Towards Intensifying Perceived Pressure in Midair Haptics: Comparing Perceived Pressure Intensity and Skin Displacement Between LM and AM Stimuli

Ultrasound Midair Haptics (UMH) can present various noncontact tactile patterns by focusing ultrasound on human skin. With UMH, a steady pressure sensation can be presented by periodically shifting a stimulus point (ultrasound focus) at several hertz. Such stimulus with a periodic focal shift is called Lateral Modulation (LM). The perceived intensity of this pressure sensation was several times stronger than the applied radiation force (e.g., 0.22 N for 27 mN of radiation force). Further intensifying the pressure sensation by LM expands the range of reproducible tactile sensations such as a hard object; however, a stimulus design guideline for the intensification has not been established because the perception mechanism of the LM-evoked pressure sensation is still unclear. Towards intensifying the pressure sensations in UMH, this study investigates the effects of the main frequency components of skin vibrations produced by LM and that of the amplitude on the perceived pressure intensity. We first confirmed that the perceived pressure intensity of LM 5 Hz was stronger than that of 5 Hz amplitude modulation (AM). AM is a simple vibration with a fixed stimulus position. We also measured the 5 Hz vibration amplitude of the skin during stimulation and confirmed no significant difference in the amplitude between LM and AM. The results showed that a 5 Hz skin vibration and the amplitude alone cannot explain the perceived intensity of the pressure sensation by LM. These results indicate that other factors in LM such as focal shifts would be necessary to present stronger pressure sensations.