Effects of exposure to high noise levels

Sound travels through the auditory canal and causes the eardrum to vibrate. These vibrations are amplified by three little bones (the ossicles) and transmitted to the cochlea which is the hearing organ in the inner ear. Sound entering the cochlea cause hair cells to vibrate and the energy is converted into electrical signals which are sent via the auditory nerve to the brain which interprets them as sound.Excessive noise can lead to damage to the hair cells and eventually noise induced hearing loss when the cells become permanently damaged.Excessive noise energy initially invokes a protection reflex which damps the nerve impulses making the ear less sensitive to low noise levels. This is known as threshold shift. This can either be temporary or permanent.Noise induced hearing loss (NIHL) This is caused when the hair cells in the inner ear are permanently damaged and fail to respond to the movement of the cochlea. Once damaged the hair cells do not re-grow.Acute effects.These effects are usually reversible with the ear being affected for short periods of time only – typically 24 hours.Acute effects include:Acute acoustic trauma from gunfire, explosions affecting the ear drum and the ossicles. In extreme cases the ear drum can perforate.Temporary threshold shift from short exposure affecting the cochlea Tinnitus (ringing in the ears) caused by intense stimulation of the auditory nerves and hair cells.

Chronic effectsThese effects are usually caused by long term exposure to excessive noise levels and in most cases the condition is irreversible.Chronic effects include:

Permanent threshold shift  Noise induced hearing loss affects the ability to hear human speech and is irreversible but can be compensated for to a certain extent. Tinnitus can become chronic without warning and is mainly irreversiblePermanent threshold shift usually shows a greater shift than temporary.

Presbycusis

 This is the term used to describe hearing loss due to ageing. Changes in the ossicles prevents them from transmitting the higher frequencies and an audiogram will show a loss of hearing starting at about 1 kHz and falling down to 8 kHz. Between the ages of 30 and 60 the hearing loss can increase from about 5 dB to 20 dB.