Acoustic and mechanical properties of nitrile butadiene rubbers (NBR) with the variation of the carbon black contents were investigated. All the composition contents except carbon black were kept constant in the specimens. Acoustic measurements were performed in the frequency range of 300-800 kHz. Increase of the carbon black contents in the rubber resulted in an increase of the mechanical properties such as density, hardness, tensile strength, and glass transition temperature. The sound speed and attenuation in the specimen were mainly affected by the carbon black. The sound speed increased with the carbon black content and depended on the square root of the ratio of hardness to density. However, the sound attenuation was not affected by the carbon black content variation but by the existence of the carbon black. The results with carbon black content variation were also compared with those of the sulfur content variation having a fixed carbon black content (K. I. Jung et al., J Appl Polym Sci 85, 2764-2771, 2002). These results show that the variation of acoustic properties can be used to nondestructively evaluate mechanical properties and to estimate the carbon black content variation in NBR. (C) 2004 Wiley Periodicals, Inc.