Nippon Telegraph and Telephone Corporation (NTT) has developed a new technology that creates strong and diverse tactile sensations in mid-air by adding vibrations at specific frequencies to the strong force sensations generated by concentrating ultrasound on the skin. By advancing this new technology, it will become possible to experience realistic tactile sensations in XR (Cross Reality) environments without wearing any devices. Part of this achievement is the result of joint research with the University of Tokyo.
When it comes to methods for realizing technology that conveys the sensation of touching objects to people without wearing devices, development is progressing in technology that transmits tactile sensations to ultrasound focal points by focusing (concentrating) ultrasound on a narrow area of the skin without directly touching an object.
However, conventional methods had two major challenges: the force perceived by ultrasound was weak, and only dull tactile sensations could be presented. NTT has now tackled these issues and developed an ultrasonic presentation technology that creates diverse tactile sensations in a non-contact manner.
For the issue of weakness of force, the company investigated stimulation conditions to enhance the force sensation generated by ultrasound and discovered that "5 Hz rotation of the ultrasound focal point" is the decisive factor in methods that make people feel stronger force. For the dullness issue, they utilized the factors that generate strong force clarified in the aforementioned issue to develop an ultrasonic tactile synthesizer that creates diverse tactile sensations.
People normally feel diverse tactile sensations by integrating the tactile stimuli of various frequencies applied to the skin. NTT focused on such tactile perception mechanisms and has now developed technology that creates diverse tactile sensations in a non-contact manner by freely synthesizing ultrasonic stimuli in multiple frequency bands.
Generally, when people touch an ultrasound focal point, they feel a weak force of about 0.01 N (Newtons) at most. When this focal point is rotated at 5 Hz on the skin, the perceived force is enhanced approximately 20 times.
This rotational stimulation includes various elements such as skin vibration with the same period as the rotation and expansion of the stimulation range. However, conventional research had not identified the decisive factor among the rotational stimuli of the ultrasound focal point that strengthens the perceived force.
In other words, it had not been clarified whether the decisive factor for enhancing the force felt when rotating the ultrasound focal point on the skin was "5 Hz rotation of the stimulation point" or "5 Hz skin vibration."
Therefore, to investigate the factors that strengthen the perceived force, the researchers generated stimulation that rotated the position of a constant-intensity ultrasound focal point at 5 Hz on the skin, and vibration stimulation that changed the ultrasound intensity at 5 Hz between zero and maximum values and compared the strength of force felt by each. As a result, 5 Hz rotational stimulation was shown to be six times stronger than 5 Hz vibration stimulation, revealing that the main factor strengthening the force felt by ultrasound is "5 Hz rotation."
On the other hand, people perceive the tactile sensations of various objects by receiving stimuli containing various frequencies on their skin. NTT has now selected 5 Hz, 30 Hz, and 200 Hz, which are frequencies to which tactile receptors react sensitively, and developed a new ultrasonic tactile synthesizer that creates diverse tactile sensations by freely synthesizing ultrasonic stimuli in these frequencies.
In this synthesizer, first the force presented by a focal point rotating at 5Hz, a stimulus that generates strong force, is modulated at 30 Hz and 200 Hz to vibrate the ultrasound focal point on the skin, then the amplitude of the presented force modulation is adjusted. This determines the strength of the vibration finally presented, so the mechanism synthesizes ultrasonic stimuli in three frequency bands and presents them to the user's skin.
Using this new technology enables the presentation of diverse sensations to the user. It reproduces sensations of touching an object by presenting force through the 5 Hz rotation of a focal point, as well as reproducing the vibrations applied to the finger when the surface of an object is stroked. Through this, it is possible to freely regulate the strength and roughness of sensations, making them smooth, silky, or rough.
This research achievement was presented at an international conference in the haptics field-the "Eurohaptics conference 2024" and was nominated for the Best Paper Award and other honors. It was also exhibited and introduced at the NTT Communication Science Laboratories Open House 2025 held from May 20.
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.