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Recipe for acoustic transparency.

A sheet of aluminum immersed in water normally acts as a good reflector of sound waves. But Pieter S. Dubbelday and Forrest M. Eggleston of the Naval Research Laboratory in Orlando, Fla. have developed an aluminum-based material that is practically transparent to sound. This novel composite, which consists of porous aluminum impregnated with polyurethane, has roughly the same density as water. Moreover, sound waves travel through this material at nearly the speed of sound in water.

Researchers are now studying how different combinations of polyurethane and foamed aluminum -- characterized by various pore sizes -- produce composites with different degrees of transparency. Although unsuitable for use as the skin of a sonar-evading submarine, this "alumer" composite may prove useful as an acoustically transparent backing plate that supports but doesn't interfere with, say, an underwater microphone.
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Title Annotation:aluminum-based material practically transparent to sound
Author:Peterson, Ivars
Publication:Science News
Date:May 11, 1991
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