British Standard 4142 (BS 4142) contains the commonly accepted method of assessing noise impact from new industrial and commercial developments on existing residential properties. It describes a methodology for determining noise levels and procedures for assessing whether the noise is likely to give rise to complaints from residents.
The Standard was first published in 1967 and has been amended in 1975, 1980, 1982 and 1990 prior to the current version. The Standard is slightly unusual in that it is not based on substantive research, but is based on accumulated experience.
BS 4142 requires the following work to be carried out to rate a noise. The background noise level (LA90) is measured at adjacent noise sensitive properties in the absence of the noise to be assessed. The background noise level is measured at an appropriate time to represent the noise level when the source to be assessed will be operating e.g. if the noise source is 24 hour operation, the background noise levels would be taken to represent the quietest period of the night. The noise from the new source is then assessed at the noise sensitive premises. The specific noise level is predicted (or measured) at the nearest noise sensitive property in terms of LAeq. The specific noise level is assessed for the relevant time period, one hour during the day and five minutes at night. The rating level is then calculated by adding 5 dB to the specific noise if the noise is tonal, impulsive or irregular enough to attract attention (only one 5 dB correction is added even if all of the above factors are present). The measured background level is then subtracted from the rating level. The Standard provides the following guidance on the difference between rating level and background noise level:
- A difference of around +10 dB or more indicates that complaints are likely.
- A difference of around +5 dB is of marginal significance.
- If the rating level is more than 10 dB below the measured background noise level then this is a positive indication that complaints are unlikely.