Fig. 1. (a) Three-dimensional reconstruction of a finless porpoise head; (b) three-dimensional reconstruction of a pygmy sperm whale head; (c) three-dimensional reconstruction of an Indo-Pacific humpback dolphin.

Fig. 2. (a) Three-dimensional acoustic structure reconstructions of a finless porpoise; (b) three-dimensional acoustic structure reconstructions of a pygmy sperm whale; (c) three-dimensional acoustic structure reconstructions of an Indo-Pacific humpback dolphin. Upper jaw, sound source, air components (including the air sacs and nasal passage) are represented in different colors.

Fig. 3. (a) Sound speed reconstructions of finless porpoise for sound emission system in horizontal section; (b) sound speed reconstructions of finless porpoise for sound emission system in vertical section^[34]; (c) sound speed reconstructions of pygmy sperm whale for sound emission system in horizontal section; (d) sound speed reconstructions of pygmy sperm whale for sound emission system in vertical section^[36]; (e) sound speed reconstructions of the Indo-Pacific humpback dolphin for sound emission system in horizontal section; (f) sound speed reconstructions of the Indo-Pacific humpback dolphin for sound emission system in vertical section^[38].

Fig. 4. Sound velocity distribution along the y axis corresponds to the line in the right part of Fig. 3(d). Mu, muscle; Me, melon; T, theca^[36].

Fig. 5. (a) Mean spectrum of the clicks from –3 dB, –6 dB, and –10 dB groups for the Indo-Pacific humpback dolphin; (b) the mean spectrum of the clicks from –3 dB, –6 dB, –10 dB and –20 dB groups for the finless porpoise.

Fig. 6. (a) Spectrogram of constant frequency whistles of the bottlenose dolphins; (b) the spectrogram sinusoidal whistles of the bottlenose dolphins; (c) the spectrogram of convex or hill whistles of the bottlenose dolphins; (d) the spectrogram of concave or valley whistles of the bottlenose dolphins; (e) the spectrogram of upsweep whistles of the bottlenose dolphins; (f) the spectrogram of down sweep frequency whistles of the bottlenose dolphins^[58,59].

Fig. 7. (a) Two captive free swimming bottlenose dolphins in Xiamen; (b) bottlenose dolphins under training; (c) pie chart of the classified whistles of two bottlenose dolphins under free swimming; (d) pie chart of the classified whistles of two bottlenose dolphins under training conditions^[58,59].

Fig. 8. (a) Beam directivity of a sound pulse with a centroid frequency of 130 kHz for No-Skull model of harbor porpoise; (b) the beam directivity of a sound pulse with a centroid frequency of 130 kHz for a complete model of harbor porpoise^[31].

Fig. 9. (a) Propagation plot of a short-duration impulse source for Baiji in vertical section at time 1; (b) propagation plot of a short-duration impulse source for Baiji in vertical section at time 2; (c) propagation plot of a short-duration impulse source for Baiji in vertical section at time 3; (d) enlarged details of (a); (e) enlarged details of (b); (f) enlarged details of (c)^[74].

Fig. 10. Solid displacements of the 20 maxilla points of baiji^[74].

Fig. 11. (a) Propagation plot of the transient sound waves at time 1 under a full model case of pygmy sperm whale; (b) propagation plot of the transient sound waves at time 2 under a full model case of pygmy sperm whale; (c) propagation plot of the transient sound waves at time 3 under a full model case of pygmy sperm whale; (d) propagation plot of the transient sound waves at time 1 under a model case without air components; (e) propagation plot of the transient sound waves at time 2 under a model case without air components; (f) propagation plot of the transient sound waves at time 3 under a model case without air components; (g) the beam directivity of a sound pulse with a peak frequency of 125 kHz for the full model case of the pygmy sperm whale; (h) the beam directivity of a sound pulse with a peak frequency of 125 kHz for the model case of pygmy sperm whale without air components^[35].

Fig. 12. (a) Acoustic field of no-melon and full head cases at time 1; (b) acoustic field of no-melon and full head cases at time 2; (c) acoustic field of no-melon and full head cases at time 3; (d) acoustic field of no-melon and full head cases at time 4; (e) the beam directivity of a sound pulse with a centroid frequency of 130 kHz for no-melon case of harbor porpoise; (f) the beam directivity of a sound pulse with a centroid frequency of 130 kHz for the full head case^[31].

Fig. 13. (a) Compressing effect of models I, II, III, IV, and V on acoustic field of the sound pulse with a peak frequency of 125 kHz inside the head, where θ represents the orientation angle of the vestibular sac and NA represents the normalized area of the forehead tissues with respect to those of the original model I; (b) beam directivities of the five cases; (c) sound beams’ –3 dB beam widths and main beam angle distribution of the five cases ^[34].