ACOUSTIC PROPERTIES OF RIGID CLOSED-CELL FOAMS

Acoustic properties of typical rigid closed-cell foams have been studied experimentally as functions of skeleton materials and cells. Longitudinal sound speed perpendicular to the rise direction for PAPI polyurethane foam can be related to foam density by an equation C₁=ap^b. Polyurethane foams present acoustic orthotropy as a-result of elongated cells in the rise direction. For an air-backed foam layer in a water-filled pulse tube, phase angles of complex reflection coefficients are always near 180 as cell sizes are about 0.5 mm. Frequency and temperature dependences of sound speed, as well as attenuation at audiofrequencies in foams, are governed primarily by properties of skeleton materials.