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Acoustic properties of odontocete head tissues, including sound velocity, density, and acoustic impedance, are important parameters to understand dynamics of its echolocation. In this paper, acoustic properties of head tissues from a freshly dead short-finned pilot whale (Globicephala macrorhynchus) were reconstructed using computed tomography (CT) and ultrasound. The animal's forehead soft tissues were cut into 188 ordered samples. Sound velocity, density, and acoustic impedance of each sample were either directly measured or calculated by formula, and Hounsfield Unit values (HUs) were obtained from CT scanning. According to relationships between HUs and sound velocity, HUs and density, as well as HUs and acoustic impedance, distributions of acoustic properties in the head were reconstructed. The inner core in the melon with low-sound velocity and low-density is an evidence for its potential function of sound focusing. The increase in acoustic impedance of forehead tissues from inner core to outer layer may be important for the acoustic impedance matching between the outer layer tissue and seawater. In addition, temperature dependence of sound velocity in soft tissues was also examined. The results provide a guide to the simulation of the sound emission of the short-finned pilot whale.

The wave propagation, sound field, and transmission beam pattern of a pygmy sperm whale (Kogia breviceps) were investigated in both the horizontal and vertical planes. Results suggested that the signals obtained at both planes were similarly characterized with a high peak frequency and a relatively narrow bandwidth, close to the ones recorded from live animals. The sound beam measured outside the head in the vertical plane was narrower than that of the horizontal one. Cases with different combinations of air-filled structures in both planes were used to study the respective roles in controlling wave propagation and beam formation. The wave propagations and beam patterns in the horizontal and vertical planes elucidated the important reflection effect of the spermaceti and vocal chambers on sound waves, which was highly significant in forming intensive forward sound beams. The air-filled structures, the forehead soft tissues and skull structures formed wave guides in these two planes for emitted sounds to propagate forward.

Harbor porpoises (Phocoena phocoena) use narrow band echolocation signals for detecting and locating prey and for spatial orientation. In this study, acoustic impedance values of tissues in the porpoise's head were calculated from computer tomography (CT) scan and the corresponding Hounsfield Units. A two-dimensional finite element model of the acoustic impedance was constructed based on CT scan data to simulate the acoustic propagation through the animal's head. The far field transmission beam pattern in the vertical plane and the waveforms of the receiving points around the forehead were compared with prior measurement results, the simulation results were qualitatively consistent with the measurement results. The role of the main structures in the head such as the air sacs, melon and skull in the acoustic propagation was investigated. The results showed that air sacs and skull are the major components to form the vertical beam. Additionally, both beam patterns and sound pressure of the sound waves through four positions deep inside the melon were demonstrated to show the role of the melon in the biosonar sound propagation processes in the vertical plane.

The relative role of the various structures in the head of the baiji (Lipotes vexillifer) is examined. A finite element approach was applied to numerically simulate the acoustic propagation through a dolphin's head to examine the relative role of the skull, air sacs, and melon in the formation of the biosonar beam in the vertical plane. The beam pattern obtained with the whole head in place is compared with the beam pattern when the air sac is removed and the other structures (skull and melon) are in place, with only the skull removed, and finally with only the melon removed. The beam pattern with the air sacs and skull intact and the melon removed closely resembled the beam pattern for the complete head, suggesting that the melon has a minor role in the formation of the beam. The beam pattern for the other two cases had very little resemblance to the beam pattern for the whole head. The air sacs seem to have a role of directing propagation of the signal toward the front and the skull prevents the sound propagating below the rostrum. The beam patterns along with a correlation analysis showed that the melon had only a slight influence on the shape and direction of the beam. The resultant beam exiting the head of the dolphin is the result of complex reflection processes within the head of the animal.

Computed tomography (CT) imaging and sound experimental measurements were used to reconstruct the acoustic properties (density, velocity, and impedance) of the forehead tissues of a deceased pygmy sperm whale (Kogia breviceps). The forehead was segmented along the body axis and sectioned into cross section slices, which were further cut into sample pieces for measurements. Hounsfield units (HUs) of the corresponding measured pieces were obtained from CT scans, and regression analyses were conducted to investigate the linear relationships between the tissues' HUs and velocity, and HUs and density. The distributions of the acoustic properties of the head at axial, coronal, and sagittal cross sections were reconstructed, revealing that the nasal passage system was asymmetric and the cornucopia-shaped spermaceti organ was in the right nasal passage, surrounded by tissues and airsacs. A distinct dense theca was discovered in the posterior-dorsal area of the melon, which was characterized by low velocity in the inner core and high velocity in the outer region. Statistical analyses revealed significant differences in density, velocity, and acoustic impedance between all four structures, melon, spermaceti organ, muscle, and connective tissue (p < 0.001). The obtained acoustic properties of the forehead tissues provide important information for understanding the species' bioacoustic characteristics.

The reconstruction of the acoustic properties of a neonate finless porpoise's head was performed using X-ray computed tomography (CT). The head of the deceased neonate porpoise was also segmented across the body axis and cut into slices. The averaged sound velocity and density were measured, and the Hounsfield units (HU) of the corresponding slices were obtained from computed tomography scanning. A regression analysis was employed to show the linear relationships between the Hounsfield unit and both sound velocity and density of samples. Furthermore, the CT imaging data were used to compare the HU value, sound velocity, density and acoustic characteristic impedance of the main tissues in the porpoise's head. The results showed that the linear relationships between HU and both sound velocity and density were qualitatively consistent with previous studies on Indo-pacific humpback dolphins and Cuvier's beaked whales. However, there was no significant increase of the sound velocity and acoustic impedance from the inner core to the outer layer in this neonate finless porpoise's melon.