What in the event you could hearken to music or a podcast without headphones or earbuds and without disturbing anyone around you? Or have a personal conversation in public without other people hearing you?
Newly published research from our team at Penn State introduces a approach to create audible enclaves—localized pockets of sound which might be isolated from their surroundings. In other words, we’ve developed a technology that might create sound exactly where it must be.
The flexibility to send sound that becomes audible only at a particular location could transform entertainment, communication, and spatial audio experiences.
What Is Sound?
Sound is a vibration that travels through air as a wave. These waves are created when an object moves backwards and forwards, compressing and decompressing air molecules.
The frequency of those vibrations is what determines pitch. Low frequencies correspond to deep sounds, like a bass drum; high frequencies correspond to sharp sounds, like a whistle.
Controlling where sound goes is difficult due to a phenomenon called diffraction—the tendency of sound waves to unfolded as they travel. This effect is especially strong for low-frequency sounds due to their longer wavelengths, making it nearly unattainable to maintain sound confined to a particular area.
Certain audio technologies, similar to parametric array loudspeakers, can create focused sound beams aimed in a particular direction. Nonetheless, these technologies still emit sound that’s audible along its entire path because it travels through space.
The Science of Audible Enclaves
We found a brand new approach to send sound to 1 specific listener using self-bending ultrasound beams and an idea called nonlinear acoustics.
Ultrasound refers to sound waves with frequencies above the range of human hearing, or 20 kHz. These waves travel through the air like normal sound waves but are inaudible to people. Because ultrasound can penetrate many materials and interact with objects in unique ways, it’s widely used for medical imaging and lots of industrial applications.
In our work, we used ultrasound as a carrier for audible sound. It will probably transport sound through space silently—becoming audible only when desired. How did we do that?
Normally, sound waves mix linearly, meaning they simply proportionally add up into an even bigger wave. Nonetheless, when sound waves are intense enough, they will interact nonlinearly, generating recent frequencies that weren’t present before.
That is the important thing to our technique: We use two ultrasound beams at different frequencies which might be completely silent on their very own. But once they intersect in space, nonlinear effects cause them to generate a brand new sound wave at an audible frequency that might be heard only in that specific region.
Crucially, we designed ultrasonic beams that may bend on their very own. Normally, sound waves travel in straight lines unless something blocks or reflects them. Nonetheless, by utilizing acoustic metasurfaces—specialized materials that manipulate sound waves—we are able to shape ultrasound beams to bend as they travel. Just like how an optical lens bends light, acoustic metasurfaces change the form of the trail of sound waves. By precisely controlling the phase of the ultrasound waves, we create curved sound paths that may navigate around obstacles and meet at a particular goal location.
The important thing phenomenon at play known as difference frequency generation. When two ultrasonic beams of barely different frequencies overlap—similar to 40 kHz and 39.5 kHz—they create a brand new sound wave on the difference between their frequencies—on this case 0.5 kHz, or 500 Hz, which is well throughout the human hearing range. Sound may be heard only where the beams cross. Outside of that intersection, the ultrasound waves remain silent.
This implies you possibly can deliver audio to a particular location or person without disturbing other people because the sound travels.
Advancing Sound Control
The flexibility to create audio enclaves has many potential applications.
Audio enclaves could enable personalized audio in public spaces. For instance, museums could provide different audio guides to visitors without headphones, and libraries could allow students to review with audio lessons without disturbing others.
In a automotive, passengers could hearken to music without distracting the driving force as they hearken to navigation instructions. Offices and military settings could also profit from localized speech zones for confidential conversations. Audio enclaves is also adapted to cancel out noise in designated areas, creating quiet zones to enhance focus in workplaces or reduce noise pollution in cities.
This isn’t something that’s going to be on the shelf within the immediate future. Challenges remain for our technology. Nonlinear distortion can affect sound quality. And power efficiency is one other issue—converting ultrasound to audible sound requires high-intensity fields that may be energy intensive to generate.
Despite these hurdles, audio enclaves present a fundamental shift in sound control. By redefining how sound interacts with space, we open up recent possibilities for immersive, efficient, and personalized audio experiences.
