Properties of sound.
Hearing basics. The range of frequency (pitch) perceived by a person with normal hearing ranges from about 20 to 20,000 cycles per second. Many animals can hear sounds at frequencies higher or lower than humans.
Human hearing is most sensitive at those frequencies (pitches) that tend to be more useful to our activities-speech perception, for example. Hearing sensitivity is greatest (we can hear sounds of lowest intensity) at approximately 1,000 cycles per second, equivalent to middle C on a piano. Sensitivity to sounds of higher or lower pitch is less and drops off gradually until a normal person cannot hear any sound at frequencies above 20,000 or below 20 cycles per second.
How to tell if it's too noisy. Few people report to work with a sound-level meter. However, some general indications can be used to identify noise-hazardous situations and equipment:
* Is the area, operation or equipment posted as noise-hazardous in accordance with DoD/Navy regulations? Certain locations, such as the exterior of combat vehicles, are likely to be noise-hazardous without being posted. (Bright, orange signs are inconsistent with good camouflage.) However, relevant manuals and the guidelines that follow should provide good indications.
* Is it necessary to significantly raise your voice to communicate at approximately arm's length (about 1 yard/1 meter)? If so, the sound level likely is in the range of 84 dBA or higher. Use of protective equipment is warranted.
* Do you or your colleagues notice symptoms of acute noise exposure/temporary threshold shift during or immediately after work in the area? Symptoms may include ringing in the ears, difficulty in communication, and raised voice levels.
Speech range and why frequency matters. Most human speech occurs in the range of 500 to 4,000 cycles per second (hertz, Hz). The fundamental frequency of men's voices tends to be lower in the range, whereas women and especially children tend to project in the upper end. Regardless of the fundamental frequency, the components of individual speech sounds play a vital role in communication.
Vowel sounds (a-e-i-o-u) and some consonants/ consonant combinations (such as r-g-gr-h-k) tend to be at the lower pitch range. If you say these sounds while gently feeling your throat, you may feel a low-pitched vibration. On the other hand, most of the consonants might be called "soft" or "windy" sounds because they are formed by subtle air movements across the tongue and teeth. If you listen while forming sounds such as those represented by the letters th-s-f-t-p-ts, you will notice that these are both higher pitched and formed in the front of the mouth. The "windy" consonants often are the sounds that help differentiate one word from another (examples: hiss vs. his, that vs. sat).
Effect of noise exposure. Noise exposure above occupational limits is associated with creating a shift in hearing acuity, typically measured as the softest sound audible at a given frequency (pitch). Initial shifts likely are to be temporary, and subjects typically recover all or most of their hearing after a certain period of time. A ringing in the ears and/or increase in the lowest level of sound detected during and immediately after unprotected noise exposure is a common symptom of temporary threshold shift. Change in voice volume immediately after exposure (typically as noticed by observers with less exposure) or a change in the volume of a car radio after work exposure also are likely indications of a temporary threshold shift. (See if the volume on your car radio seems very high when you drive to work the day following significant noise exposure.) However, initial temporary threshold shifts in hearing acuity likely will be followed by a permanent change in acuity, described as permanent threshold shifts.
Initial changes in hearing loss (temporary threshold shifts) are likely to become permanent unless further exposures are controlled. There is no medical treatment or cure for permanent noise-induced hearing loss.
Monitoring audiograms (hearing tests) typically is done without noise rest to determine if the hearing of a noise-exposed individual is being affected. Follow-up audiograms are conducted after 14 or more hours without noise exposure to rule out the effect of temporary threshold shifts, or are repeated after a threshold shift is measured to verify the shift is permanent.
The rate of hearing loss depends on factors such as the intensity of noise, its duration, the opportunity for quiet rest periods (audiological recovery) between exposures, and individual sensitivity. Reduce the noise exposures where possible, and ensure people use hearing protection when noise control is not possible or fully effective.
Regardless of the frequency spectrum of noise exposure, noise-induced hearing loss usually occurs first in a frequency of approximately 3,000 to 6,000 cycles per second. The extent of noise-induced hearing loss will progress and affect other frequencies unless exposures are controlled. Because initial hearing loss occurs at frequencies at the higher end of the speech spectrum, changes in these regions serve as initial indicators before communication is impaired severely.
Noise-induced hearing loss also is often accompanied by tinnitus, a sound perception often described as "ringing in the ears." The mechanism of tinnitus isn't fully understood but appears to be a neurological response associated with damage to components of the inner ear. Tinnitus takes the form of a high-pitched whining, buzzing, hissing, humming, or whistling sound in some patients. It also sometimes is described as a ticking; clicking; roaring; crickets, tree frogs, or locusts tunes or songs; or beeping. Some have described it as a whooshing sound, as of wind or waves.
Tinnitus may be temporary or a permanent condition, in which the ringing sounds never stop, even in quiet environments. Unlike hearing loss, which results in a lack of sensitivity to sound, tinnitus can become an incurable, continuous, 24/7 reminder of past noise damage that never goes away.
Steady state versus impulse noise. Most common noise sources are described as "steady state," or fluctuating relatively slowly in volume. Most machinery noise is described as "steady state." Some noise sources, particularly gunfire, explosive blasts, or impacts occurring rapidly, are considered impulse noise because of their rapid increase in intensity and quick decline. In general, noise sources with sharp peak levels occurring less often that once per second are considered as impulse noise. Occupational exposure limits are different for impulse and steady-state noise. It is, if anything, even more important to use hearing protection in the vicinity of impulse noise, such as gunfire, because of the risk of permanent hearing loss associated with even a few unprotected exposures to high levels of such noise.
Other impulse noise effects. Very high transient pressures can create effects beyond those produced by relatively less intense impulse noises. The most common effects of noise exposure are on the hearing mechanism within the inner ear. However, intense blast impacts physically can damage or even rupture the eardrum. Unlike other noise-induced hearing effects, a ruptured eardrum typically is acutely painful. Luckily, an eardrum, unlike other components of the inner ear's hearing mechanism, may heal, or, like the ossicles, can be repaired.
Other effects of noise--general stressor. Noise is considered a general stressor, with other physiological impacts besides hearing loss. Effects may include increased distraction (impaired task performance), increased blood pressure, sleeping problems, and a general increase in stress. The Canadian Center for Occupational Health (http://www.ccohs.ca/oshanswers/ phys_agents/non_auditory.html) provides a layman's review.
Off-duty and recreational noise exposures.
Noise exposures on and off duty are interactive in their potential impact on hearing loss. Recreational activities such as hunting, target practice, and listening to excessively amplified music create noise exposures that may contribute to hearing loss. Additionally, noise exposure during work may contribute to a temporary threshold shift that stimulates users of MP3 players or even car radios to increase the sound volume to potentially hazardous levels. Even more hazardous is the use of headsets at very high volumes to drown out surrounding background noise. The same common-sense approaches should be used to limit both recreational and on-duty noise exposures:
* Limit the volume where feasible. Users should be sensitive to the volume of amplified music and pay attention to the same warning signs that suggest excessive volume in the work environment (ringing in the ears after use, speech interference, and sound intensity that distorts the quality of music). Consider purchasing relatively quiet tools and equipment for such appliances as lawnmowers and power tools.
* Use protective equipment where the sound level can't readily be limited. Activities likely to require use of personal protective equipment include the use of lawnmowers, chainsaws, and recreational shooting.
By Mark Geiger, M.S., CIH, CSP, OpNav Safety Liaison Office
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|Date:||Dec 22, 2008|
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