A decibel is a tenth of a bel, a unit for the ratio between two levels on a logarithmic scale. Sound pressure level in dB is 20 × log10(p ÷ p0), where p0 is 20 micropascals, the standard reference for sound in air. Because the scale is logarithmic, +10 dB means ten times the sound energy and +3 dB means double. Sounds are usually measured in dBA, which is weighted to match human hearing.
Why a logarithmic scale?
The loudest sounds people encounter carry many millions of times more energy than the quietest sounds they can hear. A logarithmic scale squeezes that range into numbers from about 0 to 140. The cost is that decibels do not add like ordinary numbers.
| Change in level | Sound energy (intensity) | Sound pressure |
|---|---|---|
| +3 dB | × 2 | × 1.41 |
| +6 dB | × 4 | × 2 |
| +10 dB | × 10 | × 3.16 |
| +20 dB | × 100 | × 10 |
Adding decibels
Two sources do not add up to double the decibels. Two equal sources together are 3 dB louder than one: two 80 dB machines make about 83 dB, not 160. If one source is 10 dB louder than the other, the quieter one adds less than half a decibel. Convert each level to energy, add, and convert back: L = 10 × log10(10L1/10 + 10L2/10).
dB, dBA, dBC and dBZ
Human hearing is less sensitive to low and very high frequencies at moderate levels. Frequency weightings are filters that make a meter respond more like the ear. The international sound level meter standard, IEC 61672-1, defines three:
- dBA (A-weighting): strongly reduces low frequencies. Used for hearing safety, workplace limits and most environmental noise.
- dBC (C-weighting): nearly flat, reducing only the very lowest and highest frequencies. Used for peak levels and to reveal bass-heavy noise.
- dBZ (Z-weighting): zero weighting, a flat response.
| Frequency | A-weighting | C-weighting | Z-weighting |
|---|---|---|---|
| 31.5 Hz | -39.5 dB | -3.0 dB | 0.0 dB |
| 63 Hz | -26.2 dB | -0.8 dB | 0.0 dB |
| 125 Hz | -16.2 dB | -0.2 dB | 0.0 dB |
| 250 Hz | -8.7 dB | -0.0 dB | 0.0 dB |
| 500 Hz | -3.2 dB | +0.0 dB | 0.0 dB |
| 1k Hz | +0.0 dB | +0.0 dB | 0.0 dB |
| 2k Hz | +1.2 dB | -0.2 dB | 0.0 dB |
| 4k Hz | +1.0 dB | -0.8 dB | 0.0 dB |
| 8k Hz | -1.1 dB | -3.0 dB | 0.0 dB |
| 16k Hz | -6.7 dB | -8.6 dB | 0.0 dB |
Calculated from the analytic expressions for the A and C weightings in IEC 61672-1, normalised to 0 dB at 1 kHz. The standard's tables round these values to one decimal place. Our decibel meter uses the same formulas.
dBFS, dB SPL and dB HL
- dB SPL (sound pressure level): sound in the air, referenced to 20 micropascals. This is what a sound level meter measures.
- dBFS (decibels relative to full scale): digital audio, where 0 dBFS is the largest value a recording can hold and everything else is negative. Our mic test reads in dBFS. Turning dBFS into dB SPL needs a calibration for the specific microphone.
- dB HL (hearing level): used on audiograms, where 0 dB HL is the average threshold of hearing at each frequency, so it differs from dB SPL.
Fast, Slow, Leq and L90
Sound level meters also smooth the level over time. Fast uses a 0.125-second time constant and Slow uses 1 second (IEC 61672-1). Leq is the steady level with the same total energy as a varying sound over a period, the basis of noise-dose limits. L10 and L90 are the levels exceeded 10% and 90% of the time; L90 describes the background.
See real examples on the decibel chart, or measure your own with the decibel meter.
Common questions
What is a decibel in simple terms?
A decibel is a unit for how loud a sound is compared with a reference, on a logarithmic scale. Every 10 dB is ten times the sound energy.
Is 3 dB twice as loud?
3 dB is twice the sound energy. How loud a sound seems to you is a separate question: perceived loudness rises much more slowly than energy, so a 3 dB increase does not sound twice as loud.
What is the difference between dB and dBA?
dBA is a decibel reading with A-weighting applied, which reduces low and very high frequencies to match human hearing. A plain dB reading often means unweighted (dBZ), but check what the source used.
Why can't I add decibels normally?
Decibels are logarithmic. Two equal sounds together are 3 dB louder, not double the number. Use the calculator on this page.
What is 0 dB?
0 dB SPL is the reference pressure of 20 micropascals, close to the quietest sound a young, healthy ear can detect. It is not silence; negative decibel values are possible.
Last reviewed 14 November 2026 by Sri Murugan Systems. Found a mistake? Tell us.