What is noise? Noise was once described as “unwanted sound”. To the average person, “noise” is not only unwanted, but annoying or loud. Sound, or noise, is produced by the rapid compression and decompression of air. When you clap your hands, you are producing a sound (applause) by compressing and decompressing the air between the palms of your hands. The faster you clap, the higher frequency or pitch of the sound you are producing.
When sound waves enter your ear canal, they hit against the eardrum, a flexible piece of tissue that moves back and forth as the sound pressure builds up and falls off. Attached to the eardrum are three small, connected bones (the middle ear) which transmit movements of the ear drum to the inner ear. Inside the inner ear is a fluid that sloshes back and forth in the rhythm with the ear drum and middle ear. Special nerve cells leading to the brain have hair-like structures floating in the inner ear fluid. Movement of the eardrum, middle ear and inner ear fluid cause these “hairs” to sway back and forth. When they move, nerve impulses are sent to the brain where they are interpreted as sound soft or loud, pleasant or unpleasant, high pitched or low pitched.
When the sound is loud, the sound energy hitting the eardrum is high. This causes the hair cells to bend a lot, sending a more powerful impulse to the brain. If the sound is high pitched, the hair cells have very little time between sound waves to recover their normal, upright position.
When sound is both loud and high pitched, these inner ear nerve cells never get the chance to recover, and they die. If enough of them die, we notice that we don't hear as well as we did in the past. We may not hear faint sounds, or we may hear people speaking normally, but can't make out or understand what is being said. Once these cells die, they never regenerate. This type of deafness is permanent and irreversible—it's called “noise-induced hearing loss.”
Noise that results in damage to this hearing mechanism can come from any source—work, home, entertainment. Loud music from a live performance or a set of earphones, if loud and repeated, can have the same detrimental effect as a noisy machine at work. The noisy machine doesn't have to be at work either—it can be from leaf blowers, or a hobby such as race cars, power boats or a basement woodworking shop.
In the workplace, noise exposures are regulated by OSHA. Over a standard work-shift, the maximum allowable average noise exposure is 90 decibels (dB). Because this standard is not particularly protective, the idea of an Action Level was used. If full shift noise exposures are between 85 and 90 dB, a Hearing Conservation Program has to be started. This includes training and education, providing and using ear plugs or ear muffs to act as a barrier between the noise and the inner parts of the ear. Along with this, baseline and annual hearing tests are required and engineering means must be used to try to reduce the overall noise level to a point below 85 dB.
Communities often have noise ordinances. Unlike OSHA's standards, they are not intended to protect hearing. They are designed to prevent discomfort, such as limiting the use of noisy equipment to certain hours of the day or night.
Exposure to noise above 90 dB on a regular basis carries a real risk of hearing loss. The louder the noise and/or the longer the time of exposure, the greater the risk. Limiting the amount of time you spend in a noisy environment, and limiting the amount of noise will go a long way toward keeping your hearing healthy.
In a future article, we'll look at some techniques for reducing noise levels and protecting your hearing from damage.
OSHA has set a maximum 8-hour exposure to noise in the workplace at 90 decibels at 1,000 cycles per second (Hertz, Hz). Because this is not a particularly protective level, they instituted an Action Level (AL) of 85 decibels (dBA). If you have a workplace in which 8-hour time weighted average exposures are at or above 85 dBA, but less than 90 dBA, the regulations require a hearing conservation program, which includes training, hearing protective devices, baseline and annual hearing tests and feasible engineering controls to reduce the noise levels to a point below 85 dBA.
Communities often have noise standards as well, but these are usually set for comfort, or disturbance.
In a future article, we will take up engineering controls and how noise levels can be reduced.