Casella CEL CEL-272 - Manual (Page 5)

Sound Exposure and Noise Dose
Sound Exposure is often
thought of in terms of noise
dose because it is similar in
concept to the "dose-exposure"
relationship used in occupa-
tional medicine when defining
permissible human exposure lim-
its to toxic substances. Both
terms are in common use, but
the new international standards
specify sound exposure.
The value of sound exposure
measured (E
A ) is the product of
exposure time and the amount
of A-weighted energy contained
in the sound. Sound energy is
proportional to the square of
sound pressure and has the
units Pascal-squared (Pa
2
), so
that the units of sound exposure
become Pascal-squared sec-
onds (as stipulated by the ISO
Standard).
However, Pascal-squared hours
(Pa
2
h) are normally used in or-
der to obtain more manageable
numbers (IEC Standard). There-
fore E
A can be defined as
follows:
E A =
∫
t
1
t
2
P
A
2 ( t ) .dt
Where:
t
1 = Integration start time
(hours),
t
2
= Integration end time
(hours),
P A = Instantaneous
A-weighted sound
pressure (Pa).
A sound exposure value of
1 Pa
2 h is equivalent to a con-
tinuous sound level of
approximately 85 dB(A) for 8
hours (see chart below). A
sound exposure value of
3.2 Pa
2
h is exactly equivalent to
a continuous sound level of
90 dB(A) for 8 hours, and the
CEL-272 has been standardised
to indicate 100% Dose for this
value.
Noise Dose is not an absolute
measurement but a percentage
of the recommended maximum
daily personal noise exposure.
In this way, 100% Noise Dose
will be equal to the exposure
limit for the person, while values
lower than 100% indicate lesser
exposure than the allowable
dose and values above 100%
show excessive exposure.
The Daily Sound Exposure
(E
A,d
) is the total A-weighted
sound exposure sustained in a
"typical working day", while the
Daily Sound Exposure Level
(L
EP,d ) is the E A,d expressed as
an equivalent continuous sound
level (L
Aeq
) over a normalised
working day of 8 hours.
The formula for L Aeq is:
L Aeq = 10log
1
T
∫
(
0
T
P
A
P
0
) 2
dt
Which for a normalised 8 hour
day becomes:
L EP,d = 10log
1
T
0
E
A,d
( P
0
) 2
Where:
T
0
= 8 hours,
P 0 = 2 x 10
-5
Pa
Therefore the formula can be
written as:
L
EP,d
= 10 Log E
A,d
+ 85
A projected sound exposure
can be calculated when a sam-
ple exposure has been
obtained over a shorter period
than the standard 8 hour work-
ing day. If this exposure is
expected to continue for the
remainder of the standard day,
the sample can be projected for
the whole day as follows:
E
A,d
= E
A,Te
d
T
e
( Pa 2 h )
Where:
E
A
,
Te
= Measured exposure
over sample period T e ,
d = Length of actual
working day in hours,
T e = Exposure time of
sample in hours.
The relationships between
Sound Exposure (E
A ), Noise
Pa h
2
L
EP,d
dB(A)
E
D %Dose
900045
A
80 82 84 86 88 90 92 94 96 98 100 102 104 106
0.3 0.4 0.5 0.6 0.8 1.0 2.0 3.0 4.0 5.0 6.0 8.0 10 20 30 40 50 60 80 100
10 20 30 40 50 60 80 100 200 300 400 600 8001000 2000 30004000
1st Action
Level 1.0 2nd Action
Level 3.2 Peak Action Level 200 Pa
Triggers Overload Indication
5 Black
Register
Marks 5 CEL Blue