A Survey of Mid-Air Ultrasonic Tactile Feedback Ismo Rakkolainen*, Antti Sand, Roope Raisamo Information Technology and Communication Sciences Tampere University Tampere, Finland *e-mail: ismo.rakkolainen@tuni.fi Abstract— This paper contains a succinct survey on the recent major advances in contactless ultrasonic tactile feedback. It is a haptic technology which enables easy mid-air interactions with rich multisensory feedback and creates effects which are other- wise impossible. It can become a disruptive technology for multi- media interfaces, applications, and mobile computing. We sum- marize and discuss the advantages, problems and applications of the technology. This survey provides an introduction to the topic for anyone interested in applying or researching it. Keywords: haptics; ultrasonic tactile feedback; mid-air haptics I. INTRODUCTION Effective feedback helps users to get information, notifica- tions and warnings. Visual or auditory feedback is often used in multimedia systems, but also mid-air ultrasound-based tactile feedback has become possible for human-computer interaction (HCI) in recent years. It has raised a lot of interest in research and commercial applications and it is employed for feedback for contactless multimedia interfaces and buttons, virtual reality (VR), touch-free interaction with cars, appliances, etc. Ultrasound tactile actuation has two core advantages over the normal vibrotactile actuation in HCI. Firstly, it enables tac- tile mid-air sensations on the hand, without needing to give any visual attention to a control, or touch any device. Secondly, it can map closely to other modalities or gestures to enhance hu- man-system interaction. Ultrasound haptics maintains the free- dom of movement, is unobtrusive, and can feel magical to the user. There are many surveys on haptics in general (e.g., [7], [58], [4]), but they all only shortly mention ultrasonic mid-air haptics. There are only two earlier surveys on the ultrasonic phased array haptics that we are aware of. One survey [14] discusses mostly experiments in MHz range for the purposes of physiology and medicine, and another short HCI-focused sur- vey [2] is outdated. The technology has advanced a lot since these surveys were made. The leading company in the field, Ultrahaptics Ltd., has a knowledge base on the topic [53]. Our contribution is an up-to-date survey on mid-air ultra- sound haptics. We give an overview of ultrasound haptics in Section 2 and interaction with it in Section 3. We discuss the limitations and some major advances in Section 4. We identify some key applications and use cases in Section 5. Finally, con- clusions are given and future directions are discussed. II. HAPTIC TECHNOLOGIES A. Haptic Devices Haptics is an integral part of our lives and activities. Hu- mans use the sense of touch to grasp, explore, walk and mani- pulate in the real world. The sense of touch is delicately and marvelously built, it is a very complex system, and it pervades the whole body. It comprises of cutaneous inputs from the vari- ous types of mechanoreceptors in the skin and kinesthetic in- puts from the muscles, tendons and joints. It provides updated information, e.g., on 3D shape and texture of objects, the posi- tion of the limbs, balance and the muscle stretch [4]. Mechano- receptors have various densities in various body parts. The sense of touch associates a certain tactile stimulation with pres- sure, vibration, pain, temperature or pleasure. Haptic display is an interface for communication between human and computer. The sense of touch must be artificially recreated, e.g., in interactive computing, virtual worlds and robot teleoperation. Mechanoreceptors in the human body are stimulated to produce expedient sensations of touch. This can enhance realism and human performance. Usually tactile feed- back is provided in direct contact to skin, which seems intuitive for the sensation of touch. Some haptic devices can provide also force feedback. Many technologies can be used, e.g., tac- tile gloves, exoskeletons or proxy devices. The fidelity of current tactile display technologies is very rudimentary compared to audiovisual displays or to the capabi- lities and complexity of human tactile sensing [4]. The short- comings of tactile display technologies amount to several or- ders of magnitude. Many shortcuts and approximations must be used in order to mass-produce haptic displays for general use. Contactless haptic feedback is possible with pressurized air jets or air vortex rings, which are simple but rough feedback methods with some inherent lag. Laser-induced thermoelastic effect or electromagnetic-based haptic interface are also pos- sible, but they require a wearable prop on finger or on hand. Focused ultrasonic acoustic air pressure is one contactless feedback method. Focused ultrasound for HCI tactile sensa- tions employs typically 40 or 70 kHz frequencies, which is just inaudible sound. Here we discuss only previous work, which is relevant for these frequencies or for the ultrasound haptics for HCI. Funded in part by project MIVI, Business Finland #8004/31/2018. This is the accepted manuscript of the article, which has been published in 2019 IEEE International Symposium on Multimedia (ISM), 94-98. http://dx.doi.org/10.1109/ISM46123.2019.00022