In this article, polypropylene (PP)/clay/carbon nanotube (CNT) composites were prepared via a solution blending method. Sound transmission loss (STL), determined with an impedance tube, was used to characterize their soundproofing properties. The STL for the PP/4.8 wt % clay/0.5 wt % CNT composite was about 15-21 dB higher than that for pure PP at high frequencies (3200-6400 Hz) and about 8-14 dB higher at low frequencies (580-620 Hz). X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to study the crystallinity and the microstructure. A synergistic effect on the STL was established between the structure of the homogeneous dispersion and strong interfacial adhesion. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013