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Three different internationally standardized characteristics are used for sound measurement: weighting networks A, C, and Z or "zero" weighting. The A and C weighting networks are the sound level meter's means of responding to some frequencies more...
The C-weighted scale is used as an alternative to the Z-weighted measurement on older sound level meters on which Z-weighting is not an option , particularly for characterizing low-frequency sounds capable of inducing vibrations in buildings or...
Ultrasound is not listed in Figure 6 because it has a frequency that is too high to be audible to the human ear. See Appendix C for more information about ultrasound and its potential health effects and threshold limit values. How We Hear The ear is the organ that makes hearing possible. It can be divided into three sections: the external or outer ear, the middle ear, and the inner ear. Figure 7 shows the parts of the ear.
Figure 7. Anatomy of the Human Ear The function of the ear is to gather, transmit, and perceive sounds from the environment. This involves three stages: Stage 1: Modification of the acoustic wave by the outer ear, which receives the wave and directs it to the eardrum. Sound reaches the eardrum as variations in air pressure. Stage 2: Conversion and amplification of the modified acoustic wave to a vibration of the eardrum. These vibrations are amplified by the ossicles, small bones located in the middle ear that transmit sound pressure to the inner ear. The vibrations are then transmitted as wave energy through the liquid of the inner ear the cochlea. Stage 3: Transformation of the mechanical movement of the wave into nerve impulses that will travel to the brain, which then perceives and interprets the impulse as sound.
The cilia of nerve cells in the inner ear, called hair cells, respond to the location of movement of the basilar membrane and, depending on their position in the decreasing radius of the spiral-shaped cochlea, activate the auditory nerve to transmit information that the brain can interpret as pitch and loudness. Impaired function at any of these stages will affect hearing. Hearing Loss To categorize different types of hearing loss, the impairment is often described as either conductive or sensorineural, or a combination of the two.
Conductive hearing loss results from any condition in the outer or middle ear that interferes with sound passing to the inner ear. Excessive wax in the auditory canal, a ruptured eardrum, and other conditions of the outer or middle ear can produce conductive hearing loss. Although work-related conductive hearing loss is not common, it can occur when an accident results in a head injury or penetration of the eardrum by a sharp object, or by any event that ruptures the eardrum or breaks the ossicular chain formed by the small bones in the middle ear e.
Conductive hearing loss may be reversible through medical or surgical treatment. It is characterized by relatively uniformly reduced hearing across all frequencies in tests of the ear, with no reduction during hearing tests that transmit sound through bone conduction. Sensorineural hearing loss is a permanent condition that usually cannot be treated medically or surgically and is associated with irreversible damage to the inner ear. The normal aging process and excessive noise exposure are both notable causes of sensorineural hearing loss. Studies show that exposure to noise damages the sensory hair cells that line the cochlea. Even moderate noise can cause twisting and swelling of hair cells and biochemical changes that reduce the hair cell sensitivity to mechanical motion, resulting in auditory fatigue.
As the severity of the noise exposure increases, hair cells and supporting cells disintegrate and the associated nerve fibers eventually disappear. Occupational noise exposure is a significant cause of sensorineural hearing loss, which appears on sequential audiograms as declining sensitivity to sound, typically first at high frequencies above 2, Hz , and then lower frequencies as damage continues. Often the audiogram of a person with sensorineural hearing loss will show a "Notch" at 4, Hz.
This is a dip in the person's hearing level at 4, Hz and is an early indicator of sensorineural hearing loss. Results are the same for hearing tests of the ear and bone conduction testing. Sensorineural hearing loss can also result from other causes, such as viruses e. Figure 8 shows the typical audiogram patterns for people with conductive and sensorineural hearing loss. Figure 8. It is important to note that some hearing loss occurs over time as a normal condition of aging. Termed presbycusis, this gradual sensorineural loss decreases a person's ability to hear high frequencies. Presbycusis can make it difficult to diagnose noise-related hearing loss in older people because both affect the upper range of an audiogram. An 8,Hz "Notch" in an audiogram often indicates that the hearing loss is aged-related as opposed to noise-induced. As humans begin losing their hearing, they often first lose the ability to detect quiet sounds in this pitch range.
Effects of Excessive Occupational Noise Exposure Workplace noise affects the human body in various ways. The most well-known is hearing loss, but work in a noisy environment also can have other effects. Auditory Effects Although noise-induced hearing loss is one of the most common occupational illnesses, it is often ignored because there are no visible effects.
It usually develops over a long period of time, and, except in very rare cases, there is no pain. What does occur is a progressive loss of communication, socialization, and responsiveness to the environment. In its early stages when hearing loss is above 2, Hz , it affects the ability to understand or discriminate speech.
As it progresses to the lower frequencies, it begins to affect the ability to hear sounds in general. The primary effects of workplace noise exposure include noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus. A noise-induced temporary threshold shift is a short-term decrease in hearing sensitivity that displays as a downward shift in the audiogram output. It returns to the pre-exposed level in a matter of hours or days, assuming there is not continued exposure to excessive noise.
If noise exposure continues, the shift can become a noise-induced permanent threshold shift, which is a decrease in hearing sensitivity that is not expected to improve over time. A standard threshold shift is a change in hearing thresholds of an average of 10 dB or more at 2,, 3,, and 4, Hz in either ear when compared to a baseline audiogram. Employers can conduct a follow-up audiogram within 30 days to confirm whether the standard threshold shift is permanent. Under 29 CFR Recording criteria for cases involving occupational hearing loss can be found in 29 CFR The effects of excessive noise exposure are made worse when workers have extended shifts longer than 8 hours. With extended shifts, the duration of the noise exposure is longer and the amount of time between shifts is shorter. This means that the ears have less time to recover between noisy shifts. As a result, short-term effects, such as temporary threshold shifts, can become permanent more quickly than would occur with standard 8-hour workdays.
Tinnitus, or "ringing in the ears," can occur after long-term exposure to high sound levels, or sometimes from short-term exposure to very high sound levels, such as gunshots. Many other physical and physiological conditions also cause tinnitus. Regardless of the cause, this condition is actually a disturbance produced by the inner ear and interpreted by the brain as sound. Individuals with tinnitus describe it as a hum, buzz, roar, ring, or whistle, which can be short term or permanent.
Acoustic trauma refers to a temporary or permanent hearing loss due to a sudden, intense acoustic or noise event, such as an explosion. Bureau of Labor Statistics BLS publishes annual statistics for occupational injuries including hearing loss reported by employers as part of required recordkeeping. This represents more than 18, workers who experienced significant loss of hearing due to workplace noise exposure. Figure 9. Distribution of Occupational Injury and Illness Cases Nonfatal occupational injuries accounted for the overwhelming majority of cases reported for the SOII in Most illness cases fall into the "All other illnesses" category, which includes such things as repetitive motion cases and systemic diseases and disorders.
Source: Bureau of Labor Statistics, U. Department of Labor, October Other Effects As a general guideline, the work area is too noisy if a worker cannot make himself understood without raising his or her voice while talking to a co-worker 3 feet away. Other consequences of excessive workplace noise exposure include interference with communications and performance. Workers might find it difficult to understand speech or auditory signals in areas with high noise levels. Noisy environments also lead to a sense of isolation, annoyance, difficulty concentrating, lowered morale, reduced efficiency, absenteeism, and accidents. In some individuals, excessive noise exposure can contribute to other physical effects. These can include muscle tension and increased blood pressure hypertension. Noise exposure can also cause a stress reaction, interfere with sleep, and cause fatigue. Ultrasonics Ultrasound is high-frequency sound that is inaudible i.
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