The decibel (dB) is the unit we use to describe how loud a sound is. It is everywhere – on headphone packaging, in noise regulations, on studio meters – and it is also widely misunderstood. Here is what it really measures.
A ratio, not an amount
A decibel is not an absolute unit like a metre. It expresses the ratio between two quantities on a logarithmic scale:
- For power or intensity: L = 10 · log₁₀(P / P₀)
- For pressure or voltage: L = 20 · log₁₀(p / p₀)
The reference value (P₀ or p₀) defines which "dB" you are talking about. That is why you will see suffixes such as dB SPL, dBA, dBFS or dBV.
Why a logarithmic scale?
Human hearing covers an astonishing range. The loudest sound we can bear has roughly a million times the sound pressure of the faintest sound we can detect – a trillion times the intensity. A linear scale would be useless; a logarithmic one compresses this range to about 0–130 dB and matches how we perceive loudness, since our ears also respond roughly logarithmically.
Three rules of thumb follow:
- +3 dB = double the power (two identical machines instead of one).
- +6 dB = double the sound pressure.
- +10 dB = ten times the power, and is heard as roughly twice as loud.
So 90 dB is not "a bit louder" than 80 dB: it carries ten times the energy. Use the decibel calculator to add levels correctly – two 80 dB sources make 83 dB, not 160 dB.
dB SPL: sound pressure level
In acoustics, dB SPL uses a reference pressure of 20 micropascals (µPa) – approximately the quietest 1 kHz tone a young, healthy ear can hear. So 0 dB SPL is not silence; it is the threshold of hearing. Normal conversation is about 60 dB SPL, a busy street about 85 dB, a rock concert 110 dB. Browse every level on our decibel scale.
dBA and other weightings
Our ears are much less sensitive to very low and very high frequencies than to the 2–5 kHz range. A-weighting filters a measurement to mimic this, and most noise regulations use dBA. C-weighting is flatter and used for peak and low-frequency measurements (dBC). When comparing published numbers, check the weighting.
Leq: averaging over time
Real sound fluctuates. Leq (equivalent continuous level) is the steady level that would contain the same total energy over a period. Workplace exposure limits and environmental noise maps are expressed as Leq values, often A-weighted (LAeq).
dBFS: digital audio
In recording, dBFS (decibels relative to full scale) measures level relative to the maximum a digital system can represent. 0 dBFS is the ceiling; everything else is negative. There is no fixed link between dBFS and dB SPL – it depends on microphone sensitivity and gain – which is why phone decibel meters need calibration.
Distance matters
A decibel figure without a distance is incomplete. In open air, the level from a small source falls by about 6 dB every time you double the distance. A chainsaw at 110 dB at the operator's ear is around 90 dB ten metres away. The sound vs distance calculator does the maths.
Underwater decibels are different
Underwater acoustics uses a reference pressure of 1 µPa instead of 20 µPa, and water has a much higher acoustic impedance than air. The result is that underwater decibel figures are roughly 61–62 dB higher than in-air figures for the same intensity. A blue whale's 188 dB call is impressive – but it is not comparable with a 140 dB jet engine.
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