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Understanding the relationship between sound pressure and sound power is essential to predicting what noise problems will be created when particular sound sources are placed in working environments. An important consideration might be how close workers will be working to the source of sound. As a general rule, doubling the sound power increases the noise level by 3 dB. As sound power radiates from a point source in free space, it is distributed over a spherical surface so that at any given point, there exists a certain sound power per unit area.

This is designated as intensity, I, and is expressed in units of watts per square meter. Sound intensity is heard as loudness, which can be perceived differently depending on the individual and his or her distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases.

The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense. In other words, the sound power level of a source is independent of the environment. Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source.

To properly represent the total noise of a source, it is usually necessary to break it down into its frequency components.

One reason for this is that people react differently to low-frequency and high-frequency sounds. Additionally, for the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise.

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Engineering solutions to reduce or control noise are different for low-frequency and high-frequency noise. As a general guideline, low-frequency noise is more difficult to control. Certain instruments that measure sound level can determine the frequency distribution of a sound by passing that sound successively through several different electronic filters that separate the sound into nine octaves on a frequency scale. Two of the most common reasons for filtering a sound include 1 determining its most prevalent frequencies or octaves to help engineers better know how to control the sound and 2 adjusting the sound level reading using one of several available weighting methods.

These weighting methods e.

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The following paragraphs provide more detailed information. Octave bands, a type of frequency band, are a convenient way to measure and describe the various frequencies that are part of a sound. The center, lower, and upper frequencies for the commonly used octave bands are listed in Table II The width of a full octave band its bandwidth is equal to the upper band limit minus the lower band limit. For more detailed frequency analysis, the octaves can be divided into one-third octave bands; however, this level of detail is not typically required for evaluation and control of workplace noise.

Electronic instruments called octave band analyzers filter sound to measure the sound pressure as dB contributed by each octave band.


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These analyzers either attach to a type 1 sound level meter or are integral to the meter. Both the analyzers and sound level meters are discussed further in Section III. Loudness is the subjective human response to sound. It depends primarily on sound pressure but is also influenced by frequency. 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 than others. The very low frequencies are discriminated against attenuated quite severely by the A-network and hardly attenuated at all by the C-network. Sound levels dB measured using these weighting scales are designated by the appropriate letter i.

In contrast, the Z-weighted measurement is an unweighted scale introduced as an international standard in , which provides a flat response across the entire frequency spectrum from 10 Hz to 20, Hz. 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 other structures.

A previous B-weighted scale is no longer used. The networks evolved from experiments designed to determine the response of the human ear to sound, reported in by a pair of investigators named Fletcher and Munson. Their study presented a 1,Hz reference tone and a test tone alternately to the test subjects young men , who were asked to adjust the level of the test tone until it sounded as loud as the reference tone.

The results of these experiments yielded the frequently cited Fletcher-Munson, or "equal-loudness," contours, which are displayed in Figure 6. These contours represent the sound pressure level necessary at each frequency to produce the same loudness response in the average listener. The nonlinearity of the ear's response is represented by the changing contour shapes as the sound pressure level is increased a phenomenon that is particularly noticeable at low frequencies.

The lower, dashed curve indicates the threshold of hearing and represents the sound-pressure level necessary to trigger the sensation of hearing in the average listener. Among healthy individuals, the actual threshold may vary by as much as 10 decibels in either direction. 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.

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. The function of the ear is to gather, transmit, and perceive sounds from the environment. This involves three stages:. To categorize different types of hearing loss, the impairment is often described as either conductive or sensorineural, or a combination of the two.

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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.


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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. 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.

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. 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.

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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.

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