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

Report No. 6945
BBN Systems and Technologies Corporation
Section 2'.1.3 for each potential source - specie$ encounter situation. Thus
ratings are obtained for both the encounter sensitivity for a single animal
and the specific potential for acoustic impact for the population within an
area. The SER values obtained from the analysis are shown in bar graph format.
to supplement the data presented in the tables.
The SER Model is designed to show which species are potentially most
influenced by a given noise source by developing high values when compared to
the input SNCl value. If the SER value is comparable to, or less than the
SNC1 value, the species is likely to be minimaly influenced as a result of a
mismatch between the source noise spectrum and the species hearing sensitivity
and/or a low species population density in the area being considered. A
procedure was devised for summarizing the results of the SER analysis using
three general ratings of low, medium and high. These ratings were established
by statistically analyzing the output of the SER Model for all of the source
species encounters examined in the 4 planning areas. The average and the
standard deviation of the area SER values were determined for a total of 75
data points. The values ranged between 111 and 207 dB with a mean value of
159 dB. The standard deviation was 20 dB. With these results as a guide the
general ratings were defined as
Low - SER values 140 dB and lower
Medium - SER values between 141 and 179 dB
High - SER values 180 dB and higher.
These values are designed to provide a broad ranking of source audibility
for the various source - species encounters considered under standardized
conditions. These SER ratings are not based on behavioral observations and
are intended only as a means of ranking encounter situations where potential
behavioral responses may occur.
Only a limited number of behavioral studies are available to provide a
means of calibrating this ranking scheme. Averaged results from gray whale
disturbance studies using playback of several types of continuous noise from
industrial sources (Malme et al. 1984) showed that 50% of the whales migrating
through the test area avoided areas where sound levels were about 120 dB.
Over 90% of the whales avoided the region near the source where sound levels
were higher than 130 dB. Reviewing the results of the SER analysis for those
areas where continuous source - gray whale encounters were considered provided
the following results which show a comparison of the mean SER ratings with the
mean of the received levels at the reference range of 300 m for the various
sources considered (the number of samples and the standard deviation are also
shown) :
SER Rank N Mean SER(~B) SD Mean Lrl(dB) SD
High 2 18 1 - 138 -
Medium 8 167 - 8 125 ' 7
These results suggest that the SER ranking scheme is consistent with
behavioral observations for at least one species. However, more acoustic