Acoustic backscattering spectra of three syntactic-foam spheres

Acoustic backscattering spectra have been determined for three 60-mm-diameter spheres fabricated from product samples of syntactic foam manufactured by CMT Materials, Inc., namely HYTAC-W, HYTAC-WF, and HYTAC-XTL. The nominal diameter of each sphere was 60 mm. Measurements were made over each of two frequency bands: 80-200 and 300-600 kHz. These were accompanied by similar measurements made on a precision 75-mm-diameter sphere of tungsten carbide with 6% cobalt binder. The echo spectra were rendered absolute by each of two calibration procedures depending on how incident wave at the position of the target was obtained. One procedure assumed that the front-face specular echo from a tungsten carbide sphere is proportional to the transmit waveform to within a constant of proportionality given by the ratio of acoustic impedances of sphere and ambient medium. The other procedure measured transmit and receive response of the transducer from water surface, to account for the unknown frequency response function of the system. Results are expressed in terms of the generalized backscattering form function. Additional measurements were performed to determine the magnitude of possible temperature- and pressure-dependent effects over the respective temperature range 6-30°C and equivalent depth range 0-400 m.