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What is a decibel (dB)?

A decibel is a logarithmic unit that compares a sound's pressure to the quietest sound humans can hear (20 µPa). Every 10 dB is a tenfold jump in energy and roughly a doubling of perceived loudness — which is why the scale feels so counter-intuitive.

The decibel (dB) is one tenth of a bel, named after Alexander Graham Bell. It is not an absolute unit like a metre or a gram; it is a ratio expressed on a logarithmic scale. For sound pressure level (SPL), that ratio is measured against a fixed reference of 20 micropascals — the threshold of healthy human hearing.

The formula

Sound pressure level in decibels is L = 20 × log₁₀(p / p₀), where p is the measured pressure and p₀ is 20 µPa. Working in energy (intensity) instead, it is L = 10 × log₁₀(I / I₀). Both give the same dB figure; the factor of 20 versus 10 simply reflects that energy is proportional to pressure squared.

Why every 10 dB matters so much

The logarithm is what trips people up. Adding 10 dB multiplies sound energy by ten, but your ear only perceives it as roughly twice as loud. So 90 dB has ten times the energy of 80 dB, a hundred times that of 70 dB, and a thousand times that of 60 dB — yet it sounds only about two, four and eight times as loud respectively. This gap is exactly why loud noise damages hearing well before it feels painful.

+3 dB

Double the energy

+10 dB

10× energy, ~2× as loud

+20 dB

100× energy, ~4× as loud

dB, dBA and other weightings

A plain “dB” figure counts all frequencies equally, but human ears do not. The A-weighting (dBA) filters the measurement to match how we actually hear, de-emphasising very low and very high frequencies. Almost every safety and environmental figure — including those across this site — is A-weighted. Our dBA vs dB guide explains when each is used.

Adding decibels together

Because they are logarithmic, decibels do not add arithmetically. Two 70 dB sources make about 73 dB, not 140. To combine sources you convert to energy, sum, then convert back: L = 10 × log₁₀(Σ 10^(Lᵢ/10)). Our decibel calculator does it instantly.

Frequently asked questions

Why is the decibel scale logarithmic?

Because human hearing spans an enormous range — the loudest sound we tolerate carries about a trillion times the energy of the quietest we can detect. A logarithmic scale compresses that range into manageable numbers from 0 to ~140.

Does 0 dB mean no sound?

No. 0 dB is the reference threshold of human hearing (a pressure of 20 micropascals), not silence. Levels below 0 dB exist and are simply too quiet for a typical person to hear.

Is 60 dB twice as loud as 30 dB?

Not by energy — 60 dB carries a thousand times the energy of 30 dB. But perceived loudness roughly doubles every 10 dB, so 60 dB sounds about eight times as loud as 30 dB to the ear.

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Sources & accuracy. Reference levels are drawn from published data by NIOSH, OSHA, the CDC, the WHO Environmental Noise Guidelines and the American Speech-Language-Hearing Association (ASHA). Real-world readings vary with distance, room acoustics and the specific source, so treat every figure as a typical value rather than an exact constant. Last reviewed 2026-08-11.