Introduction to Health and Safety at Work

Introduction to Health and Safety at Work
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provide employees with information and training
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if a manufacturer or supplier of equipment, to pro-
vide relevant noise data, on that equipment (particu-
larly if any of the three action levels is likely to be 
reached).
The main purpose of the Noise Regulations is to control 
noise levels rather than measuring them. This involves 
the better design of machines, equipment and work 
processes, ensuring that personal protective equipment 
is correctly worn and employees are given adequate 
training and health surveillance.
Sound is transmitted through the air by sound waves 
which are produced by vibrating objects. The vibrations 
cause a pressure wave which can be detected by a 
receiver, such as a microphone or the human ear. The 
ear may detect vibrations which vary from 20 to 20 000 
(typically 50–16 000) cycles each second (or Hertz – Hz). 
Sound travels through air at a fi nite speed (342 m/s at 
20ºC and sea level). The existence of this speed is shown 
by the time lag between lightning and thunder during a 
thunderstorm. Noise normally describes loud, sudden, 
harsh or irritating sounds.
Noise may be transmitted directly through the air, by 
refl ection from surrounding walls or buildings or through 
the structure of a fl oor or building. In construction work, 
the noise and vibrations from a pneumatic drill will be 
transmitted from the drill itself, the ground being drilled 
and from the walls of surrounding buildings.
15.4.1 Health effects of noise
The human ear
There are three sections of the ear – the outer (or exter-
nal) ear, the middle ear and the inner (or internal) ear. 
The sound pressure wave passes into and through the 
outer ear and strikes the eardrum causing it to vibrate. 
The eardrum is situated approximately 25 mm inside the 
head. The vibration of the eardrum causes the propor-
tional movement of three interconnected small bones in 
the middle ear, thus passing the sound to the cochlea 
situated in the inner ear.
Within the cochlea the sound is transmitted to a 
fl uid causing it to vibrate. The motion of the fl uid induces 
a membrane to vibrate which, in turn, causes hair cells 
attached to the membrane to bend. The movement 
of the hair cells causes a minute electrical impulse to 
be transmitted to the brain along the auditory nerve. 
Those hairs nearest to the middle ear respond to high 
frequency, while those at the tip of the cochlea respond 
to lower frequencies.
There are about 30 000 hair cells within the ear and 
noise-induced hearing loss causes irreversible damage 
to these hair cells.