Hardness perception by tapping: Effect of dynamic stiffness of objects

Humans can judge the hardness of an object by tapping its surface using a fingertip. The damped natural vibration caused by tapping is a vibrotactile cue for hardness perception. We investigated how dynamic characteristics of an object or vibration influence the hardness perceived by tapping. Using multivariate analyses, the subjectively reported hardness was linked with the dynamic stiffness of an object. Dynamic stiffness, which characterizes the impulsive response of an object, was acquired across 40-1,000 Hz for fourteen types of material cuboid through a hammering test. These blocks were also ranked by seven participants based on their hardness perceived when the participants tapped them with a finger. It was found that the dynamic stiffness did not equally affect the hardness perception across the wide frequency range. Its sensitivity displayed a peak around 200-400 Hz and decreased or disappeared at greater frequency bands in which human perceptual capability is limited.