Box 16B Measurement of noise

There are two main ways of assessing the influence of noise: by physically measuring sound pressure levels, and by recording the discomfort or annoyance caused by noise. Physically noise is treated as an acoustic phenomenon called sound. A sound event as a physical phenomenon can be fully described by four parameters:

  1. the strength or sound pressure, mostly expressed in terms of the amplitude of the sound pressure waves, and is usually measured as sound pressure levels in decibels (dB);
  2. the frequency or pitch, measured in Hertz (most noises consist of a mixture of sounds with various pitches and frequencies, and hence do not have a recognisable pitch in any musical sense);
  3. the fluctuation of sound with time (also known as the time history), measured as sound pressure level as a fluctuation of time;
  4. sound character, which describes the particular features of a sound (eg, tonal and harmonic qualities).

Decibels are measured on a logarithmic scale, so that an increase of the sound pressure level (SPL) by a factor of 10 would result in a 10 dB increase in sound level, an increase of SPL by a factor of 100 would result in a 20 dB sound level increase, a factor of 1000 would result in a 30 dB increase, and so on. The decibel scale ranges from 0 (the threshold of normal human audibility) to 130 (the threshold of pain). For most purposes the frequency scale is weighted by the frequency sensitivities of the human ear, known as A-weighting, transforming the (unweighted) sound pressure (dB) into A weighting sound pressure level ('dB(A)'). The dB(A) scale strongly attenuates the low frequencies and moderately attenuates the very high frequencies. The way in which the dB(A) scale corresponds to everyday noises is shown in Figure 16.2, where it can seen that the range of everyday noises varies from roughly 45 dB(A) to 115 dB(A).

The main descriptors used in the assessment of environmental noise are shown in Table 16.1. For describing the impact of noise on humans, the so-called Equivalent Sound Pressure Level (Leq) needs to be calculated, that is, the mean value of sound intensity over time expressed in decibels. The significance of the Leq is to be seen in the hypothesis that a noise that varies through time is equivalent in its disturbance on humans to a steady constant source of noise, over the same interval of time, if the noise level (sound pressure level) of the constant source of noise is equal to the Leq value of the noise that varies through time. However, Leq is not enough for the characterisation of environmental noise. It is equally important to measure and display the maximum values of the noise fluctuations (Lmax), preferably combined with a measure of the number of noise events (Ln) (WHO, 1993a). For most people, noise pulses are more annoying than a steady pulse of noise.

Apart from the descriptors listed in Table 16.1, there are a number of other descriptors used in European countries, notably with regard to aircraft noise (eg, in the UK, The Netherlands and Norway). The use of such a variety of descriptors makes international comparisons of noise exposure difficult, if not impossible. However, as far as can be judged from the serious efforts being made by the scientific community, through various international bodies, such as the International Organization for Standardisation (ISO) and the European Committee for Standardisation (CEN), it appears there is a preference towards those descriptors based around the concept of Leq.

The actual annoyance to people caused by noise is normally measured by sociological investigations, in the laboratory, or by analysing complaints by type of noise source to local authorities. Such figures should, however, be treated with caution, as people tend to be disturbed by, and react differently to, different types of noise. It is possible to relate annoyance to noise measured in terms of Leq to obtain 'dose-effect curves'. These have assumed a prominent role in quantifying the impact of environmental noise on people. The general conclusion from dose-effect curves is that people are becoming 'considerably annoyed' if environmental noise levels are in excess of about 40 dB(A) at night and 50 dB(A) during the day.


Figure 16.2 - Maximum A-weighted sound pressure level (dB)

Source: Müller, DG X1, CEC, personal communication based on US EPA, 1979

Note: For modes of transport, the speed, distance from source and weight of carrier are indicated.