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Synchronization of audio-tactile stimuli represents a key feature of multisensory interactions. However, information on stimuli synchronization remains scarce, especially with virtual buttons. This work used a click sensation produced with traveling waves and auditory stimulus (a bip-like sound) related to a virtual click for a psychological experiment. Participants accomplish a click gesture and judge if the two stimuli were synchronous or asynchronous. Delay injection was performed on the audio (haptic first) or the click (audio first). In both sessions, one stimulus follows the other with a delay ranging from 0−700ms . We use weighted and transformed 3-up/1-down staircase procedures to estimate people's sensitivity. We found a threshold of 179ms and 451ms for the auditory first and haptic first conditions, respectively. Statistical analysis revealed a significant effect between the two stimuli' order for threshold. Participants' acceptable asynchrony decreased when the delay was on the haptic rather than on the audio. This effect could be due to the natural experience in which the stimuli tend to be first tactile and then sonorous rather than the other way around. Our findings will help designers to create multimodal virtual buttons by managing audio-tactile temporal synchronization.

Ultrasonic travelling waves possess the capability to induce net shear forces on the finger pulp, which finds utility in various applications. This study specifically investigates their potential in generating vibrotactile cues through force modulation. Two experiments are outlined herein. In the first, we assess the detection threshold, estimated at 1.5 μm peak-peak for a modulation frequency of 75 Hz. Remarkably, this threshold closely aligns with that observed in conventional vibrotactile indentation and remains consistent regardless of the force modulation method employed. In the second experiment, we correlate the amplitude of a travelling wave tactile display with the vibration amplitude of a shaker to achieve equivalent vibration intensity. Our findings reveal a substantial amplitude ratio range experienced on the travelling wave device, spanning from 24 to 155, which is pivotal in optimizing vibrotactile stimulator design. Additionally, we demonstrate that vibrations below 1 μm yield negligible net lateral forces on the finger pulp. These results underscore the potential of ultrasonic travelling waves in enhancing tactile feedback systems.