Biomedical subjects

D H Chambers

Publications and source records attributed to D H Chambers.

3 recordsLinked to original sources

PubMed2002

Analysis of the time-reversal operator for scatterers of finite size.

Recently, it was shown that the time-reversal operator for a single, small spherical scatterer could have up to four distinguishable eigenstates [Chambers and Gautesen, J. Acoust. Soc. Am. 109, 2616-2624 (2001)]. In this paper, that analysis is generalized for scatterers of arbitrary shape and larger size. It is shown that the time-reversal operator may have an indefinitely large number of distinguishable eigenstates, with the exact number depending on the nature of the scatterer and the geometry of the time-reversal mirror. In addition, the case of a multiple number of well-separated scatterers is investigated, with the result that the total spectrum is the direct combination of the eigenstates associated with each scatterer. As an example, the singular value spectrum of the time-reversal operator for a linear array is calculated explicitly for bubbles and hard rubber spheres of finite size. Both resonance peaks and apparent crossing points can be observed in the spectrum as the size of the scatterer increases.

Journal Article↗

PubMed2001

Perceptual ratings for pureed and molded peaches for individuals with and without impaired swallowing.

Food molds have been created to alter the appearance of pureed foods by making the pureed consistencies look more like typically prepared foods. This study examined the perceptions of 15 adults with impaired swallowing and 15 adults with nonimpaired swallowing for three different types of pureed peach: a typically prepared pureed peach, a pureed peach with a thickening agent added, and a pureed peach with a thickening agent added and molded to look like peach slices. Results are presented for the attribute ratings of overall liking: liking of taste, texture, and appearance; and ease of chewing and swallowing. The results showed that the adults with impaired swallowing rated the attributes of chewing and swallowing significantly lower (i.e.. more difficult) for each type of puree. Significant differences within samples also were found for the attributes of overall liking and liking of taste. Overall, the pureed peach molds were not perceived favorably for any of the examined attributes. In addition, the molded peach was rated lower (more difficult) in its ease of chewing. The implications of these findings and ideas for further research are discussed.

Adult↗

PubMed2001

Time reversal for a single spherical scatterer.

We show that the time reversal operator for a planar time reversal mirror (TRM) can have up to four distinct eigenvalues with a small spherical acoustic scatterer. Each eigenstate represents a resonance between the TRM and an induced scattering moment of the sphere. Their amplitude distributions on the TRM are orthogonal superpositions of the radiation patterns from a monopole and up to three orthogonal dipoles. The induced monopole moment is associated with the compressibility contrast between the sphere and the medium, while the dipole moments are associated with density contrast. The number of eigenstates is related to the number of orthogonal orientations of each induced multipole. For hard spheres (glass, metals) the contribution of the monopole moment to the eigenvalues is much greater than that of the dipole moments, leading to a single dominant eigenvalue. The other eigenvalues are much smaller, making it unlikely multiple eigenvalues could have been observed in previous experiments using hard materials. However, for soft materials such as wood, plastic, or air bubbles the eigenvalues are comparable in magnitude and should be observable. The presence of multiple eigenstates breaks the one-to-one correspondence between eigenstates and distinguishable scatterers discussed previously by Prada and Fink [Wave Motion 20, 151-163 (1994)]. However, eigenfunctions from separate scatterers would have different phases for their eigenfunctions, potentially restoring the ability to distinguish separate scatterers. Since relative magnitudes of the eigenvalues for a single scatterer are governed by the ratio of the compressibility contrast to the density contrast, measurement of the eigenvalue spectrum would provide information on the composition of the scatterer.

Journal Article↗