Plucked String Stiffness Affects Loudness Perception

A great variety of interactions between senses, and between motor production and senses, have been reported in previous research. In the present study, we tested whether the mechanics of a plucked string affected how the sound it produced was perceived. To test this hypothesis, we simulated the feel of a plucked string using a high fidelity haptic force-feedback device and simultaneously simulated its acoustic emission. This way, we could independently manipulate the two sensory inputs -- how it felt and how it sounded -- together with physical correct haptic interaction and with accurate synchronization. This arrangement makes it very plausible that the two sensory inputs came from a common source. We used a two-interval forced-choice discrimination procedure to determine the point of subjective equality of the loudness between a stiff and a soft plucked string. When the stiffness of the string was low, the sound was perceived to be softer. Interestingly, this effect was found only when the first string was less stiff than the second string plucked during a comparison. The results are consistent with the inverse effectiveness principle of multisensory integration.