Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
4. SOUND TRANSMISSION CHARACTERISTICS
This section contains a brief summary of sound transmission theory
relevent to the problem of predicting the effective ranges of the various
sources discussed in the preceding section. A summary of sound transmission
in air is presented as well as a discussion of shallow water sound propagation
and transmission through the air-water interface. A discussion of sound
transmission model development and application is presented along with
examples of predicted transmission loss characteristics for the Alaskan
environment.
4.1 Sound Transmission in Air
Sound transmission from a source in an unbounded atmosphere is attenuated
only by geometrical spreading of the sound energy and by absorption of sound
energy by air molecules. Sound transmission from a source near a non-rigid or
permeable boundary is also influenced by reflection and refraction losses and
by wave transmission along the boundary surface. Interference between these
direct, reflected, and ground wave paths causes fluctuations in level and in
frequency response for near ground transmission. In addition, the refraction
caused by wind and temperature gradients produces shadow zones with very poor
sound transmission in the upwind direction and often enhanced sound transmission
downwind. These effects are very site and weather condition specific
and hence it is not feasible to predict them on a general basis. As a result,
for the purpose of predicting the average atmospheric sound transmission,
gradient effects will be neglected and only spreading loss and atmospheric
absorption will be considered in a simplified sound transmission model.
The loudest non-explosive airborne noise sources have been shown to be
aircraft. The most significant mode of sound transmission to a point on the
ground usually involves a direct path from the source to a receiver that is
elevated well above the refracting and scattering effects of near-surface
transmission. Because of this, by considering only spherical spreading,
atmospheric absorption, and ground reflection effects, one can develop an
adequate transmission loss (TL) equation for estimating the received level on .
the ground from an aircraft passing nearby. The relationship can be stated as:
where: Lr = Received level spectrum near the ground
L, = Source Level spectrum at 1 m from the source
R = Slant range in m
a = Atmospheric absorption spectrum in dB/m
= Ground reflection factor, dB.
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Since for most aircraft noise transmission calculations, a reference sound
level at 300 m is used rather than a 1 m source level, Eq. ( 12) can be
rewritten in a more convenient from as: