POSTECH: Eliminating Low-frequency Noise Using Chiral Metabeam

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Soft whirring sound of low-frequency vibrations cannot be easily detected because it is not loud. But once detected, it can be hard to ignore. Often, residents complain of the annoyance caused by low-frequency vibrations that can be heard between adjacent apartment units in Korea.

A research team led by Professor Junsuk Rho (Department of Mechanical Engineering, Department of Chemical Engineering), Ph.D. candidate Jeonghoon Park (Department of Mechanical Engineering), and Professor Anna Lee (Department of Mechanical Engineering) at POSTECH has developed a method to completely eliminate low-frequency vibrations using a chiral structure. The chiral structure, also called mirror symmetry, is symmetrical like the left and right hands but has a unique characteristic of not overlapping.

Recently published in Communications Physics, the findings from this study are applicable to machinery and construction as well as for the development of vibration and noise reduction systems.

Since elastic waves*1 of structures appear in many wave modes, suppression of all possible vibration modes has been rarely achieved. Previous studies on reducing vibrations using metamaterials – with properties that do not exist in nature – also focused only on one mode of vibration. However, such systems posed a risk of amplifying the spread of vibrations which were not originally intended.

In this study, the research team succeeded in blocking all vibration modes that spread in a specific frequency band. The researchers developed a mechanism that can effectively reduce any vibration by implementing a low-frequency complete bandgap*2 using a chiral structure.

Professor Junsuk Rho explained, “It is significant that the range of metamaterials studied in the nanometer (nm, 1 billionth of a meter) has been expanded to a size that can be used in daily life.” He added, “The new system will be applicable to many fields including mechanical structures (e.g., automobiles and aircraft), buildings, and civil engineering in the future.”

This study was conducted with the support from the Mid-career Researcher Program, Global Frontier Projects, Regional Leading Research Center Program, and from the Korea Research Institute of Ships & Ocean engineering (KRISO) funded by the Ministry of Oceans and Fisheries.

1. Elastic wave
A wave where energy is transferred in an elastic medium due to a change in the disturbance state of the medium, for example, sound, water, and seismic waves. Elastic waves are also called mechanical waves.

2. Bandgap
The energy space between the valence and conduction bands where the density of the electron states is zero in semiconductors and insulators. It refers to a frequency region that cannot be penetrated via elastic waves in an elastic structure.

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