Fig 1. Experimental apparatus and procedure.

(a) The experimental setup consisted of a CRT display showing visual stimuli, which were viewed through a mirror, and a force-feedback device, which was concealed beneath the mirror with its reference point calibrated at the center of the reflected display surface. (b) The contact sound recorded in an anechoic chamber was used to extract a set of modal frequencies for synthesizing the impact sound in correspondence with participants’ tapping actions. (c) The procedure of Experiment 1. When participants held the stylus and struck the standard surfaces (a light gray square), they could hear different contact sounds via headphones. Their left hands manipulated the keyboard to adjust the stiffness of a silent test surface while their right hands were holding the stylus and tapping the surface.