Reconstruction of the forehead acoustic properties in an Indo-Pacific humpback dolphin (Sousa chinensis), with investigation on the responses of soft tissue sound velocity to temperature

Abstract

Computed tomography (CT) imaging and ultrasound experimental measurements were combined to reconstruct the acoustic properties (density, velocity, and impedance) of the head from a deceased Indo-Pacific humpback dolphin (Sousa chinensis). The authors extracted 42 soft forehead tissue samples to estimate the sound velocity and density properties at room temperature, 25.0 °C. Hounsfield Units (HUs) of the samples were read from CT scans. Linear relationships between the tissues' HUs and velocity, and HUs and density were revealed through regression analyses. The distributions of the head acoustic properties at axial, coronal, and sagittal cross sections were reconstructed, suggesting that the forehead soft tissues were characterized by low-velocity in the melon, high-velocity in the muscle and connective tissues. Further, the sound velocities of melon, muscle, and connective tissue pieces were measured under different temperatures to investigate tissues' velocity response to temperature. The results demonstrated nonlinear relationships between tissues' sound velocity and temperature. This study represents a first attempt to provide general information on acoustic properties of this species. The results could provide meaningful information for understanding the species' bioacoustic characteristics and for further investigation on sound beam formation of the dolphin.

Acoustic properties of a short-finned pilot whale head with insight into temperature influence on tissues' sound velocity

“…Furthermore, temperature is also a factor that affects sound velocity, and thereby the capacity of focusing sound for tissues. Sound velocities of soft tissues and lipids extracted from melon in odontocete forehead decrease with increasing temperature (Blomberg and Jensen, 1976;Litchfield et al, 1979;Goold et al, 1996;Goold and Clarke, 2000;Song et al, 2017), which is consistent with the results of our study (Fig. 5).…”

“…This enabled us to compare sound velocities of different tissue types and investigate the acoustic function of forehead tissues in vivo. The corrected sound velocity of one melon sample in 1077.6 m/s is much less than that in previous studies (Blomberg and Jensen, 1976;Litchfield et al, 1979;Goold et al, 1996;Goold and Clarke, 2000;Soldevilla et al, 2005;Song et al, 2017). Sound velocity and density in samples composed of melon, muscle, or connective tissue of the short-finned pilot whale are shown in Table III with different temperatures, together with those in previously investigated species, such as Cuvier's beaked whale (Soldevilla et al, 2005), Yangtze finless porpoise (Wei et al, 2015), and pygmy sperm whale (Song et al, 2015).…”

“…In most previous studies, the sound velocity and density of head tissues in it were measured at room temperature (Norris and Harvey, 1974;Wei et al, 2015;Song et al, 2015). There is a complex relationship (i.e., linear or non-linear) between temperature and the sound velocity of soft tissues or lipids extracted from melon in odontocete head (Blomberg and Jensen, 1976;Litchfield et al, 1979;Goold et al, 1996;Goold and Clarke, 2000;Soldevilla et al, 2005;Song et al, 2017). Thus, temperature is an important factor for the acoustic property.…”

“…In this study, we employed similar methods to reconstruct the acoustic properties (i.e., sound velocity, density, and acoustic impedance) distributions of the head tissue from a freshly dead short-finned pilot whale. Because the temperature may affect the sound velocity in soft tissues and therefore the sound propagation in odontocete foreheads (Blomberg and Jensen, 1976;Litchfield et al, 1979;Goold et al, 1996;Goold and Clarke, 2000;Soldevilla et al, 2005;Song et al, 2017), the relationship between the temperature and sound velocity were examined. The results enable us to further simulate the acoustic-structure model for better understanding the biosonar system of the short-finned pilot whale.…”

Biosonar signal propagation in the harbor porpoise's (Phocoena phocoena) head: The role of various structures in the formation of the vertical beam

“…Such potential effect on tissue properties would add some extent of uncertainty to the model predictions, in order to quantify the uncertainty in the model and demonstrate how much the model output would have changed with potential range of sound velocity, the sound velocity of the melon was calculated at 23 C, 27 C, 32 C, 37 C according to the relationship between temperature and sound velocity measured by Song et al (2017). In the study by Song et al (2017), the relationship between the soft tissue's sound velocity and temperature demonstrated the nonlinearity of responses to temperature. The distribution of sound velocity of the melon at 23 C, 27 C, 32 C, 37 C can be estimated.…”