HEARING PROTECTION

Wayland McKenzie and Mary Andersen
Occupational Medicine, UMC

Sound has three basic properties that are important in hearing protection; loudness, frequency and duration. Loudness (intensity) is measured in decibels, abbreviated dB. Most people can just barely hear a noise at 15 dB. Normal speech is about 60 dB. The OSHA limit, at which hearing damage occurs after too much exposure, is 90 dB. A typical tractor or other farm equipment usually is about 100 dB, reduced to 85 in a properly sound-insulated cab. A chain saw is about 120 dB, and the level above which you feel pain, as when a gun is fired near by, is 140 dB.

The length of time you listen to a loud noise is important in how much damage it causes, as anyone who has had to listen to a loud band can testify. If you leave the dance hall right away, your ears stop ringing almost immediately, but if you're there all night, your ears may still be ringing the next morning. Based on extensive hearing research, OSHA has established a standard for the length of time a person should be exposed to certain sound levels:

          Duration (Hours per day)           Sound Level (dBA)*
            8                                   90
            6                                   92
            4                                   95
            3                                   97
            2                                  100
            1 1/2                              102
            1                                  105
            1/2                                110
            < 15 min                           115

* decibels on an "A-weighted" scale. For now, assume this is equal to dB. To learn how to estimate dBA on this scale, consult Code of Federal Regulations 29 CFR 1910.95.

You will notice that a properly sound-insulated tractor does not fall under the OSHA guidelines, because it produces less than 90 dBA. However, a tractor without an insulated cab, producing about 100 dBA, should only be operated a total of 2 hours per day to avoid hearing loss. If you need to use such a tractor, you need hearing protection that will reduce the noise exposure to acceptable levels, or else plan your work schedule so that you won't be exposed for more than two hours per day. Remember, too, that if the machinery is not working properly, the noise level may rise above its listed dBA to the point where it does hurt your hearing. Notice that chainsaws and gunshots exceed the upper limit for ANY exposure. You should wear hearing protection when exposed to these noises, using the information provided by the manufacturer to make sure it reduces noise to safe levels. For more information about OSHA rules, read 29 CFR 1910.95 or contact the Missouri Dept. of Labor and Industrial Relations, 722 Jefferson St., Jefferson City, Mo. 65101. Tel. (800)392-0208.

The frequency of sound is important to your hearing, too. Frequency, measured in Hertz, or waves per second, indicates the number of sound waves that strike your eardrum each second. Since each sound wave louder than 90 dB can damage the ear, a 90 dB sound of 2000 Hertz could possibly do twice as much damage as an equally loud (90 dB) sound of 1000 Hertz. There is an advantage in frequency differences that should not be missed. Human speech normally ranges from 500-2000 Hertz, while machinery noise typically emits more than 2000 Hertz. Since hearing protectors block some frequencies better than others, it is possible to have effective hearing protection from machinery noise that does not significantly affect your ability to hear someone trying to speak to you. Hard as it may be to believe, even the very inexpensive little foam plugs do a fairly good job of blocking machinery noise while only slightly reducing the ability to hear human speech. By the way, cotton is almost totally ineffective in offering hearing protection-it's better at blocking voices than it is at blocking high frequency noise.

Finally, it's well worthwhile to have your physician give you a hearing test periodically. These inexpensive and quick tests may help you to detect a developing problem before it becomes too serious. If you have a family history of hearing loss or earwax buildup you certainly should do this anyway. If you have problems with ringing or pain in your ears, don't delay having this checked.