Ultrasonic wave propagation in a composite consisting of thin-walled glass microballoons dispersed in a PMMA matrix in a random homogeneous manner has been studied in the frequency range of 0.5 ≤ n ≤ 5.0 MHz and the volume fraction of inclusions range of 5% ≤ C ≤ 45%. The elastic moduli of the composite, <λ + 2μ> and <μ>, were deduced from the measurements of longitudinal and shear wave velocities. Through a comparison of the ex perimental results with the upper bound on the corresponding stiffness of a PMMA matrix with an equal volume fraction of voids, the additional stiff ness attributable to the presence of the microballoons is deduced. For exam ple, when C = 40%, the additional stiffnesses are roughly 20% of the cor responding matrix stiffness, λ+2μ and μ.
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