KAIST Develops New Noise Blocking Technology Including 'Gap Noise' in Open Spaces

A new concept noise blocking technology that can effectively reduce gap noise in open spaces has been developed.

KAIST announced on the 6th that Professor Jeon Won-joo's research team from the Department of Mechanical Engineering has developed a new concept acoustic metamaterial called the ‘Complex Impedance Tile,’ which allows heat exchange and airflow through gaps or openings in structures while effectively blocking noise.

(From left) Professor Wonju Jeon, Ph.D. candidate Eunjin Yang, Ph.D. candidate Jiwan Kim, Department of Mechanical Engineering, KAIST. Provided by KAIST

Sound has the characteristic of being transmitted even through small holes or gaps. Sound escaping through gaps propagates into wide spaces, making it difficult to prevent external noise from being heard inside or internal noise from leaking outside without blocking the gaps.

For example, structures in various industrial fields such as home appliances like dryers, air conditioners, and air purifiers, mechanical and power equipment like cooling towers and transformers, and urban air mobility such as air taxis and drones require open spaces (openings or gaps) for heat exchange or airflow.

At this time, noise generated from noise sources such as fans and motors located inside products and equipment inevitably leaks outside, and the noise that escapes outside lowers the quality of daily life and can cause problems such as hearing impairment in extreme cases.

However, the Complex Impedance Tile developed by the research team functions by adjusting the acoustic impedance to a desired complex value to effectively block noise.

Acoustic impedance refers to the inherent acoustic properties of the medium (air, water, etc.) through which sound propagates. Since sound is generally expressed as the product of the medium's density and the speed of sound, its value is real, and once the medium is fixed, it is impossible to freely adjust the value to a desired one.

In contrast, the Complex Impedance Tile developed by the research team can control not only the magnitude of the reflected sound when sound hits the boundary surface and is reflected but also its direction.

This differs from existing technologies that only absorb sound on structural walls by appropriately adjusting the magnitude and direction of sound so that sound hardly escapes through gaps and is confined and diminished within the structure.

The research team predicted through precise computational simulations that applying the Complex Impedance Tile could reduce the sound escaping outside by more than 90%, and verified the noise reduction performance through fabrication and experiments.

In particular, it was confirmed that using the Complex Impedance Tile reduces the sound escaping outside much more significantly than completely absorbing sound 100% on the inner walls of the structure.

Professor Jeon Won-joo explained, “The Complex Impedance Tile can prevent sound from escaping outside without blocking openings or gaps at all, and because it has a thin thickness, it can be applied to systems installed in relatively narrow spaces.”

He added, “Above all, since it utilizes the structural shape characteristics rather than materials, it is easy to maintain despite humidity or temperature changes and has the advantage of easy fabrication. This will enable it to be used as a new solution for noise reduction in various systems, from electronic products such as hair dryers and vacuum cleaners to urban air mobility, which is attracting attention as a future transportation means.”

Meanwhile, this research was conducted with support from the Mid-Career Researcher Support Program of the National Research Foundation of Korea and the KAIST Leap Research Program. The research results were published on March 1 in the international journal in the field of mechanical engineering, Mechanical Systems and Signal Processing. Doctoral students Yang Eun-jin and Kim Ji-wan from the KAIST Department of Mechanical Engineering participated as co-first authors in writing the paper.