‘dB’ is the standard abbreviation for decibel – the unit of sound pressure level and noise exposure measurement. The decibel scale is a logarithmic ratio between any two sound pressure levels. The addition or subtraction of decibels is different to normal linear calculations.

Adding 3 dB to an existing sound pressure level doubles it, subtracting 3 dB halves it. For example, adding two sound pressure levels of 80 dB does not result in a sound pressure level of 160 dB. It gives a level of 83 dB, i.e. twice the 80 dB sound pressure level.

Noise measurement can be weighted to different frequencies:
- 'dB(A)' refers to A-weighting – used to measure average noise levels
- 'dB(C)' refers to C-weighting – used to measure peak, impact or explosive noises.

Find advice and guidance on noise at work on the HSE website