Condit Dam Removal Condit Dam Removal - Access Washington

Condit Dam Hydroelectric Project
Final Supplemental EIS
Attenuation Resulting From Air Absorption (A air )
Some of the energy in a sound wave is absorbed by the atmosphere. The amount of
absorption depends on the frequency of the sound and the temperature and relative humidity
of the atmosphere. Because of the more effective absorption at higher frequencies,
atmospheric absorption would also tend to lower the pitch of noise generated at the site. This
effect is small and ignored for short distances, but becomes significant as the distance
between the source and the receiver increases.
Attenuation of noise levels generated by construction activities associated with the Proposed
Action due to air absorption (A air ) over specific distances from the sources was calculated
using the following equation (Piercy and Daigle 1991):
where:
A air = α d
1,000
α = air attenuation coefficient (dependent on temperature and relative humidity)
d = distance from the noise source to the receiver in meters
Because A air depends on temperature and relative humidity, attenuation may differ slightly
between summer and winter months. Therefore, A air was calculated under summer (i.e.,
20°C, 30 percent relative humidity) and winter (i.e., 0°C, 90 percent relative humidity)
atmospheric conditions.
Attenuation by Foliage (A foliage )
Trees and bushes normally provide very little noise attenuation as a result of shielding.
However, if the foliage is dense enough to completely obstruct the view and also intercepts
the path of acoustic propagation (as in a dense forest), some attenuation can be quantified. A
practical upper limit to noise attenuation by foliage (A foliage ) is reached at a path length
through the foliage of approximately 200 meters (656 feet). In addition to distance, the
magnitude of A foliage depends on the frequency of the sound.
Attenuation of noise levels generated by construction activities associated with the Proposed
Action due to foliage was calculated using the following equation (Piercy and Daigle 1991):
A foliage = 0.04 dB / m
where: dB = decibels
m = length of sound propagation path through foliage in meters (maximum 200
meters)
Noise Level Calculations
After appropriate noise sources were identified, noise levels at 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8,
and one mile from the Proposed Action caused by construction activities were modeled
considering the noise reductions caused by distance, topography, foliage, and atmospheric
stability and absorption (Piercy and Daigle 1991), Mestre and Wooten 1980), as described in
the previous sections. The key assumptions used for the modeling were as follows:
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