2-4. Acoustic characteristics analysis
Acoustic characteristics of call duration, peak frequency, and
maximum/minimum frequency were estimated from at least 100 sample
signals of a single call. Because classified call datasets contained
signals that either overlapped a couple of vocalizations or exhibited a
low signal-to-noise ratio, we extracted acoustic characteristics from
randomly selected sample signals containing a single call with a
sufficient signal-to-noise ratio. The extraction of acoustic
characteristics commenced by plotting figures of the waveform,
spectrogram, sound pressure level, and spectrogram of the envelope
waveform as a function of time (Figure 2). The call types of leopard
seal analyzed in this study included trill sounds, which exhibited the
amplitude modulation patterns. The amplitude modulation rate of the
trill waveform is characterized by the pulse repetition rate, which is
more effectively represented using the envelope-spectrogram technique
(Klinck et al., 2008).