Abstract
Experimental measurements of acoustic backscattering of FM pulses from a family of precision-solid and hollow-aluminum spheres of 7 in. o.d. are presented and compared with computed echo pulse forms. The present paper is an extension of work reported by Diercks, Goldsberry, and Hickling for 5-in.-diam spheres [J. Acoust. Soc. Am. 40, 1258 (A) (1966)]. The spheres studied had i.d./o.d. ratios of 0.95, 0.8, and 0.0, with either air or water inside the hollow spheres. The incident FM pulses had a nondimensionalized center frequency of ka = 20 (approximately 54 kHz) with nondimensionalized frequency ranges of 15–25 and 10–30 during the pulse. Incident pulse lengths were selected in the nondimensionalized form Δτ = 0.5, 1.0, or 5.0, where Δτ relates the range extent of the pulse to the radius of the sphere according to 2Δτ = c2Δt/a, where 2Δt is the pulse length in seconds, c is the velocity of sound in water, and a is the radius of the sphere. Detailed comparison of experimental and theoretical results shows generally good agreement in frequency structure and amplitude for FM pulses having a range extent equal to or shorter than the sphere diameter. Comparisons for longer pulses are also shown. [This work was done under contract with the U. S. Office of Naval Research, Code 463, and the U. S. Naval Ordnance Test Station, Pasadena, California.]
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