Low-frequency echo-reduction and insertion-loss measurements from small passive-material samples under ocean environmental temperatures and hydrostatic pressures

System L is a horizontal tube designed for acoustical testing of underwater materials and devices, and is part of the Low Frequency Facility of the Na val Undersea Warfare Center in Newport, Rhode Island. The tube contains a f ill fluid that is composed of a propylene glycol/water mixture. This system is capable of achieving test temperatures in the range of -3 to 40 deg Cen tigrade, and hydrostatic test pressures in the range 40 to 68 950 kPa. A un idirectional traveling wave can be established within the tube over frequen cies of 100 to 1750 Hz. Described here is a technique for measuring the (no rmal-incidence) echo reduction and insertion loss of small passive-material samples that approximately fill the tube diameter of 38 cm. (Presented als o is a waveguide model that corrects the measurements when the sample fills the tube diameter incompletely.) The validity of the system L measurements was established by comparison with measurements acquired in a large acoust ic pressure-test vessel using a relatively large panel of a candidate mater ial, a subsample of which was subsequently evaluated in system L. The first step in effecting the comparison was to least-squares fit the data acquire d from the large panel to a causal material model. The material model was u sed to extrapolate the panel measurements into the frequency range of syste m L. The extrapolations show good agreement with the direct measurements ac quired in system L.