When the density and compressibility of an object are similar to the corresponding properties of the surrounding fluid and the incident sound field is a standing wave, the Born approximation may be used to calculate the acoustic radiation force and torque on an object of arbitrary shape. The approximation consists of integration over the monopole and dipole contributions to the force acting at each point within the region occupied by the object. The method is applied to axisymmetric objects, for which the force and torque may be expressed as a single integral along the axis of symmetry. The integral is evaluated analytically for spheres and cylinders. The accuracy of the Born approximation is assessed by comparison with complete solutions for compressible spheres and prolate spheroids that are based on expansions of the incident, scattered, and transmitted fields in terms of eigenfunctions of the corresponding separable coordinate system. Results are presented for objects with various densities and compressibilities relative to the surrounding fluid, as well as different shapes, sizes, and orientations of the object with respect to the standing wave field. The method also accommodates spatial variations of the density and compressibility within the object.

If an object and the host fluid have similar material properties, and the dominant contribution to the acoustic radiation force is due to energy density gradients in the sound field, the Born approximation may be used to reduce calculation of the radiation force and torque on an object of arbitrary shape to an integral over the volume of the object. Previous comparisons with full solutions based on wave function expansions demonstrate that the approximation is accurate for homogeneous spheres and prolate spheroids in a standing plane wave for objects as large as the wavelength. The Born approximation may also be applied to inhomogeneous objects having spatial variations in compressibility and density. Simplified integral expressions are presented for axisymmetric particles in a standing plane wave that are small compared with the wavelength and possess inhomogeneity that varies along their axis of symmetry. The integrals yield analytical expressions for the force and torque on finite cylinders and prolate spheroids for inhomogeneity that varies linearly along their axis of symmetry. Also considered are the force and torque on a homogeneous red blood cell, which are compared with the corresponding results for a cylindrical disk and oblate spheroid having equivalent volume and aspect ratio.

The acoustic radiation force on a compressible spheroid is calculated using expansions of the scattered field in terms of both spherical and spheroidal wave functions that are matched analytically in the far field. There is no restriction on the size or impedance of the spheroid, the structure of the incident field, or the orientation of the spheroid with respect to the incident field. The form of the solution is the same as that developed previously for the radiation force on an elastic sphere, which is a summation of terms involving products of the coefficients in spherical wave expansions of the incident and scattered fields. Spheroidal wave expansions are employed to satisfy the boundary conditions and obtain the scattering coefficients. While the scattering coefficients must be obtained numerically for compressible spheroids, explicit expressions in terms of radial wave functions are available for spheroids with rigid or free surfaces. Results are compared with available analytical expressions for various limiting cases. The theoretical framework may be extended to objects of arbitrary shape.

The Born approximation developed previously to model acoustic radiation force and torque exerted on homogeneous compressible objects of arbitrary shape [Jerome et al., J. Acoust. Soc. Am. 145, 36–44 (2019)] is extended to include objects that are inhomogeneous. The same general restrictions apply to this extended model, mainly that the incident field is not too similar to a progressive plane wave, that the material properties of the object do not differ substantially from those of the surrounding fluid, and that the size of the object is not much larger than a wavelength. Two applications of the model are presented, one for objects consisting of connected homogeneous regions with different material properties, and the other for objects with continuously varying material properties. Calculations are presented for spheres, finite cylinders, and prolate spheroids.

Seagrasses are sentinel species whose structures are sensitive to variations in environmental conditions and thus reliable indicators of long-term changes in sea level and regional climate. The biological and physical characteristics of seagrasses are known to affect acoustic propagation. Gas bodies contained within the seagrass tissue as well as photosynthetic-driven bubble production result in free gas bubbles attached to the plants and in the water. The detachment of gas bubbles from the plants is also a source of ambient sound. In this work, we deployed a measurement system in a shallow (1.3 m deep) seagrass meadow that included an acoustic projector, a set of receiving hydrophones, an instrumentation pressure vessel that housed the electronics controlling the acoustic data acquisition and data storage, and a suite of environmental sensor loggers. Acoustic propagation and ambient sound were collected every ten minutes for a period of one year. Coincident measurements of water temperature, salinity, dissolved oxygen, and photosynthetically active radiation were also collected and used to interpret the acoustic data. We present preliminary results from the yearlong deployment of the acoustic system in a moderately dense seagrass bed dominated by Thalassia testudinum (turtle grass) in Corpus Christi Bay, Texas. [Work supported by NSF.]