Page 109
Ear Plugs:
These can be either disposable or re-useable and are made from a variety of
materials.
Disposable
plugs can be manufactured from glass down, plastic coated glass
down and wax impregnated cotton.
Re-usable
plugs can be made from permanent moulded plastic, paste filled
rubber and paste filled plastic. (All re-usable plugs require washing after use and
a sterile place for storage).
Semi-inserts:
These are pre-moulded ear caps attached to a headband which presses them
against the entrance to the ear canal. This type of protector can be useful for
those who spend short periods of time in ear protection zones.
Special types of protector:
Sophisticated ear protectors are now available which provide additional noise
control facilities, for example built-in electronic active control systems.
Protection Calculation - Octave Band Analysis Method
To calculate the degree of protection given by hearing protectors to the highest
accuracy it is necessary to measure the sound spectrum of the noise emitted at
the workplace, using octave band analysis.
If the result is required in dBA, the A-weighting values at each octave frequency
should be subtracted from the measured sound (Z-weighted) to find the
‘corrected level’. Each corrected level can then be added together
logarithmically to find the estimated noise level: -
Unit
Octave Band Levels
Hz
63 125 250 500 1k
2k
4k
8k
Measured LP
(Z-weighted)
dB
92
96 102 101 98
97
94
93
A- weighting correction
dB
-26 -16
-9
-3
0
1
1
-1
Corrected level
dB
66
80
93
98
98
98
95
92
Estimated noise level
104 dBA
The assumed protection of the hearing protector is given by the mean
attenuation minus one standard deviation.
So, for example, at 500 Hz the assumed protection is 33 – 6 = 27 dB.
The assumed protection level should then be subtracted from the corrected
levels (as above) to produce the estimated dBA at the wearer’s ear and the
octave band results logarithmically added as before to give the predicted level of
noise with the hearing protection.
