
Certified or self-reported
HVI tests products independently, and its certified ratings are listed in a public directory. HVI says range hoods have sone ratings too, and that they can be found in that directory.
Not every brand takes part. Several direct-to-consumer range hood brands do not appear in HVI's directory, so their sone figures are their own. In our range hood research, one such brand gave two different figures for the same hood in two places. A certified rating is the one to trust.
ENERGY STAR uses sones as well. Its ventilation fan criteria set maximum allowable sound levels in sones, along with minimum efficiency, and they require a fan to keep a set share of its rated airflow when tested at the higher 0.25 inch pressure.
How to use sone ratings when you shop
- Compare sones with sones. Do not convert. A lower number is quieter, in direct proportion.
- For a bathroom fan, aim for 1.0 sone or less, which is HVI's own threshold for quiet ventilation. Then look for a rating at 0.25 inch, because that is closer to a real installation.
- For a range hood, ignore the lowest-speed figure and find the rating at full speed, or at the airflow you need.
- Prefer HVI-certified ratings over figures that appear only on the maker's own site.
- Remember the duct. In the makers' own figures, the same bathroom fan can be far louder on a 4-inch duct than on a 6-inch one.
Common questions
What is a sone?
A unit of loudness. The Home Ventilating Institute, which certifies fan ratings, calls it an internationally recognized unit of loudness that follows a linear scale, like inches. In everyday terms, HVI says one sone is equivalent to the sound of a quiet refrigerator.
How many sones is a quiet bathroom fan?
HVI says that for quiet bathroom ventilation a fan should be rated at 1.0 sone or less. The lowest rating it will currently certify for a residential product is under 0.3 sone. Several fans in our bathroom fan guide carry that rating at their lower speeds.
How do I convert sones to decibels?
We do not, and we suggest you do not either. HVI publishes no conversion table. HVI says sones are worked out from laboratory decibel readings so that they match how people sense loudness, and it offers no method for reversing that. Treat any chart that offers exact equivalents with suspicion.
Is 2 sones twice as loud as 1 sone?
Yes. That is the point of the unit. HVI says double the sone value is double the loudness, and that a fan rated at 3 sones makes half the sound of one rated at 6. Decibels do not work that way: it takes about 10 decibels to double perceived loudness.
Why do range hoods list such low sone figures?
Because the headline figure is usually taken at the slowest speed. In our range hood guide, hoods marketed as quiet were certified at 5 to 13 sones at full speed. Look for the rating at the speed you will cook at, and prefer ratings that HVI has certified.
