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Ultrasonic haptic (UH) feedback employs mid-air ultrasound waves detectable by the palm of the hand. This interface demonstrates a novel opportunity to utilize non-visual input and output (I/O) functionalities in interactive applications, such as vehicle controls that allow the user to keep their eyes on the road. However, more work is needed to evaluate the useability of such an interface. In this study, 16 blindfolded participants completed tasks involving finding and counting UH buttons, associating buttons with audio cues, learning spatial arrangements, and determining button states. Results showed that users were generally successful with 2–4 arranged buttons and could associate them with audio cues with an average accuracy of 77.1%. Participants were also able to comprehend button spatial arrangements with 77.8% accuracy and engage in reconstruction tasks to prove user understanding. These results signify the capability of UH feedback to have real-world I/O functionality and serve to guide future exploration in this area. 

Mid-air ultrasonic feedback is a new form of haptic stimulation supporting mid-air, touch-free user interfaces. Functional implementation of ultrasonic haptic (UH) interfaces depend upon the ability to accurately distinguish between the intensity, shape, orientation, and movement of a signal. This user study (N = 15) investigates the ability to non-visually perceive two ultrasonic lines with varying lengths (3, 5, and 7 cm) and orientations (vertical and horizontal) using the palm of the hand. Key results showed that: (1) the orientation of the lines had no effect on a user’s accuracy when determining their relative lengths, (2) line length distinction significantly improved when the length difference was at least 4 cm, and (3) a clear learning curve was evident when evaluating a new user’s ability to perceive ultrasonic signals. The capabilities of UH technology identified and discussed within this study will help engineer user-friendly and functional mid-air haptic interfaces for future applications.