In our former work it has been shown that contact sounds of different materials influence the haptic perception of stiffness and surfaces with metal sound gave people the impression of a harder surface than drum sound. Because sound parameters (i.e. frequency and damping parameters) of various materials are quite different, it is not clear whether the higher level knowledge of material or the low level sound parameters have stronger effects. Here we modified the two original sounds of metal and drum by adjusting their mode frequency centroids to be the same while keeping the internal friction coefficient (i.e. the multiply of frequency and damping parameter) constant. Thus in the interview all participants still recognized the modified sounds as metal and drum and subjectively evaluated surfaces with metal sounds were stiffer than those with drum sounds. However in the task of adjusting stiffness of surfaces to match the memorized stiffness of standard surfaces with different sounds, statistical analysis showed interaction effect of material and parameter factors. Although the perceived stiffness of surfaces with original metal and drum sounds were significantly different, surface with modified metal sound was perceived no stiffer than that with modified drum sound. It suggests that sound parameters may influence the haptic-auditory encoding of stiffness stronger than the knowledge of materials.

haptic-auditory integration; stiffness perception; material; contact sound