Blue whale calling in the Rottnest trench-2000, Western ... - ANP

Figure 13: Magnitudes of several type II component Hilbert transforms, showing waveform envelope. The
envelopes show large and variable differences in level throughout the component. This is believed partly a function
of amplitude modulation of the signal under the animal control, but primarily a sound propagation effect.
3.1.3) Type II received levels
Since the type II component had the highest source level it tended to be the most common signal
identifiable in received spectrograms. Thus much of the sound propagation analysis and call
counting techniques were tailored for this component type. For sound propagation analysis it is
necessary to consider the frequency structure of the signal propagating, since the transmission
loss characteristics are frequency dependant. If the emitted sound is broadband, or has energy
equally spread across a wide frequency band, then the sound propagation needs to be calculated
at representative frequencies and the loss for each frequency at a specified range summed to give
the resulting signal at that range.
For the type II component the signal tended to be dominated by the 20-26 Hz tone, as shown on
Figure 11 (middle plot). This implies that the sound propagation modelling could concentrate on
this frequency band only, and neglect all the other spectral input, such as the 48 and 72 Hz tones
seen on Figure 11. If these other tones were 10 dB less than the 20-26 Hz level then they can be
validly ignored, since they contribute nothing to the decibel value of the received signal level. To
check if this applied across a range of signals, the start and end time of 107 type II signals with
no distorting or overlapping sources (boats, other whales etc) and from the bottomed bluey
logger, was determined and the spectral content of these components calculated, using a 0.6104
Hz resolution, Hanning window and around 16 averages per component. Only the portion of
each component which included 95% of the component energy was used (with the sections
averaging at 16 s each). These type II signals spanned a received level of 103-129 dB re 1μPa
(mean squared pressure, see section 3.2 for definition). Using decibel statistics (or the statistics
calculated on the intensities then converted back to dB), the mean difference in the level of the
spectral peak between 20-30 Hz minus that between 45-50 Hz was 18.18 dB with 95%
confidence limits from 16.5-19.3 dB. Using the same technique the mean difference between the
level of the spectral peak between 20-30 Hz minus that between 68-75 Hz was 9.94 dB with 95%
confidence limits of 8.2-11.1 dB. Thus the assumption of using only the 20-26 Hz tone in
propagation modelling was considered valid.
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