Investigating the effect of mechanical adaptation on mid-air ultrasound vibrotactile stimuli

Gesture control systems based on mid-air haptics are increasingly used in infotainment systems in cars, where they can reduce drivers' distractions and improve safety. However, studies on vibrotactile adaptation show that exposure to mechanical vibration impairs the perception of subsequent stimuli of the same frequency. Given that moving vehicles generate different types of mechanical noise, it is crucial to investigate whether mid-air ultrasound stimuli are also affected by mechanical adaptation. Here, we directly addressed this question by testing participants' perception of ultrasound stimuli both before and after exposure to different mechanical vibrations. Across two experiments, we systematically manipulated the frequency (Experiment 1) and amplitude (Experiment 2) of the adapting mechanical stimulus and measured participants' detection threshold for different ultrasound test stimuli. We found that low-frequency mechanical vibration significantly impaired perception of low-frequency ultrasound stimuli. In contrast, high-frequency mechanical vibration equally impaired perception of both low- and high-frequency ultrasound stimuli. This effect was mediated by the amplitude of the adapting stimulus, with stronger mechanical vibrations producing a larger increase in participants' detection threshold. These findings show that mid-air ultrasound stimuli are significantly affected by specific sources of mechanical noise, with important implications for their safe use in the automotive industry.