Answer
Standard Permissible Noise Levels (IMO Resolution MSC.337(91))
Under the Code on Noise Levels On Board Ships, strict limits protect shipboard personnel from hearing loss:
| Space | Maximum noise level |
|---|---|
| Machinery spaces/engine room | 110 dB(A) |
| Machinery control room/ECR | 75 dB(A) |
| Workshop outside machinery space | 85 dB(A) |
| Other workspaces | 85 dB(A) |
| Normally unoccupied machinery spaces | 90 dB(A) |
| Navigating bridge/chartroom | 65 dB(A) |
| Cabins and hospital—1,600 to below 10,000 GT | 60 dB(A) |
| Cabins and hospital—10,000 GT and above | 55 dB(A) |
Definition of Decibel
A decibel, dB, is a logarithmic unit used to express the ratio between a measured sound quantity and a reference quantity.
It is not an absolute unit like metre or kilogram. It expresses how many times greater or smaller one sound is relative to the reference sound.
dB(A) — A-weighted decibel
dB(A) is the sound-pressure level measured using an A-weighting filter.
The human ear is not equally sensitive to all frequencies. At ordinary sound levels:
We are less sensitive to very low-frequency sound
We are most sensitive to frequencies roughly in the speech range
We are also less sensitive to very high frequencies
The A-weighting filter therefore reduces the contribution of:
Low frequencies, especially below about 500 Hz
Very high frequencies
It gives a reading intended to approximate the way the human ear perceives sound for occupational-noise assessment.
Main uses onboard
dB(A) is normally used for:
Engine-room noise limits
ECR noise limits
Crew accommodation noise limits
Eight-hour and 24-hour occupational exposure
Noise-survey reports
Hearing-conservation programmes
dB(C) — C-weighted decibel
dB(C) is measured using a C-weighting filter.
C-weighting is much flatter than A-weighting. It includes substantially more low-frequency sound and is suitable for very high sound levels.
It is commonly used for:
Peak-noise measurement
Impact or impulse noise
High-intensity machinery noise
Low-frequency noise evaluation
Selection and assessment of hearing protection
Comparison with dB(A) to identify low-frequency content