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

Report No. 6945
BBN Systems and Technologies Corporation
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Sound Speed, m/sac
Figure 4.2. Range of Sound Speed Profiles for Study Areas.
transmission loss characteristics using published information on bottom
characteristics and sound speed profiles.
The source and receiver depths used in the modeling work were 5 m and
10 m respectively to represent the average depth of ship and boat propellers
and a representative depth of marine mammal habitat. It was necessary to
perform both frequency and depth averaging of the model output to obtain
transmission loss characteristics that were not over.1~ influenced by single
frequency interference patterns. For most of the analysis the model output
for three frequencies was averaged, corresponding to the upper, middle, and
lower frequencies of a 1/3 octave band. In this way, results for the 100,
315, and 1000 Hz 1/3 octave bands were obtained. In addition, the received
levels were depth-averaged from 5 to 15 m.
An example of the output of the IFD Model is shown in Fig. 4.3. Here
propagation in a region of the Norton Basin Planning Area with a depth of 33 m
was considered. Figure 4.3A presents the predicted transmission loss at 3
frequencies for the strong surface layer profile shown in Fig. 4.2. Using
information obtained from the literature (Mackenzie 1973), the model
incorporated a bottom composition of silt-sand with a thickness of 2 m and a
sub-bottom layer of basalt. The transmission loss for the same region under
neutral gradient conditions is shown in Fig. 4.3B. The transmission loss
characteristics can be seen to be similar out to a range of 3 km. Beyond this
range the loss can be seen to be significantly less for the neutral gradient
condition, with the greatest difference occurring at 1 kHz. These results
indicate that the range of influence of the loudest industrial noise sources
can be changed considerably by seasonal effects on the sound speed profile.
Transmission loss data reported by Mackenzie (1961) for transmission at 200 Hz
using a shallow source and receiver are also shown in Fig. 4.3B. Unfortunately
no data are available at other frequencies for these conditions in this area.