In this paper, the effects of soundproofing by polymer and carbon-nanotube (CNT) composites were investigated. The specimens for sound insulation measurement were fabricated with Acrylonitrile Butadiene Styrene (ABS)/CNT composites. Tests showed that sound transmission loss of ABS/CNT 15 vol.% composite was higher by 21.7% (4.1 dB) than that of pure ABS specimen at a frequency of 3400 Hz. It was found that the principal factor influencing the improvement of sound insulations of ABS/CNT composites was increased stiffness by CNT additives. To demonstrate the practical applicability of polymer/CNT composites, tests were conducted for the reduction of operational noise from mechanical relay.