Noise Measurement and Nuisance Levels

Sound is produced by mechanical vibration of a surface, which sets up rapid pressure fluctuations in the surrounding air. Between the quietest audible sound and the loudest tolerable sound there is a million to one ratio in sound pressure level. It is because of this wide range that a noise level scale based on logarithms is used in noise measurement. This is the decibel or dB scale. Audibility of sound covers a range of about 0 to 140 decibels (dB) corresponding to the intensity of the sound pressure level.

The ability of the human ear to recognise a particular sound is dependent on the pitch or frequencies present in the source. Sound pressure measurements taken with a microphone cannot differentiate in the same way as the ear, consequently a correction is applied by the noise measuring instrument in order to correspond more closely to the frequency response of the ear which varies within the audible range of 20 Hz to 20000 Hz. This is known as 'A weighting' and written as dB(A). The use of this unit is internationally accepted and correlates well with subjective annoyance to noise. The logarithmic basis of noise measurements means that when considering more than one noise source their addition must be undertaken in terms of logarithmic arithmetic. Thus, two noise sources each of 40 dB(A) acting together would not give rise to 40 + 40 = 80 dB(A) but rather 40 + 40 = 43 dB(A). This 3 dB(A) increase represents a doubling in sound energy but would be only just perceptible to a human ear.

The following chart gives typical noise levels in terms of dB(A) for some everyday situations.

 Sound Level  Typical Situation 
   0 dB(A)   Threshold of hearing
 20-40 dB(A)  Rural night-time background
 35 dB(A)  Quiet bedroom
 35-45 dB(A)  Wind farm, noise levels at a distance of  350m from source
 35-45 dB(A)  Busy road 5km from source
 55 dB(A)  Car at travelling at 65km/h 100m from source
 60  dB(A)  Busy general office
 60 dB(A)  Conversation
 65 dB(A)  Truck travelling at 50km/h, 100m  from source
 75-85 dB(A)  City Traffic at 5m
 95 dB(A)  Pneumatic drill at 7m
 105 dB(A)  Jet aircraft at 250m
 140 dB(A)  Threshold of pain