Equipment decibel levels

Even though we cannot see sound, it has a force with real dimensions and three definite properties: intensity, frequency and duration.

* Intensity. This property is the loudness of a sound, or the pressure it exerts through the ear. It is measured in units called decibels (dBs).

The ear is a remarkable organ, responding to sounds ranging from dripping water to amplified music (see table below). The normal range of hearing begins at about 0 decibels, a level at which a person with excellent hearing is able to detect a sound. Typically a person begins to identify sounds when a level of 10 to 15 dBs is reached. This is the threshold of hearing. The other end of the scale is known as the threshold of pain (140 dB), or the point at which the average person experiences pain.

In assessing noise, a special measure called dBA indicates damage to hearing. The dBA rating is provided for many pieces of agricultural and outdoor power equipment. The higher the dBA number, the greater the risk of damage to hearing.

* Frequency. This property is the number of sound waves (high and low pressure areas) produced by a noise source passing a given point per second. We measure frequency in cycles per second (cps), also called Hertz (Hz). The higher the number, the higher the frequency.

The human voice has a range of about 200 to 4,000 Hz. A noise-induced hearing loss first causes the loss of the ability to hear sounds at 4,000 Hz. Then hearing loss proceeds until the ear cannot hear frequencies between 500 and 3,000 Hz, a range crucial to understanding conversation. One of the first signs of hearing loss is the inability to understand people (especially in a crowd) or other sources of voice communication, such as the television or radio. You become "hard of hearing," and sounds seem muffled.

The most dangerous sounds are those of high intensity (dB level) and high frequency. This is because a large number of sound waves are transmitted to the ears with a force greater than your ears can tolerate. You can't reverse noise-induced hearing loss, and a hearing aid does little good. Therefore, prevention is by far the best treatment. * Duration. This is the amount of time you are exposed to a sound level. In the table above, the right-hand column lists various high sound levels, and the left-hand column indicates the length of exposure that is safe for the corresponding noise level during a day. These figures have been determined after years of research on noise-induced hearing loss and are accepted as the standard for allowable noise-level exposures.

The average person can be exposed to a sound source producing 90 dBA for a maximum of 8 hours. If the sound level is 100 dBA, then the maximum exposure is 2 hours. An unprotected ear can be exposed to 115 dBA for a maximum of only 15 minutes a day. Your ears should not be directly exposed for any length of time to sounds greater than 115 dBA. For every 5 dB increase above 90 dBA, the permissible exposure time is reduced by half. For example, if you operated a tractor with a 95 dBA rating, you would be risking a hearing loss after 4 hours of exposure. If the tractor had a 90 dBA rating, you could use the tractor for 8 hours before reaching the same risk level.

Reducing your exposure You can reduce your exposure to loud noise using a variety of techniques. Engineering controls, when practical and economically feasible, are the most effective ways to reduce noise exposure because engineering controls can reduce sound level at the source. Some examples are:

* Replacing worn, loose or unbalanced machine parts to cut down on the amount of vibration generated.

* Making sure that machine parts are well-lubricated to cut down on the noise exposure created by friction.

* Installing a good, high-quality muffler on all engine-powered equipment to reduce vibration produced by airflow.

* Isolating yourself from the noise source with an acoustically designed cab if the equipment accepts one. In recent years, equipment manufacturers have designed cabs that reduce noise-level exposure to safe limits. Many of the new tractor cabs, for example, can reduce an operator's noise exposure by at least 10 to 15 dBs.

Of course, you must control noise not eliminated by engineering by using altered work schedules. When practical, arrange work schedules so that workers do not exceed the allowable exposure limit to a high noise source. For example, for a tractor that produces a noise level of 95 dBA, the safe exposure is 4 hours per day per person. Try to arrange work schedules that let workers exchange work activities so that no one person is exposed to the noise for more than 4 hours.

Personal protective equipment is the final alternative for workers to cut down on noise exposure. The two basic types of hearing protection are ear muffs and ear plugs. Ear muffs are the most effective. The attenuation (noise reduction) that ear muffs provide varies widely due to differences in size, shape, seal material, shell mass and type of suspension. Some may attenuate sound by as much as 40 dBs. To get good quality muffs, deal with a reputable firm. Examine them for comfort, construction, seal and attenuation. Manufacturers supply attenuation data for their products, so you can evaluate their effectiveness.

Ear plugs are available as pre-formed inserts made of rubber, plastic or foam, and hand-formed inserts of disposable materials such as wax or Swedish wool. For landscape use, wax or Swedish wool have little value from a sanitation standpoint (you must change them daily, and you must shape them by hand before inserting) and because of their lower attenuation level.

Pre-formed ear plugs may be cheaper, but due to the difference in the shape of a person's ear canal, trained personnel should fit each individual for plugs. You also must know how to properly insert the ear plug. When purchasing ear plugs, follow the directions closely so that you obtain a snug, tight fit in the ear canal when you insert the plug. Warning: You should never use cotton to try to reduce noise exposure. Cotton cannot block out high-frequency sound and will provide no protection from high sound levels.

Ear-protective devices will not block out all sounds. They will block out only those sounds that are dangerous to hearing. Machinery sounds different when you are wearing ear protection, but with continuous use, you can learn the new sounds and still be able to determine whether the machinery is operating properly.

Operators who have suffered previous hearing losses may find that they are unable to detect certain sounds necessary to assure proper machine operation when using ear protection. In these cases, it may be necessary for the operator to remove the ear protection to check for these sounds. But remember to properly replace the ear protection to protect against further hearing loss.

Have your hearing tested 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 the necessary steps to reduce exposure and eliminate further damage to your hearing.

David E. Baker is the assistant program director of the Agricultural Engineering Extension at the University of Missouri--Columbia.

Currently no standard exists in the United States as to where manufacturers must measure decibel levels when reporting that information about their equipment. Thus, to compare these noise levels, you must pay attention to where each equipment manufacturer measures its equipments' decibel levels. For example, a European standard--which applies to equipment shipped for sale to Europe--specifies measurements at the operator's ear. "That term, at the operator's ear, is an important distinction when comparing the published noise levels of two pieces of equipment," says Terry Herlihy, product manager for Jacobsen Division of Textron Inc. "There are many methods that can be used for measuring sound."

Herlihy explains that manufacturers may choose to use bystander noise levels and sound-pressure measurements made at 25- to 50-foot distances from the equipment producing the sound. "These should not be confused with decibel measurements made at the ear of the operator," he cautioned. Thus, you should take special care when comparing noise levels to make sure you compare ratings that were measured similarly.

When comparing differences in dBA levels, note that a difference as small as 3 dBA is significant because a small increase or decrease in decibel level makes asubstantial difference in how listeners perceive the sound. For example, a 10-decibel increase in sound makes it seem twice as loud to the ear--a 100-percent increase. NOTE: Grounds Maintenance is not attempting to assess the sound levels of the equipment mentioned in this article as to whether its noise level is considered "low decibel." You should carefully consider each manufacturers' statements about noise levels before making a purchase or deciding whether you need additional hearing protection.

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