A team of researchers at UC Santa Barbara, led by Yon Visell, an associate professor specializing in interactive technologies, and Gregory Reardon, a doctoral student researcher, has made a groundbreaking discovery that sheds light on the limitations of holographic haptic displays. Their findings, published in the journal Science Advances, reveal a new phenomenon that explains why tactile sensations produced by these displays are diffuse and weak, often described as a "breeze" or "puff of air."
Holographic haptic displays use phased arrays of ultrasonic emitters to concentrate ultrasound in the air, allowing users to touch, feel, and control three-dimensional virtual objects in mid-air without the need for a physical device or interface. While these displays have tremendous potential for use in various applications, including augmented reality, virtual reality, and telepresence, their tactile sensations have been found to be less distinct than expected.
The researchers employed laser vibration measurement, simulations, and perceptual studies to thoroughly investigate ultrasound-excited waves that occur in the skin during haptic holography. They discovered that holographic displays cause extensive skin vibration patterns known as shear shock waves. These waves are formed when ultrasonic waves are focused and scanned in mid-air, creating vibrations in the skin that can interfere with one another, increasing their strength in some regions through constructive interference.
Visell explains that the creation of shock waves causes a trailing wake pattern to extend beyond the intended focal point, diminishing the spatial accuracy and clarity of tactile perceptions. He compares the concentrated sound beam to a fast-moving boat on the water, with the shock wave pattern being the wake trailing the boat. Current holographic haptic displays produce shock wave patterns so dispersed in the skin that the sensations feel diffuse.
To visualize the vibrations caused by focused ultrasound on the skin, the researchers used a PSV Polytec Scanning Vibrometer to image the vibrations of human hand surfaces during holographic haptic feedback. The findings confirmed the strong presence of ultrasound-evoked shear shock wave patterns in the skin.
Visell emphasizes that their study reveals how holographic haptic displays require new knowledge in acoustics and innovations in design to improve their realism and immersiveness. By understanding the underlying physics of ultrasound-generated shear shock waves in the skin, the team hopes to enhance the design of haptic holographic displays, making them more engaging for users.
The researchers anticipate that improved haptic displays could enable users to augment their physical surroundings with a limitless variety of virtual objects, interactive animated characters, or graspable tools that can be not only seen but also touched and felt with the hands.
The team's discovery of the previously unknown shock wave phenomena that underpin haptic holography is a significant step forward in developing haptic holographic displays that will allow users to interact more realistically and immersively in the future metaverse. As research in this field continues, the findings of Visell and his team will undoubtedly contribute to the advancement of virtual reality experiences, making them more engaging and lifelike for users.
