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7.
Sound
Pressure
Level
‐
Technical
notes
1.
The decibel (dB)
The range over which the human ear responds to sound pressure (noise) is extremely large; in
Pascals (Pa) it is 20
Pa (the threshold of hearing) to 100 Pa (the threshold of pain). The
measurement of sound pressure has been made more convenient by the use of the decibel,
which is logarithmic. However, decibels are non
‐
linear and therefore cannot be added
together. A simple rule is that doubling the amplitude of the noise under test causes the
level to rise by 3dB.
2.
SLM
‐
sound level meter: Instantaneous sound pressure level (SPL) is used for spot checks to
establish instantaneous noise levels. SPL is defined by the logarithmic equation:
SPL (in dB): 20 log
10
0
P
P
Where P = rms measured sound pressure level
P
0
= rms reference sound pressure level (20
Pa)
3.
Leq
‐
level equivalent (continuous): Leq is used to assess the rms average noise level over a
preset period of time, often the starting point of a noise assessment. To take a Leq
measurement the period of time over which it is to be made must be selected. The longer
the period of measurement time, the more accurate the Leq reading will be, a typical
period is 8 hours (the length of a working day).
4.
SEL – Sound exposure level (L
E
)
SEL measurements are almost identical to Leq measurements but normalized or compressed to 1
second. This allows the total sound energy of an event, such as train passing a platform, to
be evaluated. Another event, such as the next train, which lasts for a different amount of
time, can be measured in the same way. The two readings can be compared to assess how
much total noise the passengers standing at the platform were exposed to by each train.
