Acoustic measurements of insertion loss and echo reduction at normal incidence were made for thin plates of eight materials for evaluation of their use in underwater sound transducers in the frequency range 50–500 kHz. Materials tested included Lucite; silicone rubber RTV– 560; the commercial polyurethane products PRC–1527, CPC–19, Scotchcast 8, and Scotchcast 221; and two of these materials with talc added during curing to vary the properties. Values of longitudinal sound velocity, attenuation constant, and characteristic impedance are derived from comparison of theoretical and measured characteristics.
Subject Classification: [43]30.30; [43]40.55; [43]55.75; [43]85.40.
Experimental underwater measurements of the echo reflected or backscattered from randomly rough model pressure relief surfaces in a laboratory tank are presented as a function of range and relative roughness. Measurements of peak echo pressure amplitude were made at a frequency of 100 kHz at normal incidence; Rayleigh parameters of the surfaces were 0, 1.95, 3.90, and 7.82. The results illustrate the transition from mirror image reflection from a plane surface to spherically spreading backscattering from a surface having irregularities larger than the wavelength, with intermediate range dependence for surfaces having moderate relative roughness.
Acoustical measurements are presented on teaching studios and practice rooms of several music buildings located at four southwestern universities. Data were taken on a wide range of wall construction and recommendations on room response and acoustical isolation between rooms are presented.
Experimental measurements are presented for the target strength and directivity patterns of the reflections from stainless steel spherical shells filled with a mixture of carbon tetrachloride and freon (F113) in the frequency range 30-130 kHz. Five spheres were tested having nominal outside diameters of 6, 7, 10, 12, and 18 in. with varying wall thicknesses. Target strength measurements were made separately on echo components arising from reflections from the front and rear surfaces of the sphere; the focused rear-surface reflections showed target strengths up to 21 dB higher than those corresponding to the front surface, controlled by the sphere wall material and thickness, and by the index of refraction of the fluid. Subject Classification; [43]30.30.
Interest in the low-frequency absorption characteristics obtainable with resonator-type panel absorbers stimulated reverberation chamber measurements on a group of absorber designs covering a considerable range of resonant frequencies, using commercially available panel materials of 2 to 11 percent open area. One series of absorbers investigated consisted of three panels of different perforation covering three different porous materials and backed by a hard wall. Absorption peaks in the frequency range from 500 to 160 cps were found with depth of 2 to 412 inches behind the panels, partially or completely filled with the porous material. A further series of absorber designs consisted of perforated panels covered with cloth of suitable resistance and mounted with air-space backing, after the manner of Ingard and Bolt. With air-space depths of 312 to 1412 inches, absorption peaks in the frequency range from 700 to 85 cps were found. Measurements showing the effect of cloth resistance and of partitioning of the air space into small cells will be presented.
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