Noise is a vibration in the air transmitted by the eardrum and bones of the middle ear to the inner ear. The inner ear is full of liquid, which is set in motion by the noise. This motion is picked up by thousands of tiny hair cells. The hair cells, when the motion of the liquid bends them, send nerve messages to the brain and so we hear sound.

The cells bend and sway in the sound waves like trees in the wind. However if the wind blows too hard or for too long, the tree may loose its resilience and become permanently bent or broken. In the same way, the hair cells in the ear can be damaged.

At first, given quiet periods, like the trees, they rebound and recover. But repeated noise will damage them permanently. The damaged cells cannot transmit messages to the brain. The result is deafness to a greater or lesser degree.

The higher pitched sounds are the fi rst to be affected and so a person is deafened to sounds such as frying bacon and the higher pitched sounds of speech, such as 'S', 'T', 'K' and 'C'. This partial deafness leads to misunderstandings when listening to others speak.

The national standard for exposure to noise during a working day of eight-hours is 85 dBA and 140 dBA (lin) for a peak noise level (See the National Standard for Occupational Noise and the National Code of Practice for Noise Management and Protection of Hearing at Work NOHSC:1007:(2000) and NOHSC:2009(2000).

The law also states that noise above 85 dBA must be reduced by control design, isolation, enclosure and replacement procedures. However, the standard