A new advance in audio technology could soon let people hear music or talk without wearing headphones in public. Taking ideas from science fiction, such as the cone of silence in Dune and the whispering billboards in Blade Runner 2049, scientists are working on ways to direct sound only to a single person.
Sound manipulation has been around for some time. Strange features in buildings, such as those in the U.S. Capitol’s Statuary Hall, can cause whispers to travel far. However, it is hard to control sound intentionally in the human hearing range (20 to 20,000 hertz) since these frequencies spread out quickly.
Experiments using lasers to change light into sound through water vapor have been done, but they did not allow for precise localization. Ultrasonic waves are more promising because they are silent by themselves but can create sound when they interact with each other in a nonlinear way, as happens when water hits hot oil.
Although directional speakers, both military and commercial, take advantage of this effect, their sound can still be heard along their path. Audio privacy was still not possible.
Recently, researchers at Penn State introduced a new idea called “audible enclaves.” They used 3D-printed acoustic metasurfaces, which have tiny zigzag air channels, to direct ultrasonic waves into curved paths. When two waves meet, they create sound only at that particular location.
“It’s as if you have an invisible headset on,” explains lead researcher Yun Jing. If you stand in the right place, you can hear the music or speech, but if you move just a foot away, you won’t hear anything. Though the current setup uses inexpensive parts and has low audio fidelity, it’s a working proof of concept that could revolutionize how we listen—privately, wirelessly, and without anything in or on our ears.
