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

Report No. 6945
BBN Systems and Technologies Corporation
depend on movement patterns in the course of normal feeding, migration or
other activities. Movement patterns of the animals will have a strong
influence on the duration on sound exposure, since many noise sources are
either fixed or moving more slowly than the species exposed.
In the absence of specific evidence of an appropriate integration time
for behavioral reaction to continued noise exposure it seems appropriate to
assume a duration that is controllable by the species involved. Of the
several species potentially impacted by the noise sources in the Alaskan
marine environment, gray whales have been studied sufficiently to permit
determination of noise response criteria for some types of industrial noise
sources. In the course of these studies, it was observed that whales
responded to loud sound fields by changing their swimming pattern to reduce
the noise exposure. The swimming speed of gray whales when migrating was
observed to be 5 to 10 km/hr (Malme et al. 1984). The radius within which
behavioral reactions are expected is generally within 10 km of the source in
most of the Alaskan OCS planning areas studied. Thus a two hour reference
time is assumed to be appropriate in considering the average exposure interval
for gray whales. For the purpose of this study, a two hour reference period
is used for other species also, recognizing that changes may be needed when
more specific behavioral response data become available. The impact of using
an incorrect value for the reference exposure period (acoustic integration
time) is not severe in its effect on the predicted L eq '
If the effective
exposure period is as little as 40 min or as great as 6 hours, rather than the
assumed 2 hours, this will result in a maximum error of 5 dB in the estimated
The Le concept was developed for prediction of the response of
relatively Yixed human population centers to intrusive industrial noise
sources that were either stationary or were moving in a defined spatial
pattern. In order to apply this concept to the usual moving receiver - moving
source situation applicable to marine mammals, it is necessary to devise a
procedure which will standardize the conditions under which L is estimated.
This can be done by considering that an acoustic source near a
eq
specific site
can influence an animal passing through the area by producing a behaviorally
significant noise contribution that is proportional to the effective source
level, inversely proportional to the transmission loss, and proportional to
the probability of encounter. The effective source level is the constant
level (referred to 1 m) that would produce the same acoustic energy over a
2 hr period as the actual time-varying source over the same interval. It can
be specified in terms of the maximum source output level modified by a time
duration correction factor.
The transmission loss can be standardized by considering a reference
range which is representative of many actual exposure conditions. A practical
reference range can be calculated by emp oying the concept of the effective
source density per kilometer-squared (km 1
). For a single sound source located
in a region of horizontally uniform sound propagation conditions, it can be
shown that the mean sound pressure level for a circular area of 1 km2 is
developed at an average range of 300 m when spreading losses alone are
considered (10 Log, 15 Log, and 20 Log characteristics). Thus we propose to
use 300 m as a reference distance for comparing various sound sources at a