One of the most important ways to obtain information and know what is happening around us is through sound. We talk with others, get weather information by listening to wind, thunder, and rain, know if a machine is working properly or if a pig is stuck in a hole in the fence, and we listen to beautiful music. Thus, sound can be useful and pleasing, or it can be unpleasant, irritating, and damaging to one's health. The latter, or unwanted sounds, are called noise.
Movement of people, animals, machines, and other things cause pressure waves in the air. If these pressure waves are within a certain range of frequencies, our ears interpret them as sound.
Pressure waves or sound waves are similar to waves created by dropping a stone in a pond of water. A large stone causes higher waves. Similarly, loud sounds are caused by larger pressure waves. For example, a jet plane produces big and small pressure waves; a whisper causes only small pressure waves. The size or amplitude of these waves, called sound pressure level, is measured in decibels, abbreviated dB. The unit dB(A)* is used to indicate the loudness of sound as received or heard by the ear. The decibel scale is setup such that the sound level doubles for each increase of ten numbers on the scale, i.e. 90dB(A) is twice as loud as 80dB(A).
Frequency is the number of pressure waves occurring per second. A bass violin produces low frequency sound waves, whereas, a high note of a violin produces a high frequency sound. Sound levels in dB(A) from various sources are shown in Table 1. *The letter "A" indicates the scale used to measure the sound on the sound meter.
| Table 1. Typical Sound Levels | |
|---|---|
| dB(A) | Source of Sound |
| 140 | Threshold of Pain |
| 130 | Jet Takeoff (200 feet) |
| 120 | Jackhammer |
| Chain Saw | |
| 110 | |
| 100 | Tractors w/o soundproof cabs and farm machines |
| 90 | |
| 85 | Tractors with soundproof cabs |
| 80 | Heavy Traffic |
| 70 | Vacuum Cleaner (10 feet) |
| 60 | |
| 50 | Speech |
| 40 | Leaves Rustling |
| 20 | Soft Whisper |
| 10 | Threshold of Hearing |
| 0 | Weakest Sound |
Sound is transferred to the ear by sound waves emitted from a vibrating source such as a tractor.
As illustrated in Figure 1, our ears consist of three major parts: the outer, middle, and inner ear. The outer ear funnels sound waves through the ear canal, where they strike the eardrum. The vibration of the eardrum is transmitted to the nerve of the inner ear by three small bones which make up the middle ear. The sensitive nerve of the inner ear converts the vibrations to nerve impulses. The nerve impulses are sent to the brain, which then identifies the sound.
Sound levels are included as an industrial standard of the Occupational Safety and Health Act (OSHA), Table 2. Note that as the sound level increases, the permissible duration of exposure decreases. Eight hours of exposure is permitted at 90 dB(A). However, OSHA requires that employees be placed on a hearing conservation program if they are exposed to average noise levels of 85dB or greater over an 8 hour work day. There is much speculation as to whether the levels suggested are really safe. This is particularly true on the farm where the exposure may be much longer than 8 hours. The National Safety Council recommends 85 dB(A) for 8 hours of exposure as the safe limit for farm operations.
| Table 2. Permissible Noise Exposures | |
|---|---|
| Duration per day (hours) | Sound level, dB(A) |
| 8 | 90 |
| 6 | 92 |
| 4 | 95 |
| 3 | 97 |
| 2 | 100 |
| 1-1/2 | 102 |
| 1 | 105 |
| 1/2 | 110 |
| 1/4 | 115 |
Since 1970 the Nebraska Tractor Test Center has measured sound levels, at the operator's ear, of a representative number of tractors. The average sound level of all new tractors tested in 1970 was over 98 dB(A) at maximum power and nearly 95 dB(A) at 50 percent of maximum pull. Some older tractors produced as much as 111 dB(A) when tested. Although a few models are within the 85dB(A) range, most tractors being tested today, without cabs, still are at or over 91dB(A). These ound levels undoubtedly increase with age. Other machines such as self-propelled combines, corn pickers, hammermills, and dryers produced sound levels exceeding 100 dB(A).
Loud sounds can cause a significant loss of hearing. The amount of hearing loss experienced is related not only to the loudness of the sound but also to the frequency (pitch) and to the length of time exposure. Higher frequency sounds are much more damaging than low ones. Thus, the ability of the ear to hear high frequencies is usually the indication of damage. People who are experiencing a hearing loss may first notice this as an inability to hear higher pitched sounds such as the notes of music. Eventually the ability to understand speech is affected. Some danger signals to be aware of are (1) your ears ring or you experience head noises a few hours after you get off the tractor following a day's work, or (2) your speech seems muffled when you talk after being around loud noises for extended periods.
Noise reduction can be accomplished by either reducing the amount of noise produced or by placing some type of barrier between the noise source and the ear.
Generally speaking, reducing the amount of noise produced by machines is an engineering challenge. Progress is being made in designing machines that produce less noise. Yet, the sound produced may still exceed safe levels when operators are exposed for long periods.
Good maintenance practices are an effective way to reduce noise exposure because they reduce noise at the source. Examples are:
Tractor cabs may or may not provide protection from noise. Those isolated from tractors by vibration mounts, covered on the inside with sound proofing or deadening material, and tightly closed to prevent air-borne noises from entering do protect the operator. Noise level in cabs without these features may be higher than if no cab was used.
An effective and inexpensive method of protection is to reduce the sound level at the ear by using acoustical earmuffs. These specially designed earmuffs cover the ears and provide a barrier to noise. Properly designed earmuffs reduce the sound to a safe level but still permit sound to reach the ears for informational and safety purposes. Actually, you can hear conversation and sounds that might indicate trouble in the machinery just as well with earmuffs as without earmuffs.
Protective devices such as ear plugs, which are designed to fit in the ear, are effective if properly fitted. When purchasing ear plugs make sure to follow the directions for insertion so that a snug, tight fit is obtained. Plugs must be kept clean to prevent ear infection. Because of their size they are easily lost so consider purchasing those that have a carrying case. Cotton in the ears is practically worthless as a protective device. Table 3 more fully describes typical hearing protection.
Wise tractor operators consider the inconvenience of using ear protection such as earmuffs a small price to pay for protection of their hearing.
Operators of noisy farm equipment have an easy choice-they should get and use proper hearing protection. The cost and inconvenience of protecting hearing is practically nothing compared to wearing hearing aids or not being able to hear at all. While any noise reduction is better than none, it is best to be on the safe side and use more than you think you'll need.
Most people readily adjust to the use of hearing protectors and in a short time forget they are wearing them. In most cases, other advantages, such as less fatigue and stress, will increase as one becomes accustomed to them. Don't overrate the disadvantage on the basis of a short trial period.
Obviously, the time to take action is before any permanent damage is done. Have your hearing checked to determine your present condition, then avoid noisy situations...and protect your ear from those you can't avoid.
Plugs are compressed or shaped prior to insertion; expandable to provide a snug fit. One size fits all.
Plugs made of flexible material preformed to fit the ear. May have a joining cord to prevent loss. Are sold in one size and must be fit (sized) for each ear.
Adjustable headband with rigid cups and cushions that seal around the ear. Muffs and plugs may be worn together for additional protection.
Figure 1. Parts of the ear.
Reviewed by Dr. Thomas G. Carpenter and Dr. Randall K. Wood
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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