Page 241 - Sustainable On-Site CHP Systems Design, Construction, and Operations

214 De s i g n

Sound Noise Level (dB) Effect
0 Hearing begins
10 Just audible
Library 20 Very quiet
Light traffic (100 ft) 50 Quiet
Air-conditioning unit 60 Intrusive
Freeway traffic 70 Annoying
Heavy truck or city traffic (50 ft) 90 Very annoying, hearing
damage after 8-h exposure
Auto horn (3 ft) 120 Maximum vocal effort required
Air raid siren 140 Painfully loud
Rocket launching pad, no ear 180 Irreversible hearing loss
protection

Source: Adapted from various sources, including the Noise Pollution Clearinghouse.
TABLE 12-3 Sound Levels and Human Response

Sound pressure at any receptor point reflects not only the absolute power of sound,
but also the characteristics of sound waves after they leave the source and travel to a
receptor. As sound waves travel from the source to the receptor, they expand and lose
intensity in much the same way air pollutant concentrations lose intensity as they
migrate from the exhaust stack. For example, a sound pressure level of 70 dB at the
source would be diminished at a distance of 50 ft to a level of 46 dB. In other words, an
intrusive sound at its source would be considered to be relatively quiet at a farther
distance, due to the loss of intensity over space.
In addition to the distance between source and receptors, terrain, vegetation, and
the presence of structures also alter the intensity of a sound wave as it reaches a
receptor. Various modeling tools exist which allow the project engineer to estimate
sound pressure at various receptor points. Some models incorporate the Gaussian func-
tion, while other incorporate enhanced nonlinear estimating functions that may pro-
vide a more accurate estimate of sound distribution. Use of these models will support
the development of a strong application, should the permitting agency require demon-
strations of expected compliance with noise standards.

Noise Standards
Noise thresholds are more apt to be regulated through the enforcement of local
ordinances, rather than national or regional regulation. While noise thresholds vary by
locality, they do tend to consistently be more stringent in areas with sensitive receptors.
They also tend to be more stringent during critical periods of the day. For example, the
City of Seattle, Washington, enforces a noise ordinance which limits the maximum per-
missible noise level of a source to 55 dB if the source and receptor are both located in a
residentially zoned neighborhood during daytime hours (7:00 a.m. to 10:00 p.m.). If
the source is located in an industrially zoned neighborhood, but the receptor is in a