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Percentile Sound Levels
This is the sound pressure which has been subject to the A-weighting frequency
filter and Fast response time weighting that is exceeded for n percent of the
time.
Percentile levels are used greatly when measuring environmental noise and
referred to as Ln. The value of n may be anything from 1 to 99 and is the noise
level exceeded for n% of the measurement time.
By definition of percentiles, L1 must be greater than or equal to L2 which must
be greater than or equal to L3 etc. It is often the case that only a few Ln values
are ever used.
10
20
30
40
50
60
70
80
90
1
2
3
4
5
6
7
8
9
10
dB
Time (mins)
Lp
L10
L50
L90
The figure above shows indicative L10, L50, L90 and the Sound Pressure Level
(Lp) values.
L10 is the noise level exceeded for 10% of the measurement duration. This is
often used to give an indication of the upper limit of fluctuating noise, such as
that from road traffic.
L50 is the noise level exceeded for 50% of the measurement duration. It is
simply the ‘middle point’ exceeded for 50% of the measurement duration and
has been incorporated in some American Community Noise Assessments.
L90
is typically taken as the ambient or background noise level. As an example,
it can be used in BS4142: ’Rating industrial noise affecting mixed residential and
industrial areas’.
(L10 - L90) known as the Traffic Noise Index is used in the UK to measure
annoyance responses to traffic:-
TNI
4 L
10
L
90
−
(
)
⋅
L
90
30
−
(
)
+
dB
:=
