Noise Induced Hearing Losses
Dr. Don Smith
Farm Management Specialist

This is due in part to the many potential sources of loud noise on the farm including: tractors, combines, grinders, choppers, shotguns, conveyors, grain dryers, chain saws, etc. Prolonged exposure to excessive noise can cause permanent hearing losses unless noise-control measurers are taken.

The human ear is composed of three major sections: the external, middle, and inner ear. Each section has a specific function in the hearing process.

The middle ear consists of three tiny bones, or ossicles, that are suspended in an air-filled space. These function as a mechanical lever system that delivers sound from the ear canal to the inner ear. Noise does not affect the middle ear unless there is an impact, sound, or pressure so great that it dislodges or fractures the ossicles.

The inner ear, or cochlea, is susceptible to damage from continued exposure to high-level noise. The cochlea is a fluid-filled hydraulic system driven by the piston action of the last ossicle. The vibrating motion of the ossicle produces a wave motion in a membrane that runs the full length of the cochlea.

Situated on top of the moving membrane are thousands of small hair-like structure with nerves connected to each hair cell. When a hair cell is bent because of membrane motion, the nerve fires and the message is transmitted to the brain. Hair cells near the base transmit information about high-frequency or high-pitch sound, while those at the apex provide information about low-pitch sound.

If hair cells in a particular region of the cochlea are destroyed, the nerves will not fire ,and the brain will not receive any information. If some of the hair cells are destroyed, the brain may receive a distorted message that it cannot interpret.

Typically, hair cells are damaged or destroyed when their supporting structure are overworked. With continued exposure to high-level noise the membrane motion is great, and the cell that support the hair cells swell. Eventually, they rupture, and the hair is destroyed or damaged. Only a few hair cells may be lost at a time, but with repeated exposure over days, months, and year, the cumulative effect can be substantial.

Even though we cannot see sound, it is a force with real dimensions and three definite properties: intensity, frequency, and duration. Intensity is the loudness of a sound, or the pressure it exerts through the ear. It is measured in units called decibels with the abbreviation of dB. Typically, a person begins to identify sounds when a level of 10 to 15 dBs is reached. This is known as the threshold of hearing. The other end of the scale is known as the threshold of pain which is normally reached at 140 dB. It has been established that hearing damage will likely occur with excessive exposure to noise levels above 90 dB.

Noise induced hearing commonly results in a high frequency hearing loss. Some of the affects of noise induced hearing losses include:

• Speech (especially women and children) may sound mumbly to the hearing impaired person and this affects their comprehension.

• The ability to identify the source of sound is greatly reduced, and the listener seems inattentive.

• When there is background noise, the listener with a high-frequency hearing loss cannot separate one voice from another and therefore cannot understand anyone speaking.

• With noise-induced hearing loss, there is often a reduced tolerance for loud sounds. Thus, the level of a sound must be high before it is understood, but if it is slightly higher, it may be intolerably loud.

• With noise-induced hearing loss, hearing aids can help, but they do not totally restore the quality of hearing that was present before the loss. Noise induced hearing losses are more difficult to correct with a hearing aid since certain frequencies of sound are destroyed more than others and common hearing aids amplify a wide range of sounds.

To minimize hearing damage persons should use three strategies: reduce the level of noise at the source, reduce the time of exposure to noise, and finally use hearing protection devices such as ear plugs. If you are continually exposed to high sound levels, you should have a hearing test periodically. This test, called an audiogram will reveal signs of hearing loss as a result of high sound level exposure. If a hearing loss is noted, take necessary steps to reduce exposure, and thus reduce further damage to your hearing.

For more information on this or other subjects visit the University Extension Center in your county, call me, Dr. Don Smith, at (660) 727-3339, or visit the Clark County University of Missouri Extension Internet Web Site at http://outreach.missouri.edu/clark/ for publications online. University of Missouri Extension programs and services are open to all and most are free or at a nominal charge.