Page 119
Sound Pressure Level (Lp)
Like sound pressure this is the difference between the ambient sound pressure
and the instantaneous sound pressure caused by a noise source, however the
sound pressure level is expressed in decibels (dB).
L
p
20log
P
rms
P
o
dB
:=
where Prms is the root mean square sound pressure and Po is the reference
sound pressure of 20µPa.
Specific Noise
This is the equivalent continuous A-weighted sound pressure level at the
assessment position produced by the specific noise source over a given
reference time interval.
Speed of Sound (c)
The speed at which sound travels through a given medium.
The speed of sound in air is determined by the air itself, it is not dependent on
the amplitude or frequency of the sound wave and can be calculated using the
following equation: -
c
air
331.3 1
temp degC
(
)
273.15
+
⋅
ms
1
−
:=
Threshold Sound Level (Lt)
This is the level at below which the noise dose is not accumulated.
Where available, Castle instrumentation has selections of OFF, -5dB or -10dB.
As an example if the threshold is set at -5dB, then arithmetically subtract 5dB
from the selected criterion level to determine the level at below which sound
pressure levels can be ignored and not included in the dose accumulation: -
If Criterion Level = 85dB and Threshold = -5dB then the threshold sound level is
80dB.
This only affects %Dose calculations.
