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).