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Noise and Military Service: Implications for Hearing Loss and Tinnitus Appendix C Definitions Audiogram: Graph of hearing threshold level as a function of frequency (ANSI, 1995). Baseline audiogram: The initial audiogram to which subsequent audiograms are compared for the calculation of significant threshold shift. The Occupational Safety and Health Administration requires that a baseline audiogram be obtained from an examination administered before employment or within the first 30 days of employment and that is preceded by a period of at least 12 hours of quiet. Continuous noise: Noise with negligibly small fluctuations of level within the period of observation (ANSI, 1995). Cross-sectional study: A study that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defined population at one particular time (Last, 1995). Decibel (dB): The unit used to express the level of sound. The decibel is a logarithm of a ratio of two quantities, the denominator of which has been specified such that 0 dB approximates the threshold of hearing in the middle frequencies for young adults. The reference quantity in the denominator of the ratio is either a sound pressure of 20 micropascals (µPa) or a sound intensity of 10−12 watts/m2.
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Exchange rate: An increment of decibels that requires the halving of exposure time, or a decrement of decibels that allows the doubling of exposure time for a given noise dose. For example, a 3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in noise level. With a 5-dB exchange rate, exposure time may be doubled for each 5-dB decrease in noise level. Frequency: The number of times that a periodic process, such as a sound wave, repeats each second. Hearing threshold level (HTL): For a specified signal, the amount in decibels by which the hearing threshold for a listener exceeds a specified reference equivalent threshold level (ANSI, 1994). Hertz (Hz): A unit of frequency equal to one cycle per second. Impact noise: Impact noise is generated by the collision of one mass in motion with a second mass that may be in motion or at rest (ANSI, 1994). Impulsive noise: Impulsive noise is characterized by a sharp rise and rapid decay in sound levels and is less than 1 second in duration, with 1 second between successive stimuli. For the purpose of this document, it refers to impact or impulse noise. Intermittent noise: Noise levels that are interrupted by intervals of relatively low sound levels. Inverse square law: A law in physics stating that the magnitude of a physical quantity varies inversely with the square of its distance from its source. For sound, sound level will diminish by 6 dB from a point source for every doubling of distance. Longitudinal study: An epidemiological study in which subsets of a defined population can be identified who are, have been, or in the future may be exposed or not exposed, or exposed in different degrees, to a factor or factors hypothesized to influence the probability of occurrence of a given disease or other outcome. The main feature of longitudinal studies is observation of large numbers of people over a long period with comparison of incidence rates in groups that differ in exposure levels (after Last, 1995). Noise: (1) Undesired sound. By extension, noise is any unwarranted disturbance within a useful frequency band, such as undesired electric waves
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Noise and Military Service: Implications for Hearing Loss and Tinnitus in a transmission channel or device. (2) Erratic, intermittent, or statistically random oscillation (ANSI, 1994). Noise dose: The noise exposure expressed as a percentage of the allowable daily exposure. For the Occupational Safety and Health Administration, which uses a 5-dB exchange rate (per 29 C.F.R. 1910.95) and a permissible exposure limit of 90 dBA, a 100-percent dose equals an 8-hour exposure to a continuous 90 dBA noise. A 50-percent dose (requiring enrollment in a hearing conservation program) corresponds to an 8-hour exposure to 85 dBA or a 4-hour exposure to 90 dBA. Different permissible exposure limits and exchange rates will change the dose computations accordingly (Berger et al., 2000). Noise reduction rating (NRR): A single value in decibels that indicates a hearing protector’s noise reduction capabilities, averaged across the range of audible frequencies, as measured under optimum laboratory conditions. By law, the NRR must appear on the label of all devices sold as personal hearing protectors in the United States. Permanent threshold shift (PTS): Permanent increase, measured in decibels, in the threshold of audibility for an ear (ANSI, 1995). Prevalence: The number of events (e.g., instances of a given disease or other condition) in a given population at a designated time (Last, 1995). Prevalence rate (a proportion): The total number of individuals who have an attribute or disease at a particular time (or during a particular period) divided by the population at risk of having the attribute or disease at this point in time or midway through the period (Last, 1995). Significant threshold shift: A shift in hearing threshold, outside the range of audiometric testing variability (±5 dB), that warrants follow-up action to prevent further hearing loss. The National Institute for Occupational Safety and Health defines significant threshold shift as an increase in the HTL of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear, which is confirmed for the same ear and frequency by a second test within 30 days of the first test (NIOSH, 1998). Sound: Auditory sensation evoked by an oscillation in pressure, stress, particle displacement, particle velocity, and so on, in a medium with internal forces (e.g., elastic or viscous) (after ANSI, 1994).
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Noise and Military Service: Implications for Hearing Loss and Tinnitus Sound pressure: Root-mean-square instantaneous sound pressure at a point, during a given time interval. Sound pressure is measured in pascals (Pa) (ANSI, 1994). Sound pressure level (SPL): Ten times the logarithm to the base 10 of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 Pa. Sound pressure levels are measured in decibels (ANSI, 1994). Steady-state noise: Ongoing noise, the intensity of which remains at a measurable level without interruptions over an indefinite or specified period of time. Temporary threshold shift (TTS): Temporary increase, measured in decibels, in the threshold of audibility for an ear (ANSI, 1995). Threshold: The minimum sound pressure level of a pure tone that can be heard by an individual at least 50 percent of the time. Time-weighted average (TWA): An A-weighted average sound level normalized to 8 hours, meaning that the average level over the time period that is observed is adjusted (i.e., normalized) to correspond to the sound level, which if present during a continuous 8-hour period would provide the same noise dose as the measured average level, for the specified exchange rate. For example, an average sound level of 100 dBA measured during a period of 4 hours using a 5-dB exchange rate corresponds to a TWA of 95 dBA (Berger et al., 2000). Varying noise: Noise for which the level varies substantially during the period of observation (ANSI, 1995). Weighted sound pressure levels: Two weighting curves are commonly applied to measures of sound levels to account for the way the ear responds to sound (ANSI, 1994): A-weighting (dBA): A measurement scale that approximates the loudness of sound relative to a 40 dB SPL 1000-Hertz (Hz) reference tone. A-weighting emphasizes the frequencies between 1000 and 4000 Hz and reduces the contributions from frequencies below 500 Hz. C-weighting (dBC): A measurement scale that approximates the loudness of tones relative to a 100 dB SPL 1000-Hz reference tone. C-
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Noise and Military Service: Implications for Hearing Loss and Tinnitus weighting allows for relatively flat sound pressure measurements by including lower frequency sounds than does the A-weighting scale. REFERENCES ANSI (American National Standards Institute). 1994. American National Standard: Acoustical Terminology, ANSI S1.1-1994 (R 1999). New York: ANSI. ANSI. 1995. American National Standard: Bioacoustical Terminology, ANSI S3.20-1995. New York: ANSI. Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, eds. 2000. The Noise Manual. 5th ed. Fairfax, VA: American Industrial Hygiene Association. Last JM. 1995. A Dictionary of Epidemiology. New York: Oxford University Press. NIOSH (National Institute for Occupational Safety and Health). 1998. Criteria for a Recommended Standard—Occupational Noise Exposure Revised Criteria. DHHS (NIOSH) Pub. No. 98-126. Cincinnati, OH: NIOSH.
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