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

Report No. 6945
BBN Systems and Technology Corporation
APPENDIX D:
ESTIMATION OF HEARING RESPONSE CHARACTERISTICS FOR GRAY AND FIN WHALES
Figure D.l is presented as background information to help describe the
proposed procedure. This figure shows the average human speech spectrum and
hearing sensitivity characteristics as reported in the literature. One
interesting feature is that the frequency range of maximum hearing sensitivity
lies above the frequency range where the maximum speech level occurs. This is
believed to have evolved to compensate for the higher attenuation of high
frequencies in propagation through the air and for the need to maintain a
nearly constant signal-to-noise ratio through the speech range for good speech
intelligibility. Note that the upward slope of the hearing sensitivity curve
is similar to the downward slope of the speech spectrum. For the human
characteristics the frequency of maximum hearing sensitivity (Fma) is about 2
1/2 octaves above the frequency of maximum vocal output (for male speakers)
(Fmv). It is possible that a similar difference exists in the frequency bands
for marine mammal vocalization and hearing characteristics.
Two other curves are also shown in the figure which represent the sound
levels at which a tone would become annoying or would become loud enough to
cause permanent hearing damage. The pure tone amplitude range of normal
hearing response for humans can be seen to cover a range of 60 to 90 dB on a
logarithmic scale or a range of 1000 to 30,000 on linear scale, depending on
frequency.
Figure D.2 illustrates the procedure used for estimating the hearing
response of the gray whale. We assume that the characteristic will be similar
in spectrum shape to that of other mammals (Myrberg 1978) but its location in
frequency range will be determined by the acoustic requirements of the
species. The vocalization output characteristic shown was estimated from a
.
brief review of reported data. If a 2 1/2 octave difference exists between
Fmv and Fma for gray whales, the range of maximum hearing sensitivity may
occur around 700 Hz as shown. The maximum sensitivity level is estimated to
be lower than the ambient noise spectrum level for Sea State 0 in this
frequency range since. gray whale hearing sensitivity has been observed to be