Introduction to Health and Safety at Work

Physical and psychological health hazards and control
261
to hear speech clearly but the ability to hear is not 
lost completely
➤
permanent threshold shift – this results from pro-
longed exposure to loud noise and is irreversible due 
to the permanent reduction in nerve impulses to the 
brain. This shift is most marked at the 4000 Hz fre-
quency, which can lead to diffi culty in hearing certain 
consonants and some female voices;
➤
tinnitus – is the same as the acute form but becomes 
permanent. It is a very unpleasant condition, which 
can develop without warning.
It is important to note that, if the level of noise exposure 
remains unchanged, noise-induced hearing loss will lead 
to a permanent threshold shift affecting an increasing 
number of frequencies.
Presbycusis is the term used for hearing loss in older 
people which may have been exacerbated by occupa-
tional noise earlier in their lives.
15.4.2 Noise 
assessments
The Control of Noise at Work Regulations specify action 
levels at which the hearing of employees must be pro-
tected. The conclusion as to whether any of those lev-
els has been breached, is reached after an assessment 
of noise levels has been made. However, before noise 
assessment can be discussed, noise measurement and 
the statutory action levels must be described.
Noise measurement
Sound intensity is measured by a unit known as a pas-
cal (Pa – N/m
2
), which is a unit of pressure similar to that 
used when infl ating a tyre. If noise was measured in this 
way, a large scale of numbers would be required ranging 
from 1 at one end to 1 million at the other. The sound 
pressure level (SPL) is a more convenient scale because:
➤
it compresses the size of the scale by using a loga-
rithmic scale to the base 10
➤
it measures the ratio of the measured pressure, p, to 
a reference standard pressure, p
0
, which is the pres-
sure at the threshold of hearing (2 
⫻
10
⫺
5
Pa).
The unit is called a decibel (dB) and is defi ned as:
SPL