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

Report No. 6945
BBN Systems and Technologies Corporation
The model provides for transmission of only one frequency for each set of
calculations. Consequently the calculated values shown in Fig. 4.5A for 100
Hz have fluctuations in level caused by multipath interference patterns. The
results have been smoothed somewhat by averaging the TL values calculated at
depths of 1, 2 and 3 m for each range increment to derive the solid curves
shown in the figure. The dashed lines are estimated rms-averaged TL characteristics
which would be obtained by averaging several model calculations
using closely-spaced tones to smooth out the interference pattern.
,
Figure 4.5A shows that for a 100 Hz source located 10 km from the beach,
the predicted TL becomes greater than 100 dB at range of 6 km from the source
or 4 km from the beach. This is essentially the acoustic cutoff for sound at
this frequency. For a source located 3.3 km from the beach the cutoff is
reached within a few hundred meters of the beach. Note the TL at very short
ranges from the source position is about 60 dB. This high value at short
ranges is the result of the shallow soxce (1 m) and shallow receiver depths
(2 m) selected for use in the study. This geometry was selected to represent
the operating depth of the propellers of small and medium-sized vessels and
the swimming depth of pinnipeds near the haul-out sites.
Figure 4.5B presents the predicted TL characteristics of the Type 1
bottom for 315 Hz. At this frequency the bottom losses are not as severe and
transmission from a source at 10 km is not cut off until it gets very near the
beach. For a source range of 3.3 km, transmission up to the beach region can
be seen to occur. While attenuation rates near the source can be seen to be
high as a result of the shallow geometry, a TL plateau is reached wherein a
constant level is maintained or the level decreases slowly with increasing
distance from the source. This is probably the result of sound transmission
within the bottom layers and reflection and refraction out of the layers to
reinforce sound in the water column. The TL characteristics shown in Fig 4.5C
for 1 kHz are similar to those obtained at 315 Hz with somewhat lower values
of loss being predicted.
The TL characteristics obtained from the model calculations for the Type
1 Bottom were interpolated to obtain a set of curves for "predicting the TL
from a shallow source to a shallow receiver near the beach as a function of
the distance of the source from the shoreline. The results, shown in Fig.
4.5D, are presented to facilitate the estimation of received level near shore
for a vessel operating directly offshore. The received level may be estimated
as:
where: Lr = Received level in a selected 1/3 octave band
Ls = Source level at 1 m'in the selected 1/3 octave band for a
specific source (from source level tables)
TL = The transmission loss from Fig. 4.5D for the 1/3 octave band at
the range of interest (this may have to be interpolated).