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

Report No. 6945
BBN Systems and Technology Corporation
ambient noise limited, not hearing sensitivity limited during quiet sea state
conditions (Malme et al. 1984). A maximum hearing sensitivity of 40 dB was
assumed since this corresponds to the value measured for orcas, the largest
whale tested to date. It is possible that gray whale hearing is not this
sensitive since the estimated frequency of maximum hearing sensitivity is
700 Hz versus the measured 12 kHz for orcas (see Fig. 2.24). Underwater
ambient noise levels at 700 Hz are higher than at 12 kHz. Thus evolutionary
processes may have resulted in reduced sensitivity at low frequency as an
adaptation to the underwater ambient noise spectrum. Conversely, human
**
:* hearing thresholds are below the level of general ambient noise so that human
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hearing is almost always noise limited. This is expected to be true for
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Figure D.3 shows the estimated hearing characteristics for gray and fin
whales compared with the measured data for white whales (see Fig. 2.24). The
hearing characteristic for fin whales was obtained from the gray whale
characteristic by scaling the frequency range downward by a factor of 3. This
was done because their dominant vocalization output occurs at lower frequen-
cies than that for gray whales (see Table 2.7) and hence their hearing
characteristic is expected to cover a lower frequency range. This procedure
is highly speculative and the predicted characteristics are intended to be
used only to provide preliminary estimates of potential acoustic sensivitity.
Measured data must be used a$ soon as test results become available.