There is some good terminology in the National Park Service's Soundscape Pl=
an and Environmental Assessment for Zion National Park, page 25:
"deviation from natural ambient can be used to identify reductions in liste=
ning area and alerting distance. Reduction in listening area quantifies the=
loss of hearing ability to humans and animals as a result of an increase i=
n ambient noise level. Under natural ambient conditions a sound is audible =
within a certain area around a visitor or animal. If the ambient level is i=
ncreased due to a noise event, the area in which the sound is audible decre=
ases."
and on p. 26:
For example, under natural ambient conditions, an owl perched in a tree may=
be able to hear a mouse scurrying through the brush anywhere within an are=
a of 100-square-meters of the perch. If a noise event increases the ambient=
level by 3 decibels (dBA), the area in which the owl can hear a mouse woul=
d decrease by 50 percent to approximately 50-square-meters.
Reduction in alerting distance is closely related to reduction in listening=
area. The residual alerting distance is equal to the square root of the re=
sidual listening area. Instead of addressing losses in terms of an area, re=
duction in alerting distance expresses the reduction as a linear distance f=
rom a source. For example, under natural ambient conditions, a canyoneer ma=
y be alerted to the sound of a flash flood at a distance of 1-mile. If a no=
ise such as an aircraft overflight increases the ambient level by 6 dBA, th=
e distance at which the flood could be detected would decrease by 50 percen=
t to approximately 1=E2=81=842-mile or 2,640-feet.
Visitors and wildlife are impacted by their failure to hear natural sounds =
that would have been audible in the absence of noise: a bird misses the sou=
nd of a worm, a mouse misses the footfall of a coyote, a visitor misses the=
sound of a distant waterfall. Reductions in listening area and alerting di=
stance capture these types of impacts.
BTW the geometry is incorrect in Fig. 4 on p. 26.
-Dan
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