The Mayor's Ambient Noise Strategy - Greater London Authority

The Mayor’s Ambient Noise Strategy Mayor of London 219
of the pressure variation of that sound to a reference level and is
measured in decibels (dB). The logarithmic unit dB has the advantage of
compressing the numerical range, from 0 dB up to, for example, 120 dB
for a nearby pneumatic drill. Also, the human ear responds roughly
logarithmically rather than linearly to sound. That is, the ear becomes
(logarithmically) less sensitive to increments of additional sound energy.
Sound can be described by its sound pressure level in each frequency
band. However, the human ear typically notices higher frequency sounds
more readily than low frequencies. A-weighting is commonly used to
reflect this. The A-weighting scale attaches a lower weighting to lower
frequencies relative to higher frequencies. The A-weighted sound pressure
level is denoted ‘dB(A)’. C-weighting may be used when evaluating very
loud sounds, or those with a lot of energy at lower frequencies. Other
weightings may be used. With modern equipment, sound can readily be
measured across a wide frequency spectrum and subsequently analysed.
Sound levels in the environment typically vary through time. For example,
free flowing traffic on a busy motorway may be continuously audible,
while overflying aircraft may be individual events separated by a return to
pre-existing background levels. A variety of descriptors have been used to
aggregate sound through time, for different sources and in different
circumstances. Two commonly used descriptors are:
■
■
Equivalent continuous A-weighted sound pressure level - This is
the notional A-weighted level which would deliver the same energy as
the actual fluctuating source if it were to be delivered continuously
over the defined period. Notation for this measure is L Aeq, time period
. The
time period must always be stated.
Percentile parameters - This is the value exceeded for a given
percentage of the measurement period. L A90
is the dB(A) level
exceeded for 90% of the measurement period, whereas L A10
is the level
exceeded for 10% of the measurement period.
L A10
has been widely used in the UK to describe road traffic noise. L A90
is
commonly used to describe background noise. Railway noise, and,
increasingly, other environmental noise, is commonly described using L Aeq
.
A number of indices are used in different countries to describe aircraft
noise. The UK Government has moved from using NNI (Noise and Number
Index) to using L Aeq
, which is the commonest international measure. L Amax
and SEL (see glossary) are also commonly used in describing railway and
aircraft noise events. The European Environmental Noise Directive will
require mapping and reporting of a composite day-evening-night index,
L den
. L den
combines the L Aeq
for a 12 hour day period, with L Aeq
for a 4 hour