Perception of Ultrasound Haptic Focal Point Motion


Ultrasound haptic patterns can be rendered by continuously moving an ultrasonic focal point. It is not known how this focal point motion affects haptic perception. In a paper at the 2021 ACM International Conference on Multimodal Interaction, we describe two psychophysical experiments investigating the perception of an ultrasound haptic focal point moving along a circular path.

Mid-air haptic patterns can be created by rapidly moving ultrasonic focal points, e.g., along a circular path. In this work, we investigated how such motion is perceived.

Our first experiment finds that a sensation of motion is perceived at speeds up to 17 revolutions per second (17 Hz rendering frequency), similar to the so-called ‘flutter’ sensation associated with low frequency vibrations and movements.

Plot showing the mean threshold render frequencies with 95% confidence intervals. The x-axis shows circle diameter, from 4 to 7 centimetres. The y-axis shows focal point render frequency, from 0 to 18 revolutions per second. The plot shows a mean of approximately 17 revolutions per second for all circle sizes.

Our second experiment found a mostly linear relationship between movement speed and perceived intensity up to this speed.

Plot showing mean intensity estimates for both sized circles with 95% confidence intervals. The x-axis shows rendering frequency, from 5 to 19 revolutions per second. The y-axis shows normalised intensity estimates, from 0 to 1. Plot shows that magnitude estimates increase with frequency, and there are higher magnitude estimates for the larger circle.

Haptic circles are widely used in ultrasound haptic interfaces: e.g., for spherical virtual objects or to give feedback about mid-air gestures. Our results can inform the design of ultrasound haptic interfaces, so that designers can create or avoid the sensation of tactile motion. Motion may be desirable for dynamic feedback: e.g., using below the 17 revolutions per second threshold to create moving patterns to indicate changing values or to accompany animated visual icons. Conversely, designers may wish to emphasise the contiguous outline of a virtual shape by rendering significantly above 17 revolutions per second. Since perceived intensity scales with circle size and rendering frequency, our results can also be used to create perceptually similar haptic objects: i.e., balancing size and frequency to yield similar intensity.

    Perception of Ultrasound Haptic Focal Point Motion
    E. Freeman and G. Wilson.
    In Proceedings of 23rd ACM International Conference on Multimodal Interaction – ICMI ’21, accepted to appear. 2021.


This research has received funding from the 🇪🇺 European Union’s Horizon 2020 research and innovation programme under grant agreement #737087. This work was completed as part of the Levitate project.

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