annexc - Newmont Mining Corporation

Annex C6 -- Noise Supplemental Information C6 - 1
NOISE TERMINOLOGY AND DESCRIPTORS
Noise can be characterized as excessive or unwanted sound. Because a person’s response
to noise is subjective, the perception of noise varies from person to person. The human ear
responds to a wide range of noise intensities. The decibel scale used to describe noise is a
logarithmic rating system that accounts for the large differences in audible sound intensities.
This scale accounts for the human perception that loudness doubles with an increase of 10
dB. Therefore, a 70-dB sound level would sound twice as loud as a 60-dB sound level. For
increases or decreases in a noise source, people generally cannot detect differences of 1 dB.
Although differences of 2 or 3 dB can be detected under ideal laboratory situations, they are
difficult to discern in an active outdoor noise environment. A 5-dB change would likely be
perceived under normal listening conditions. A new noise source with different sound
characteristics than the existing ambient noise could be perceived even if its sound levels
were markedly lower than the ambient noise level.
Because of the logarithmic scale used to describe noise, a doubling of noise source strength
produces a 3-dB increase in average noise. For example, two adjacent, similar noise events
occurring simultaneously would result in a 3-dB increase over the sound level produced by
only one event. Such an increase would not be perceived as a doubling in noise loudness,
which requires a 10-dB increase.
When addressing the effects of noise on people, it is necessary to consider the frequency
response of the human ear, or those frequencies that people hear well. Sound measuring
instruments are therefore often designed to “weight” sounds based on the way people hear.
The frequency weighting most often used to evaluate environmental noise is the “A”
weighting because it best reflects how humans perceive sound. Measurements from
instruments using this system are reported in “A weighted decibels,” or dBA.
Noise levels are decreased by distance, obstructions such as buildings or terrain,
atmospheric absorption, and absorption by ground and vegetation. Sounds from line
sources (e.g., fairly continuous roadway traffic) decrease by approximately 3 dBA for each
doubling of distance from the source. Sounds from point sources (e.g., bulldozers) decrease
by 6 dBA when distance from the source is doubled.
Noise levels can be described using many types of noise descriptors. This noise section
primarily uses the A-weighted Leq and L90 noise descriptors for discussing the existing and
the steady future sound levels in the project vicinity. The L90 is the sound level exceeded
90% of the time and is often considered representative of the background sound level. The
Leq is the level of a constant sound that has the same sound energy as the actual fluctuating
sound. As such, it can be considered an energy-average sound level. The Leq gives most
weight to the highest and longest duration sound levels because they contain the most
sound energy. The Leq noise metric has been found to be highly correlated to community
response to noise and is the noise descriptor used most often by international organizations
and U.S. agencies when discussing hourly sound levels. Finally, the Lmax is a short-term
noise level typically used to represent the highest level associated with a noise event. The
“fast” Lmax is the highest 1/8 second sound level.
Akyem Gold Mining EIS November 2008 FINAL EIS