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Lexicon of Noise Parameters and Terminology
A wide range of parameters are used to assess human
reaction to noise. The highly variable response of individuals to noise
and the many characteristics (level, frequency content, impulsiveness,
intermittency, etc.) of different types of noise sources has led to many
attempts to provide single-number ratings of the effect of that noise. The
following list summarizes most of the parameters in common usage.
“A” frequency weighting: The method of frequency
weighting the electrical signal within a noise-measuring instrument is to
simulate the way the human ear responds to a range of acoustic
frequencies. It is based on the 40 dB equal loudness curve. The symbols
for the noise parameters often include the letter “A” (e.g., LAeq)
to indicate that frequency weighting has been included in the measurement.
Fast, Slow and Impulse time weightings:
Standardized response times were originally built into noise measuring
instruments to provide visual indication of fluctuating noise levels.
Environmental assessment standards usually specify which time weighting
(F, S or I) to use.
LAeq,T: A widely used noise parameter
that calculates a constant level of noise with the same energy content as
the varying acoustic noise signal being measured. The letter “A” denotes
that the A-weighting has been included and “eq” indicates that an
equivalent level has been calculated. Hence, LAeq is the
A-weighted-equivalent continuous noise level.
LAE: Sound Exposure Level (SEL): A
parameter closely related to LAeq for assessment of events (aircraft,
trains, etc.) that have similar characteristics but are of different
duration. The LAE value contains the same amount of acoustic energy over a
“normalized” one second period as the actual noise event under
LAFMax, LASMax or LAIMax:
Maximum A-weighted noise level measured with Fast (F), Slow (S) or
Impulse (I) time weighting. They are the highest level of environmental
noise occurring during the measurement time. They are often used in
conjunction with another noise parameter (e.g., LAeq) to ensure
a single noise event does not exceed a limit. It is essential to specify
the time weighting (F, S or I).
LAFMin, LASMin or LAIMin:
Minimum A-weighted noise level measured with Fast (F), Slow (S) or
Impulse (I) time weighting. They are the lowest level of environmental
noise occurring during the measurement time.
LAFN,T Percentile levels: The level of
A-weighted noise exceeded for N% of the measurement time. In some
countries the LAF90,T (level of noise exceeded for 90% of the
measurement time) or LAF95,T level is used as a measure of the
background noise level. Note that the time weighting (usually Fast) should
LAr,Tr Rating level: The A-weighted
equivalent continuous noise level (LAeq,T) during a specified
time period with specified adjustments for tonal, impulsive or
intermittent noise. In general, the rating level is given by:
LAr,Tr = LAeq,T +
KI + KT + KR + KS
In some countries, a subjective assessment of the
characteristics of the noise in question is made. In other countries, an
objective test is made to see whether the noise is tonal or impulsive.
For example (1) a 1/3-octave frequency band of noise
which exceeds the levels in adjacent bands by 5 dB or more for the
detection of tonal noise and, (2) a measurement of the difference between
an Impulse and A-weighted “Leq” parameter (LAIm,T)
and LAeq,T would reveal the presence of impulses.
Aircraft Noise Parameters: If aircraft noise is
assessed as just a normal noise source (as is usually the case), then the
usual environmental noise parameters required are LASMax and LAE
(equivalent to LAX in some older standards) for single events
and LAeq,T for a succession of noise events. In some cases
(e.g., aircraft certification), more detailed analysis of the 1/3-octave
spectral content of the aircraft noise is made at 0.5 second intervals.
Perceived noise level (LPN) is then calculated by converting
the sound pressure levels to perceived noisiness values according to the
ICAO Annex 16 standards.
If the aircraft noise spectrum has pronounced tonal
content, then an additional correction of up to 6.7 dB is added to the
perceived noise level (LPN) to give a tone-corrected perceived
noise level LTPN. The total subjective effect of an air-craft's
flyover must take into account the time history of the flight. This is
accounted for by integrating the tone-corrected, perceived noise level to
produce the effective perceived noise level, LEPN. Full details
can be found in the ISO 3891 standard.
LDN: Day-night average sound level. An LAeq
with 10 dB(A) penalty for environmental noise occurring from 22:00 to
07:00 to take account of the increased annoyance at night.
Frequency spectrum: In environmental noise
investigations, it is often found that the single-number indices, such as
LAeq, do not fully represent the characteristics of the noise.
If the source generates noise with distinct frequency components (tonal
noise), then it is useful to measure the frequency content in octave,
1/3-octave or narrower (Fast Fourier Transform) frequency bands. For
calculating noise levels (prediction), octave spectra are often used to
account for the frequency characteristics of sources and propagation.
Sound power is the acoustic power (W) radiated from
a sound source. This power is essentially independent of the surroundings,
while the sound pressure depends on the surroundings (reflecting surfaces)
and distance to the receiver. If the sound power is known, the sound
pressure at a point can usually be calculated, while the reverse is true
only in special cases (e.g., in an anechoic or reverberation room). So,
the sound power is very useful to characterize noise sources and to
calculate sound pressure. Like sound pressure, sound power is measured in
logarithmic units, the 0 dB sound power level corresponding to 1 pW
(picowatt = 10−12 W). The symbol used for sound power level is
LW, and it is often specified in dB(A), 1/1 octaves or 1/3 octaves.
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