where 86,400 is the number of seconds in a 24-hour day.

2. A frequency weighting is understood, unless another frequency weighting is specified explicitly.

Decibel—Unit of level when the base of the logarithm is the tenth root of 10 and the quantities concerned are proportional to power. Unit symbol, dB.

NOTE: Examples of quantities that qualify are power (in any form), sound pressure squared, particle velocity squared, sound intensity, sound-energy density, and voltage squared. Thus, the decibel is a unit of sound-pressure-squared level; in common practice, however, called sound pressure level, unless an ambiguity results from so doing.

Effective perceived noise level—Level of the time integral of the antilogarithm of one-tenth of tone-corrected perceived noise level over the duration of an aircraft flyover, the reference duration being 10 seconds. Unit, decibel (dB); abbreviation, EPNL; symbol, .

NOTE: The integral is usually approximated by summation, over the top 10 decibels of an aircraft noise signal of the antilogarithms of one-tenth of tone-corrected perceived noise level at successive 0.5 second intervals.

Equivalent continuous sound level—Ten times the logarithm to the base 10 of the ratio of time-mean-square instantaneous A-weighted sound pressure, during a stated time interval T, to the square of the standard reference sound pressure. Unit, decibel (dB); respective abbreviations, TAV and TEQ; respective symbols, LAT and LAeqT.

NOTES:

1. A frequency weighting other than the standard A-weighting may be employed if specified explicitly. A frequency weighting that is essentially constant between limits specified by a manufacturer is called “flat.”

2. In symbols, time-average (time-interval equivalent continuous) A-weighted sound level in decibels is

where pA2 is the squared instantaneous A-weighted sound pressure signal, a function of elapsed time t; in gases reference sound pressure p0 = 20 μPa; T is a stated time interval.

3. In principle, the sound pressure signal is not exponentially time weighted, either before or after squaring.

*Frequency weighting—Modification of the spectrum of an acoustical signal by means of an analog or digital filter having one of the standardized response characteristics known as A, B, C, etc., defined in IEC 61672-1. The A-weighting filter is the one most commonly used. See weighting network.

*Hearing conservation program—A system to identify noise-exposed workers and monitor their exposure and audiometric function.

*Hearing loss—Increase in the threshold of audibility due to disease, injury, age, or exposure to intense noise. Conductive hearing loss: Hearing loss caused either by blockage of the external ear or by disease or damage in the middle ear, so that the signal amplitude reaching the inner ear is reduced. Noise-induced hearing loss (NIHL): Cumulative hearing loss associated with repeated exposure to noise. Sensorineural hearing loss: Hearing loss due to a lesion or disorder of the inner ear or of the auditory nervous system. Nonoccupational hearing loss: Hearing loss caused by exposure outside of the occupational environment.

*Hertz (Hz)—A unit of frequency measurement representing cycles per second.

Muffler—Duct designed to reduce the level of sound. The sound-reducing mechanisms may be either absorptive, reactive, or a combination of both.

*Newton (N)—A unit of force. The force of one Newton accelerates a 1 kg mass at 1 m/s2.

*Noise dose—(1) According to the definition given by Occupational Safety and Health Administration (OSHA), noise dose is the ratio, expressed as a percentage of (a) the time integral, over a stated time or event, of the 0.6 power of the measured “S” (slow) exponential time-averaged, squared A-weighted sound pressure and (b) the product of the criterion duration (8 hours) and the 0.6 power of the squared sound pressure corresponding to the criterion sound pressure level (90 dB). (2) According to the definition given by OSHA, noise dose is the percentage of actual exposure relative to the amount of allowable exposure, and for which 100 percent and above represents exposures that are hazardous. The noise dose is calculated using:

where Ci is the total time of exposure at a specified noise level, and Ti is the exposure time at which noise for this level becomes hazardous.



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