Domestic researchers have proven for the first time in the world a wave phenomenon that changes in frequency depending on the direction of the sound. This research is expected to bring innovation to various fields, ranging from medical ultrasound equipment to noise reduction technology.
Pohang University of Science and Technology (POSTECH) announced on the 21st that a research team led by Professor Noh Jun-seok from the Departments of Mechanical Engineering, Chemical Engineering, Electrical Engineering, and the Graduate School of Convergence, along with Professor Kim Eun-ho from Jeonbuk National University's Department of Mechanical Systems Engineering, demonstrated a phenomenon where the frequency varies depending on the direction of waves within a single system.
This research was published in the world's leading journal in the field of physics, Physical Review Letters, on the 15th.
Many technologies we use utilize the principle of changing the frequency of waves. For example, a green laser pointer doubles the frequency of invisible infrared waves (upconversion) to create green light. A super-directional speaker mixes two ultrasonic waves to lower the frequency (downconversion), producing sounds that can be heard.
They utilize 'nonlinearity,' where the response does not simply proportionally change as the wave intensity increases but rather changes in a complex manner, which usually required the wave direction to be fixed or complicated structures and external manipulation.
To solve this, the research team designed a granular 'phononic crystal' structure, connecting small beads. This structure can slightly adjust the consolidation strength of each bead, showing entirely different responses depending on the direction even for the same wave.
This system normally blocks almost all weak energy waves but changes as the wave intensity increases. Waves coming from one side become higher in frequency, producing sharper sounds, while waves from the opposite side become lower in frequency, generating duller sounds. This is similar to how the same door makes different sounds when entering from the front versus the back.
In particular, the research team added the characteristic of 'local resonance' in which beads vibrate especially vigorously at specific frequencies, simultaneously implementing 'nonlinearity' and 'spatial asymmetry' based on direction, allowing both upconversion and downconversion to occur freely within one system.
This technology could be utilized to selectively reduce specific vibrations in earthquakes or construction sites, as well as enhance the resolution of medical ultrasound diagnostic equipment. It could also be applied to sound devices that detect sounds inaudible to humans coming from specific directions or in analog signal processing technologies.
Professors Noh Jun-seok and Kim Eun-ho, who led the research, noted, 'It is significant that a concept previously presented only as a theoretical possibility has been proven through actual experiments,' expressing hopes that it could be widely applied to next-generation frequency conversion and signal processing technologies.
Meanwhile, this research was conducted with the support of the POSCO Holdings N.EX.T Impact project, the National Research Foundation of Korea (NRF), and the Ministry of Science and ICT's support for mid-career researchers.
References
Physical Review Letters (2025), DOI: https://doi.org/10.1103/3n97-7kmd