Noise (NOI)
Sound that reaches unwanted levels is referred to as noise. The central
issue with noise is the impact, or perceived impact, on people, animals (wild
and domestic), and general land use compatibility. Exposure to high noise
levels can have a significant impact on human activity, health, and safety.
The decibel (dB) scale is used to quantify sound intensity. To understand the
relevance of decibels, a normal conversation often occurs at 60 dB, while an
ambulance siren from 100 feet away is approximately 100 dB. Noise
associated with military operations (arrival/departure of military aircraft,
firing of weapons, etc.) may create noises in higher dB ranges.
Key Terms
C‐weighted Day‐Night Average Sound Level. The C‐weighted Day‐Night
Average Sound Level (CDNL) noise metric is used for demolition and large
caliber weapons to assess the low‐frequency energy produced from such
activities. The CDNL is an annual average noise dose from range operations
and is intended for long‐term land use planning.
Day‐Night Average Sound Level. Day‐night average sound level (DNL)
represents an average sound exposure over a 24‐hour period. During the
nighttime period (10:00 p.m. to 7:00 a.m.), averages are artificially increased
by 10 dB. This weighting reflects the added intrusiveness and the greater
disturbance potential of nighttime noise events attributable to the fact that
community background noise typically decreases by 10 dB at night.
Ambient Noise. Ambient noise is the total noise associated with an existing
environment (built or natural) and usually comprised of sounds from many
sources, both near and far. Decibel. A decibel (dB) is the physical unit commonly used to describe noise
levels. It is a unit for describing the amplitude of sound, as heard by the
human ear.
Attenuation. Attenuation is a reduction in the level of sound resulting from
an object’s distance from the noise source or absorption by the surrounding
topography, the atmosphere, barriers, construction techniques and
materials, and other factors. Sound attenuation in buildings can be achieved
through the use of special construction practices that reduce the amount of
noise that penetrates the windows, doors, and walls of a building. Sound
attenuation measures may be incorporated during initial construction for
new buildings or as additional construction for existing buildings. Noise Contour. Noise contours consist of noise impact lines constructed by
connecting points of equal noise level measured in dB and identify areas on
a map that fall within that particular dB noise contour.
A‐weighted decibel. The A‐weighted decibel (dBA) is the most commonly
weighted sound filter used to measure perceived loudness versus actual
sound intensity. The human ear responds differently to frequencies. For
example, the human hearing system perceives mid‐frequency sounds as
louder than low and high frequency sounds. To accommodate this condition
when measuring sound levels, filters need to be installed into sound meters.
The results are a more accurate measurement of sound for the human
hearing system. NOISEMAP Program. The Department of Defense (DoD) noise models are
based on NOISEMAP technology, using linear acoustics and an integrated
formulation to determine the impact of noise.
Background Report
Noise‐Sensitive Uses. Noise‐sensitive uses are locations and uses typically
more sensitive to noise, including residential areas, hospitals, convalescent
homes and facilities, schools, libraries, churches, recreational areas, and
other similar land uses.
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