How is noise measured?

TUC’s Hazards at work guide to health and safety describes how noise is measured.

When noise is measured at work, emphasis is normally given to the frequencies that have most effect on the human ear. This is done by adjusting the noise meter to take more notice of these frequencies. The scale used is called a ‘weighted decibel scale’ or dB(A). According to the Health and Safety Executive (HSE), some examples of typical noise levels are:

• normal conversation 50–60 dB(A)
• a loud radio 65–75 dB(A)
• a heavy lorry about 7 metres away 95–100 dB(A)
• a jet aircraft taking off 25 metres away 140 dB(A)

Since the scale is logarithmic, a small increase in the decibel scale corresponds to a large increase in intensity. This is very important in understanding the significance of noise measurements. For example:

• If the sound level increases by 10dB then the sound intensity, that is, the amount of sound energy being transmitted to the ear, increases tenfold.
• An increase of 3dB corresponds to a doubling of intensity. 83dB is not just over 80dB but is in fact twice as intense, and is capable of producing correspondingly more damage to hearing.