If you enter a room to see someone dancing their heart out in silence, no headphones in sight, don't be alarmed. They may be jamming to music via a new 3D-printed speaker cover that can focus sound across a room to a focal point just above an inch.
Researchers from Pennsylvania State University have created a 3D-printed speaker cover that can focus sound waves to an astonishingly small point. The device works like a convex magnifying glass lens that focuses light into a narrow, concentrated beam.
Sound waves typically spread outward from their source, expanding as they travel. This characteristic means that the waves can reach far beyond their intended target, a sure recipe for clashes between neighbors. Beyond noise pollution, there are several scenarios in which discrete, precise sound transmission without personal audio equipment is necessary.
Technologies for this purpose already exist. Parametric array loudspeakers (PALs) use high-intensity ultrasonic waves (inaudible to humans) to focus audible sound into a narrow, laser-like beam. For example, companies such as Soundlazer and Akoustic Arts have demonstrated headphone-free listening using parametric array loudspeaker technology for more than a decade.
Now, while these speakers are deployed in environments such as museums and broadcast rooms, their use outside these preplanned, specialized spaces is challenging. The physics behind the technology also impacts the sound’s output quality.
“These arrays are so directional that once the sound beam comes in contact with a surface, the sound can reflect all around the room, compromising privacy,” explains Jee Woo Kevin Kim, acoustics doctoral candidate and first author of an IEEE Transactions on Ultrasonics paper on the study. “Additionally, they struggle to produce low-end frequencies, which can take away from the experience of listening to bass-heavy music, for example.”
To address these issues, the researchers turned to metasurfaces, a class of materials that can manipulate sound, light, heat, and other waves using only their thin structures. Acoustic metasurfaces already feature in sound technology.
Rather than replacing PALs, Penn State's approach is designed to enhance them. The researchers say their 3D-printed acoustic metasurface can be fitted to PAL-based speakers to create a smaller, more tightly confined listening zone while also improving bass performance, a longstanding limitation of many directional audio systems.
“To develop an acoustic metasurface, we use a large surface that works like a lens focusing a beam of light,” says Yun Jing, acoustics professor and the paper's corresponding author. “The surface modulates sound waves in such a way that they converge at a central point after leaving the speaker, allowing us to focus the audio into a precise area.”
To validate the design, the researchers first simulated and experimentally verified the acoustic metasurface's performance before manufacturing it with a 3D printer. The circular lens was then attached to a series of PALs arranged in an array, creating a focal point approximately 4 inches (102 mm) from the speaker. The team played bass-heavy electronic music while slowly moving a microphone through and around the focal region to evaluate both sound quality and confinement.
The results were striking. When positioned inside the very tight focal point, the microphone captured clear, high-quality audio. However, moving the microphone just 2 inches (51 mm) away from the center of the sound spot reduced the volume by up to 50 decibels, making the audio nearly inaudible. In practical terms, a person standing inside the sound bubble could listen to music or receive audio instructions, while someone only a short distance away would hear little to nothing.
The metasurface also appeared to solve one of the biggest shortcomings of conventional PAL systems: low-frequency reproduction. Tests showed that the system could effectively project frequencies as low as 38 Hz, approaching the deepest range of human hearing. Producing such low frequencies typically requires much larger speaker systems or dedicated subwoofers, making the result particularly notable for compact directional audio applications.
The researchers believe the technology could find applications anywhere private, localized audio is desirable. Potential use cases include ATMs, ticketing kiosks, retail displays, museums, and automobiles, where multiple occupants could listen to different audio streams without headphones or interfering with one another. Beyond consumer applications, the team suggests the approach could enable more discreet public information systems and improve privacy in shared environments where conventional speakers would broadcast sound to everyone nearby.
Before anyone starts eyeing their Bluetooth speaker and a 3D printer, it is worth noting that the technology is not a universal attachment that can transform any speaker into a privacy-focused audio system. The metasurface was specifically designed to work with parametric array loudspeakers (PALs), a specialized type of speaker that projects highly directional ultrasonic waves.
The 3D-printed lens then manipulates and focuses those already-directional waves into a tiny listening zone. Conventional speakers, including the vast majority of consumer audio products, radiate sound outward in all directions and do not generate the ultrasonic beam that the metasurface relies on. In other words, the lens is refining and concentrating the sound that the PAL has already shaped into a narrow beam, not creating directional audio on its own.
Source: Pennsylvania State University
