What Is a Decibel? How Loudness Is Measured

Quick answer

A decibel (dB) is a logarithmic unit that compares a sound level with a reference level. Because the scale is logarithmic, every 10 dB step represents a tenfold increase in sound power, and every 3 dB roughly doubles it. NIOSH recommends limiting exposure to 85 dBA averaged over eight hours.

Decibel: A logarithmic unit used to express the ratio between two quantities, most commonly sound pressure or power relative to a reference level.

Why sound uses a logarithmic scale

The human ear responds to an enormous range of sound strength. The loudest sound a person can tolerate carries trillions of times more power than the faintest they can detect. Writing numbers that large is clumsy, so acousticians compare levels on a logarithmic scale, which compresses the range into numbers such as 0 to 140.

On this scale, 0 dB is set near the quietest sound a healthy young ear can detect, and each additional 10 dB corresponds to ten times as much sound power. A sound at 60 dB therefore carries a thousand times the power of one at 30 dB, even though the numbers differ by only a factor of two.

How sound power changes with decibel steps
Increase in level	Sound power	Typical perception
+3 dB	About 2 times	A noticeable but modest increase
+10 dB	10 times	Usually heard as about twice as loud
+20 dB	100 times	Heard as about four times as loud
+30 dB	1,000 times	Heard as about eight times as loud

The perception column is a rule of thumb: people judge loudness differently, and very low or very high pitches do not follow it exactly.

What dBA means

Sound at the same power does not sound equally loud at every pitch, because the ear is less sensitive to low and very high frequencies. A-weighting adjusts a measurement to reflect that, and the result is written dBA. Occupational guidance, including the National Institute for Occupational Safety and Health (NIOSH), uses dBA, so a phone app or meter labelled dBA is measuring in a way that matches how the ear responds.

Everyday sound levels

Approximate typical levels, for orientation
Sound	Approximate level
Whisper in a quiet room	About 30 dBA
Normal conversation	About 60 dBA
Heavy city traffic from inside a car	About 80 to 85 dBA
Power lawn mower	About 90 dBA
Loud concert or siren nearby	100 to 120 dBA

These figures vary with distance and equipment, so treat them as ballpark values, not specifications.

Exposure limits: level and time together

Harm from noise depends on both loudness and how long the exposure lasts. NIOSH has established a recommended exposure limit of 85 dBA averaged over an eight-hour workday. It uses a 3 dB exchange rate: for each 3 dBA increase in level, the recommended exposure time is cut in half. Under this rule, 88 dBA allows about four hours a day and 100 dBA allows less than 15 minutes.

NIOSH earlier worked with a 5 dB exchange rate and moved to 3 dB because the shorter-step rule is more firmly supported by the evidence. The legal limit enforced for many workplaces in the United States, set by a different agency, uses 90 dBA with a 5 dB exchange rate, which is less protective than the NIOSH recommendation. The 85 dBA figure is a recommendation, not a law.

Hearing damage is gradual and permanent

Noise-induced hearing loss usually develops slowly and does not reverse. If you must raise your voice to be heard by someone an arm's length away, or if your ears ring after an event, the level was probably high enough to take precautions with. Questions about your own hearing are best put to a hearing-health professional.

Using this in practice

Lowering the level helps more than shortening the time, because of the 3 dB rule: halving a noise's power saves as much exposure as halving the time. Moving away from a source also reduces its level, and ear protection rated for the noise reduces it further. Headphone volume is worth the same attention as workplace machinery, since personal audio can reach levels in the range where the limits above apply.