Assessment of noise: technical advice note

In addition to its non-linear amplitude response, the human ear has a non-linear frequency response; it is less sensitive at low and high frequencies and most sensitive in the range 1 kHz to 4 kHz (cycles per second). The A-weighting is applied to measured sound pressure levels so that these levels correspond more closely to the subjective response. A-weighted noise levels are often expressed in dB(A).

Ambient noise is the total sound in a given situation at a given time usually composed of sound from many sources, near and far.

The A-weighted sound pressure level of the residual noise at the assessment position that is exceeded for 90% of a given time period,T.

This is the unit of measurement used for sound pressure levels and noise levels are usually quoted in decibels (dB). The decibel scale is logarithmic rather than linear. The threshold of hearing is zero decibels while, at the other extreme, the threshold of pain is about 130 decibels. In practice these limits are seldom experienced and typical levels lie within the range of 30 dB(A) (a quiet night-time level in a bedroom) to 90 dB(A) (at the kerbside of a busy street).

A facade sound level is that determined 1 metre in front of the most exposed window or door in a facade. Sound is reflected from hard surfaces in a similar manner to light by a mirror and the effect is to produce a slightly higher (about 2.5 dB) sound level than would occur if the building was not there. For façade levels at dwellings required for this assessment process, the level 1 metre from the most exposed façade must be calculated with a reflection correction.

The sound level which is measured or calculated, in the open, without any reflections from nearby surfaces. For free-field levels at dwellings required for this assessment process, the level one metre from the most exposed façade must be calculated without a reflection correction.

L _A10,T is the A-weighted sound level in dB that is exceeded 10% of the measurement period, T.

The background noise level is commonly quoted using the L _A90,T index. This is the A-weighted sound level in dB that is exceeded 90% of the measurement period,T.

The L _A10,18h noise level is arithmetic mean of all the 18-one hour levels of L _A10,1h during the period from 06:00 to 24:00. This is the standard index used within the UK to describe traffic noise. From research it has been found that subjective response to road traffic noise is closely linked to higher noise levels experienced and is correlated well with the L _A10,18h index.

The equivalent continuous sound level L _Aeq,T is the level of a notional steady sound, which at a given position and over a defined period of time,T, would have the same A-weighted acoustic energy as the fluctuating noise.

The maximum A-weighted level measured during a given time period,T with the sound meter set on FAST response.

The specific noise level plus any adjustment for the characteristic features of the noise

The ambient noise level remaining at a given position in a given situation when the specific noise source is suppressed to a degree such that it does not contribute to the ambient noise.

Receptors which are potentially sensitive to noise and vibration. Examples include dwellings, hospitals, schools, community facilities, designated areas ( e.g.AONB, National Park, SAC, SPA, SSSI, SAM), and public rights of way.