This article focuses on the relationship of noise exposure and the development of tinnitus.
The hearing mechanism is very delicate in all animals. Also called the auditory or audio-vestibular system, this system is generally thought to be composed of three parts. In actuality, there are four.
1. The External ear consists of the auricle, the skin covered cartilaginous appendage that protrudes from the side of the head and collects and funnels sound toward the ear canal. The ear canal then directs sound inward toward the delicate ear drum, or tympanic membrane.
2. The middle ear consists of the tympanic membrane and three tiny bones of hearing-the malleus, incus, and stapes. These bones modulate and transmit sound vibrations from the tympanic Silencil membrane to the fluid contained within the inner ear.
3. The inner ear consists of two distinct systems: the snail-shaped cochlea, for receiving sound, and the vestibular apparatus with its three semicircular canals, for regulating equilibrium, or balance. In each system, physical vibrations are converted into electrical impulses. The eighth cranial nerve then transmits these electrical impulses to respective auditory and vestibular centers in the brain.
4. Auditory and vestibular centers in the brain interpret electrical impulses from the eighth cranial nerve as sound or conditions of relative balance or imbalance.
The loudness of sound is expressed as sound pressure, and measured in decibels (dB.) All sound received in the inner ear results in vibrations of delicate cells known a “hair cells.” These cells rest upon a delicate membrane suspended within the inner ear fluid. Sound above known safe levels vibrates the membrane and hair cells so violently that individual cells may be disrupted or damaged beyond repair. Badly damaged cells cannot function to receive and transmit sound to the cranial nerves and brain. They no longer “hear” sound.