Ultrasonic waves are normally inaudible to the human ear. One professor at Ritsumeikan is researching a technology that superimposes ultrasonic waves to create sound that can only be heard in a very small area. The name of this technology is pinpoint audio. I talked to this professor to get a first-hand look at this technology, which is expected to be applied to personal experiences in the post-COVID era.

To learn about this exciting technology, I visited the laboratory of Professor Yoshinobu Nishiura of the College of Information Science and Engineering at Ritsumeikan University. The rows of audio and video equipment made the lab look like the command center in an old superhero movie.

I was curious about the technical mechanisms of pinpoint audio but decided to listen to the sound first. I stood where the professor told me to in front of a wall where a large display was hanging.
Almost instantly, a rhythmic, high-frequency sound began to ring in my eardrums. Until then, the room had been silent, but in that one spot, I could hear the sound clearly.

If I were to exaggerate a bit, I would say it was a magical experience. Imagine hearing a sound in a certain place amid a sea of silence. If you were to pass by this spot without knowing anything about it, you might think you were having some kind of mystic experience.

Inaudible ultrasonic waves overlap to generate sound in a target area

How is it possible to generate sound in one specific spot? We asked Professor Nishiura to explain.

"There is a tool called a tuning fork, which is used for tuning musical instruments. For example, when you strike tuning forks A and B, which have different frequencies (tones) at the same time, a humming sound, or C, with a completely different frequency (tone) is generated. Pinpoint audio is a technology that applies this phenomenon to ultrasonic waves.
We have developed an algorithm that converts the sound we want to play into three ultrasonic waves. The decomposed sounds flow from three ultrasonic speakers, but because they are ultrasonic, they are not audible to the human ear.
Only at the exact spot where the three ultrasonic waves overlap and produce a humming sound can the sound be generated,” explained Professor Nishiura.

Actually, an elliptical sweet spot about the size of a small rugby ball is created in the target space. When I visited, the sweet spot was about 2cm in diameter, but it is possible to make the audible spot larger.

Confining sounds to bring silence to a world full of sound

With pinpoint audio, you can generate sounds that can only be heard in certain places. So, what are the practical applications for this technology?

"The audio guide in a museum would be a good example. With pinpoint audio, you could listen to explanations of artworks without headphones while maintaining the silence of the museum as a whole.
Also, if we can generate sound only in front of a computer screen, we can have online meetings without headphones while keeping a more natural posture, all without worrying about the sound leaking into the surrounding area,” explains Professor Nishiura.

How can we reduce excess sound and leave only what is necessary? The pinpoint audio technology, which can "confine" sound, definitely has the potential to help address this issue of excess sound.