5958_MCA Noise v2_4.indd - Institute of Sound and Vibration ...

Code of Practice for Controlling Risks due to Noise on Ships
22
Response, dB
Frequency, Hz
10
10
100 1,000 10,000
0
-10
-20
-30
-40
-50
-60
-70
-80
C-weighting
A-weighting
Figure 1 The frequency responses of the A‑weighting (green line) and the C‑weighting (grey
line). The horizontal scale is frequency in hertz (Hz). The vertical scale is the response in
decibels (dB) relative to 1000 Hz. Note that the A‑weighting discriminates against very low
frequencies of sound; the negative sign indicates an attenuation of the low‑frequency energy
of the sound being measured.
Together, the L Aeq and L Ceq values will be useful in assessing low‑frequency
noise, and in picking suitable hearing protectors for an occupational noise
environment.
The measurement instrument must also be able to measure and report the
instantaneous peak sound pressure levels L Cpk of impulse/impact noises
to which workers might be exposed. Such noises may be defined as a
noise event of short duration which occurs as an isolated event or as one
of a series of events with a repetition rate of less than 15 per second. (An
example of impulse noise aboard a ship is the starting air release on some
slow‑speed engines; an impact noise might be the blow of a hammer.)
Meters will usually report the highest L Cpk occurring at any time during the
noise sampling period. This value should be compared to the action and
limit values of the 2007 Noise Regulations. Ensure that the sound level
meter is capable of measuring instantaneous peak sound pressure levels up
to 140 dB L Cpk , the exposure limit value.
5958_MCA Noise v2_4.indd 22 26/2/10 10:02:00