Time domain signals can manifest characteristic signals when the mid-pulse frequency is in some resonance region. This work attempts to determine unique signals characteristic of pulse scattering at frequencies associated with resonances excited from elongated targets. In an earlier work beat patterns and damped sinusoidal patterns (as a function of time) were associated with single or specific clusters of resonances. Here, damped sinusoidal patterns are associated with specific resonances in which it is shown how to extract specific resonance widths that are shown to be associated with three classes of resonances excited from elastic spheroids. This includes resonances excited broadside, end-on, and at oblique incident angles. Also determined are some beat patterns from some types of resonance clustering.

In a previous paper [C. E. Dean and M. F. Werby, J. Acoust. Soc. Am. 91, 2469 (1992)] initial calculations for scattering from spheroids composed of six materials for aspect ratios of three and six were presented. Improved results for the same cases are now shown. The difference in resonance locations as a function of material properties is noted and explained in terms of the pseudo-Rayleigh type resonances described extensively in the literature for spheroids. There is also a marked difference between the amplitudes of the resonances. The relative amplitudes of the resonances is predicted as a function of material properties based on the Rayleigh wave interpretation of these resonances. A theory is developed based on the fact that Rayleigh type resonances can only be excited as a function of the appropriate grazing annular region. This region is determined as a function of the material property and geometry. Numerous results are presented.

In an earlier work it has been shown that certain classes of resonances excited on elastic spherical solids correspond to standing wave patterns on the object surface. This observation was in fact implicit in the circumferential nature of such resonances. The demonstration was made possible by subtracting the rigid background and plotting the bistatic angular distributions in the asymptotic limit resulting in a standing wave pattern. For the spheroidal case it is more difficult to demonstrate this affect in the asymptotic limit since typically the asymptotic limit implies spherical symmetry that is not adhered to for spheroids. It is possible to demonstrate that this is also observed for resonances on spheroids by choosing a spheroidal surface and plotting the results once the rigid background is subtracted. It is also possible to observe the results by projecting the asymptotic results on to a spheroidal basis. Results are presented for several examples.

It is possible to employ the extended boundary condition equations (EBC) method due to Waterman to describe scattering from axially symmetric submerged elastic targets over a broad frequency range. The problem of scattering from cylinders with hemispherical end caps offers one a useful target to analyze. Resonance phenomena predicted by the EBC calculations are analyzed and a systematic study of results is presented. In addition, bending resonances are predicted by the theoretical calculations and compared with predictions from bar theory. A series of predictions are made for various materials and aspect ratios to aid future experimental work.

When scattering from elastic targets backscattered echoes yield interesting information in the resonance region. In particular, resonance scattering theory in the frequency domain along with the circumferential nature of resonances imply that material constituency is a characteristic of resonance location. Moreover, recent discussions of resonance signatures in the time domain [see Uberall's book to be published on resonance scattering] can also yield information concerning resonance widths and average phase velocities which can also be related to material characteristics. By adjusting the orientation of the target over a suitable angular region it is possible to ascertain certain symmetries of the target if they exist, particularly if one varies the frequencies over a suitable range of resonances. If one observes axial symmetry through such a process, then it is possible to obtain both the dimensions of the object and the aspect ratio of the object (ratio of length to width). This is assuming that the target is in a “free” environment; that is, the boundaries of the target are not a factor in calculation. Time-domain responses for specific pulse types also yield information and it is easy to see how a series of questions can form the basis of a scenario that can rule out certain targets or lead to a probability (confidence level) that specific targets are present. To determine the extent that this can be done, targets are examined which are composed of five materials for elastic solid spheroids for aspect ratios of 3 to 1 and 6 to 1 for end-on incidence and for the case of 4 to 1 for all incident angles.

It is not difficult to predict how sound scatters from a fluid-loaded elastic shell based on exact elastodynamic theory, provided the shell is a sphere or an infinite cylinder. Problems arise for more general shapes, however, and although some success has been obtained for spheroids and cylinders with hemispherical end caps, the results are rather tedious, if not disappointing, when one wishes to extend the frequency range or aspect ratio of the target. Some progress has been made for results predicted from thin-shell theories either utilizing finite-element methods in two and three dimensions or T matrices based on thin-shell theories. In this study, some common thin-shell theories that are employed for spherical elastic shells with exact normal mode theory are examined, with the goal of extending the results to elongated targets. Limitations of the various thin-shell theories are explored for both the frequency range and thickness.

Audition resulting from vibration of the skull is investigated in various respects using audiometers with bone conduction receivers and other equipment. Applying the driving element at any point sets the entire skull into vibration, and the stimulus in one ear resulting from driving the skull on that mastoid is found to exceed the stimulus in the other ear by only 3.4±2.2 decibels. The statistical accuracy of bone conduction observations is found and compared with published air conduction results. The sensitivity on the mastoid exceeds that on the forehead by 5 db on the average. Tests of instruments for measuring air conduction and bone conduction sensitivity to determine the amount of desired over undesired type of stimulation are described and the results given. The effect of a telephone receiver in producing hearing in the opposite ear is observed with two available cases of assured total deafness on one side and the data given. Beats involving bone conduction are observed. The fact that bone conduction sensations often seem in one ear is compared with similar results for air conduction stimuli under certain conditions and a general statement of all results is made. The problems of apparent hyper-sensitivity resulting from abnormality and occlusion are discussed in the light of the foregoing results.