CR:270 Series User Manual
Page 18
Therefore, in this example, the ear protection reduces the noise level from 103.2 dB(A) to 82
dB(A).
Example of the HML Method
The noise levels measured from a machine are:
L
A
or L
Aeq
(A weighted Leq)
103.2 dB(A)
L
C
or L
Ceq
(C weighted Leq)
103.4 dB(C)
The hearing protection comes with the following HML data:
H
25dB
M
19dB
L
13dB
The effective A-weighted sound pressure level at the ear can be estimated by using the
HML attenuation data.
The predicted noise level reduction (PNR) is calculated from the H,M and L values. The
formula used depends upon the difference between the L
Ceq
and the L
Aeq
.
If L
Ceq
- L
Aeq
is less than or equal to 2
)
2
(
4
)
(
A
C
L
L
x
M
H
M
PNR
If L
Ceq
- L
Aeq
is greater than 2:
)
2
(
8
)
(
A
C
L
L
x
L
H
M
PNR
The A-weighted sound pressure level at the ear is given by subtracting the appropriate
PNR value (rounded to the nearst whole number) from the measured A-weighted sound
pressure level L
A
or L
Aeq
.
In this example, L
C
- L
A
= 0.2dB therefore the first equation for PNR is used:
PNR = 19 - (25 - 19)/4 x (0.2 - 2)
21.7 dB
A-weighted sound pressure at the ear = L
A
- PNR
81.5 dB(A)
Round to the nearest whole number
82 dB(A)
