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A team of researchers in South Korea has developed a compact directional speaker that can send speech and music toward a specific listener while significantly reducing the sound heard by people nearby. The prototype uses a single vibrating driver combined with acoustic metamaterials, potentially offering a simpler alternative to directional speakers that rely on large arrays of ultrasonic emitters.
The speaker, developed by researchers led by Junsuk Rho at POSTECH, is roughly the width of a drink coaster. Tests in an anechoic chamber showed that it could maintain a focused sound beam across four octaves, from 500 hertz to 10 kilohertz.
Creating a tightly focused beam from a small conventional speaker is difficult because low-frequency sounds have long wavelengths. A 500-hertz sound wave, for example, has a wavelength of about 0.7 meters. A sound source generally needs to be comparable in size to the wavelength to steer it effectively.
The researchers instead use ultrasound as a carrier. Two ultrasonic frequencies can interact as they travel through air, producing a lower-frequency tone equal to the difference between them. For example, combining 60 and 62 kilohertz produces a 2-kilohertz audible tone. Because the audible sound is generated within the narrow ultrasonic beam, it can inherit the beam’s direction.
This principle, known as parametric acoustic generation, was first described in 1963 and later became the basis for technologies such as the Audio Spotlight. Traditional implementations, however, typically require dozens or even hundreds of ultrasonic emitters, each individually controlled and precisely timed.
Rho’s team took a different approach by combining two metamaterial components around a single vibrating plate. A 3D-printed acoustic metasurface on the front uses 21 layers of tiny channels and cavities to manipulate the ultrasonic wavefront. On the rear, 18 tuned spring-and-weight resonators suppress unwanted vibration modes that can produce sound in unintended directions.
The prototype was tested using a subway announcement played through the speaker. A microphone positioned 0.5 meters away recorded the sound directly in front of the device and at angles of 15, 30 and 45 degrees. With the resonators installed, the unwanted 2-kilohertz sound decreased progressively as the microphone moved away from the center of the beam.
The system still has limitations. The prototype does not produce enough volume for typical personal listening levels, and some unwanted sound remains around 400 hertz and 6 kilohertz. The experiments were also conducted in an anechoic chamber, leaving questions about performance around seats, passengers and other obstacles.
“By co-designing the metamaterials and the ultrasonic device as a single system, we overcame the limitations of cost, complexity and design flexibility at the same time,” Rho said.
The researchers say future work will focus on increasing output volume and keeping multiple frequencies tightly focused in the same direction. The study was published in Nature Communications.
