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Equivalent Continuous Sound Level (L
eq
)
Leq is the constant sound level over a given time that has the same sound
energy of the varying noise source over the same time period. It can be
measured using ‘A’, ‘C’ or ‘Z’ frequency weighting LAeq, LCeq, LZeq.
30
35
40
45
50
55
60
65
70
1
2
3
4
5
6
7
8
9
10
dB
Time (minutes)
Lp
Leq
See ‘Function Equations’ to see how Leq is described mathematically.
In practice when measuring noise it is possible to take Leq readings with your
instrument of short duration i.e. less than five minutes (providing all variations of
noise emissions are covered). If the measured environment changes greatly,
then the longer the Leq reading is taken, the more accurate the measurement
will be.
Exchange Rate
As the sound pressure level increases above the criterion level then the allowed
exposure time must be decreased. The allowed maximum exposure time is
calculated by incorporating an exchange rate value. The exchange rate value is
the amount by which the permitted sound level may increase if the exposure
time is halved.
Castle instrumentation generally allows a user selection of exchange rate to
allow for different legislation of 3dB, 4dB or 5dB.
Having an exchange rate of 3dB is more severe in terms of allowable exposure
times, and an exchange rate of 3dB is currently used in the UK.
The following tables show the relationship between the exchange rate, the
criterion level and the noise exposure limits: -
