The effect of the SiC reinforcements on the attenuation coefficient of SiCp/LDPE composite materials measured by pulse echo method have been investigated at various volume fractions and sizes of the SiC reinforcements. The SiCp/LDPE composite materials were fabricated by the melt injection process at 170℃ and 25MPa. The SiCp/LDPE composite materials showed full density up to 40vol% SiCp reinforcement materials, however the density decreased above 50 vol% SiCp reinforcement. The sound velocities and the attenuation coefficients of LDPE and the fabricated SiCp/LDPE composites were measured by pulse echo method at 0.5M, 1.0M, 2.25MHz. The sound velocities of LDPE and 10, 20vol% SiCp/LDPE composite did not change with increasing the frequency, but their attenuation coefficients increased rapidly with increasing the frequency. The attenuation coefficients of SiCp/LDPE composite showed the maximum value of 17 dB/cm at 30 vol% SiCp reinforcement materials. The attenuation coefficient decreased with increasing of the SiC reinforcement size from 8㎛, 24㎛ to 48㎛ in frequency range from 0.5M to 1.0MHz. The attenuation coefficient is proportional to $R^nF^m$, where R is the SiCp reinforcement size and F is the frequency. The exponent related with the dependence of SiCp reinforcement size on attenuation coefficient, n is measured -0.1, while the exponent related with the dependence of frequency on attenuation coefficient, m is measured 0.7. This result shows that the acoustic attenuation mechanism of the SiCp/LDPE composite materials is different from the attenuation mechanism due to the grain-boundary in metallic alloys. The measured loss factor is divided into two categories. One is the loss factor occurred by the polymer matrix and the other is the loss factor occurred by the SiC particulate reinforcements. The loss factor by the reinforcements is multiplied by the insertion loss from the reinforcements to the matrix, and showed the same value at the same frequency. This result shows that the acoustic attenuation mechanism by the reinforcements is strongly related to the insertion loss from the reinforcements to matrix.
