Multi-point STM: Exploring the effect of drawing speed and the number of focal points on perceived intensity, valence and arousal in ultrasonic mid-air haptics

Spatiotemporal modulation (STM) is used to render tactile patterns with ultrasound arrays. Previous research only explored the effects of single-point STM parameters, such as drawing speed ($V_d$). Here we explore the effects of multi-point STM on both perceptual (intensity) and emotional (valence/arousal) responses. This introduces a new control parameter for STM - the number of focal points ($N_{fp}$) – on top of conventional STM parameter ($V_d$). Our results from a study with 30 participants showed a negative effect of $N_{fp}$ on perceived intensity and arousal, but no significant effects on valence. We also found the effects of $V_d$ still aligned with prior results for single-point, even when different $N_{fp}$ were used, suggesting that effects observed from single-point also apply to multi-point STM. We finally derive recommendations, such as using single-point STM to produce stimuli with higher intensity and/or arousal, or using multi-point STM for milder and more relaxing (less arousing) experiences.