Thresholds of Haptic and Auditory Perception in Midair Facial Stimulation

In this study, we evaluated the perceptual threshold of airborne ultrasound focused on a facial skin. The ultrasound was applied with amplitude and lateral modulation, and the tactile and bone-conduction thresholds for them were obtained. The results showed that the tactile stimulation with lateral modulation was perceived stably, while the bone-conduction sound was perceivable only at sweet spots in the experimental setup. Even at the sweet spots, the tactile and bone-conduction thresholds were comparable. A potential application of this study is a non-contact personal warning system, where a focused ultrasound irradiates the face of a pedestrian walking at the edge of a train-station platform, for example. Considering this application, we also conducted experiments on the reaction time for the highest intensity by the used ultrasound device. The results showed that the average reaction time to the facial stimulus was 0.48 s. Though it became longer to 0.74 s when the participants were under a 2-back task, the simulation was noticed in all the cases of the experiment.