The development of haptic systems for telepresence, teleoperation, and virtual reality has long focused on creating devices that allow users to interact with virtual three-dimensional (3D) items or environments that float in the air without any physical objects. By focusing the ultrasonic output from phased arrays of transducers, emerging 3D holographic haptic displays can provide such tactile feedback in the air. The skin is deformed by non-linear acoustic radiation pressures caused by incident ultrasound.

However, current holographic haptic systems provide tactile sensations that are diffuse and weak, with apparent spatial resolutions much coarser than would theoretically be predicted from acoustic focusing.

A UC Santa Barbara research team has discovered a new phenomenon leading to upcoming holographic haptics and could lead to more compelling virtual reality experiences.

The formation of shock waves creates a trailing wake pattern.

Yon Visell is an associate professor in UCSB’s Faculty of Engineering whose research focuses on interactive technologies, particularly haptics, robotics and electronics, said: “Our new study explains why such holograms feel much more diffuse or unclear than expected.”

The study used high-resolution optical imaging, modeling and perceptual testing to examine excited waves stimulated in the skin during haptic holography. They noted that holographic displays cause extensive vibrational patterns in the skin known as shear shock waves.

According to Visell, shock waves are generated in haptic holography when ultrasonic waves in the air are focused and scanned, creating vibrations in the skin. These vibrations can interfere with each other so that their strength is amplified at some points, a process known as constructive interference.

Creating shock waves produces a trailing wake pattern that extends beyond the intended focus point, reducing the spatial accuracy and clarity of tactile impressions. According to the researchers, if the concentrated sound beam represents a fast-moving boat on the water, the shock wave pattern represents a wake behind the ship. Modern holographic haptic displays generate shock wave patterns in the skin that are so scattered that the sensations appear very diffuse.

Visell said, “Our research reveals how holographic haptic displays, which are a promising new technology for virtual reality and telepresence, require new knowledge in terms of acoustic innovations in design. By understanding the underlying physics of ultrasound-generated shock waves in the skin, we hope to improve the design of haptic holographic displays and make them more realistic and immersive for users. Such haptics could allow us to augment our physical environment with a limitless variety of virtual objects, interactive animated characters or graspable tools that can not only be seen, but also touched and felt with the hands.”

The discovery of shock wave phenomena supports haptic holography. It can also lead to haptic holographic renderings, allowing users to interact more realistically in the future metaverse.

Magazine reference:

  1. Gregory Reardon, Yon Visell et al. Shear shock waves mediate haptic holography via focused ultrasound. Scientific progress. DOI:10.1126/sciadv.adf2037