A decibel (dB) is a logarithmic unit of sound pressure level relative to the threshold of human hearing. Because it is logarithmic, every 10 dB increase represents a tenfold increase in sound intensity, and roughly a doubling of perceived loudness.

Why is the decibel scale logarithmic?

Human hearing spans an enormous range of intensities, from the faintest audible sound to a painfully loud one — a ratio of about a trillion to one. A logarithmic scale compresses that range into manageable numbers, so 0 dB is the hearing threshold and 120 dB is near the pain threshold.

At what level does sound damage hearing?

Prolonged exposure above 85 dB can cause permanent hearing loss. The NIOSH rule halves safe exposure time for every 3 dB increase: 85 dB is safe for 8 hours, 88 dB for 4 hours, 91 dB for 2 hours, and so on. Above about 120 dB, damage can be immediate.

How does distance affect decibel level?

Sound pressure level drops by about 6 dB each time the distance from a point source doubles, following the inverse-square law. So a 100 dB machine measured at 1 metre is about 94 dB at 2 metres and 88 dB at 4 metres in free field conditions.

Can I add decibels by simple arithmetic?

No. Because decibels are logarithmic, two equal sources do not double the number — they add about 3 dB. Two 80 dB sources together produce roughly 83 dB, not 160 dB. Combining different levels requires logarithmic addition, not ordinary sums.

Decibel (dB) Reference Table

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Decibels describe sound on a logarithmic scale, which makes raw numbers hard to picture. This reference anchors common dB values to real-world sounds — from a whisper to a jet engine — and pairs each with the safe daily exposure time, so a meter reading turns into something you can actually judge.

How it works

Sound pressure level in decibels is defined relative to the threshold of human hearing. Because the scale is logarithmic:

+10 dB  = 10× the sound intensity, ≈ 2× perceived loudness
+3  dB  ≈ doubling of intensity (two equal sources combine to +3 dB)
−6  dB  per doubling of distance from a point source

For exposure, NIOSH uses a 3 dB exchange rate: the safe daily duration halves for every 3 dB above 85 dB. The tool looks up the nearest reference level for any value you enter and reports its risk and safe exposure time.

Sound landmarks

0 dB — threshold of hearing.
30 dB — quiet library, whisper.
60 dB — normal conversation.
85 dB — heavy traffic; damage risk begins on long exposure.
120 dB — rock concert, ambulance siren; near pain threshold.
140 dB — jet engine at close range; immediate damage.

Anything above 85 dB for extended periods is a hearing risk — use the table to see how quickly exposure becomes unsafe.