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 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.
The function of cells in the auditory centers of the brain is to receive and interpret electrical impulses as sounds. These cells determine whether given impulses represent a bird singing, a piano playing conversation, traffic noise, or other. In the absence of sound impulses, these cells become extra sensitive to any received stimulus, regardless of its source. In the presence of silence, from whatever cause, they literally strain to interpret sounds of any type. When silence is extreme, they become so sensitive as to pick up stimuli from the normal energy flow of adjacent brain cells, and interpret these sensations as sound. This is thought to be the primary mechanism of the most common type of tinnitus.
Anything that interrupts the normal transmission of external sound vibration from its source to the auditory centers in the brain, results in a “hearing loss.” In turn, any significant hearing loss may result in tinnitus. We typically hear normal speech in the frequencies of 500 to 2000 cycles per second (cps.) Even when hearing seems to be normal for speech, however, tinnitus may occur because of a loss at higher frequencies.
The most common cause for hearing loss in our society is exposure to noise loud enough to damage hair cells in the inner ears. Repeated exposure or our ears to noise levels above 80 to 85 DB often inflicts permanent damage. This is known as acoustic trauma. Such damage generally occurs in the higher frequencies around 4000 cps. Hair cells that receive vibrations in this frequency range appear to be more vulnerable than others. The damage from acoustic trauma is often permanent.
Rather than search for tinnitus treatments or tinnitus cures after the damage is done, it’s far more sensible to avoid such damage. While help is definitely available, there are no miracle cures for tinnitus. To avoid ringing in the ears from acoustic trauma, prevention is a far better solution.
I will detail the average levels of common noise sources in a subsequent article.