Environmental Engineering Reference
In-Depth Information
Eyeball
h
A
A´
d
FIGURE 2.3
The law of the visual angle. Objects of different sizes and distances that sub-
tend the same angle will cast the same size image on the retina. Angles A and A′ are the same.
l
AW
of
tHe
V
isuAl
A
ngle
Figure 2.3 shows how objects of different sizes and different distances can form
the same size image within the eye. The angle (A′) inside the eye is the same as the
angle (A) from the eye to each of the objects. These angles, called
visual angles
, are
the conventional units used to describe object size because they are directly related
to retinal-image size, which is the only relevant variable for these purposes. Thus,
a small object close to the eye can cast the same size retinal image as a large object
seen from a much farther distance.
Another concern involves the effect of ambient noise on communication dis-
tance and an animal's ability to detect calls. For effects to birds, this can mean (1)
behavioral and/or physiological effects, (2) damage to hearing from acoustic over-
exposure, and (3) masking of communication signals and other biologically relevant
sounds (Dooling and Popper, 2007). Based on the 49 bird species whose behavioral
audibility curves or physiological recordings have been determined, Dooling and
Popper (2007) developed a conceptual model for estimating the masking effects
of noise on birds. Based on the distance between birds and the spectrum level,
bird communication was predicted to be “at risk” (~755-ft distance where noise
was 20 dB), “difficult” (~755-ft distance where noise was 25 dB), and “impossible”
(~755-ft distance where noise was 30 dB). Although clearly there is variation among
species and there is no single noise level where one size fits all, this masking effect
of turbine blades is of concern and should be considered as part of the cumulative
impacts analysis of a wind facility on wildlife. It must be recognized that noise in
the frequency region of avian vocalizations will be most effective in masking these
vocalizations (Dooling, 2007).
Barber et al. (2010) assessed the threats of chronic noise exposure, focusing on
grouse communication calls, urban bird calls, and other songbird communications.
They determined that although some birds were able to shift their vocalizations
to reduce the masking effects of noise, when shifts did not occur or were insig-
nificant masking could prove detrimental to the health and survival of wildlife.
Although much is still unknown in the real world about the masking effects of noise
