This crate provides HoloGain.
HoloGain produces multiple focal points.
S. Inoue et al, "Active Touch Perception Produced by Airborne Ultrasonic Haptic Hologram," Proc. 2015 IEEE World Haptics Conference, pp.362-367, Northwestern University, Evanston, II, USA, June 22–26, 2015.
Shun Suzuki, 2020