February 26, 2024

Physicists Discover a Remarkable New Type of Sound Wave

A research group at City University of Hong Kong (CityU) discovered a new type of sound wave: the airborne sound wave vibrates transversely and carries both spin and orbital angular momentum like light does.” If you speak to a physicist about airborne transverse sound, s/he would believe you are a layperson without training in university physics because books say that air-borne noise (i.e., sound propagating in the air) is a longitudinal wave,” said Dr. Wang. One is the momentum-space spin-orbit interaction which offers increase to negative refraction of the transverse noise, indicating that sound bends in the opposite instructions when passing through a user interface. Another one is the real-space spin-orbit interaction which creates sound vortices under the excitation of the transverse noise.
“In future, by controling these extra vector residential or commercial properties, researchers might be able to encode more information into the transverse sound to break the bottleneck of standard acoustic interaction by regular sound waves.”.

Sound vortex generation allowed by the spin-orbit interaction in genuine space. Credit: Wang, S., Zhang, G., Wang, X. et al./ DOI number: 10.1038/ s41467-021-26375-9.
Can you picture sound travels in the exact same way as light does? A research group at City University of Hong Kong (CityU) discovered a brand-new type of acoustic wave: the air-borne acoustic wave vibrates transversely and brings both spin and orbital angular momentum like light does. The findings shattered researchers previous beliefs about the sound wave, opening an opportunity to the development of novel applications in acoustic interactions, acoustic noticing, and imaging.
The research was initiated and co-led by Dr. Wang Shubo, Assistant Professor in the Department of Physics at CityU, and performed in partnership with scientists from Hong Kong Baptist University (HKBU) and the Hong Kong University of Science and Technology (HKUST). It was released in Nature Communications, entitled “Spin-orbit interactions of transverse sound.”.
Beyond the standard understanding of sound wave.
The physics textbooks inform us there are 2 type of waves. In transverse waves like light, the vibrations are perpendicular to the instructions of wave proliferation. In longitudinal waves like sound, the vibrations are parallel to the instructions of wave proliferation. The most current discovery by researchers from CityU modifications this understanding of sound waves.

Illustration of the “meta-atom” and transverse noise. Credit: Wang, S., Zhang, G., Wang, X. et al./ DOI number: 10.1038/ s41467-021-26375-9.
” If you speak to a physicist about airborne transverse noise, s/he would think you are a layman without training in university physics because textbooks say that airborne noise (i.e., sound propagating in the air) is a longitudinal wave,” said Dr. Wang. “While the airborne sound is a longitudinal wave in usual cases, we showed for the first time that it can be a transverse wave under certain conditions. And we investigated its spin-orbit interactions (an important home just exists in transverse waves), i.e. the coupling in between two types of angular momentum. The finding supplies new degrees of freedom for sound controls.”.
The absence of shear force in the air, or fluids, is the reason that noise is a longitudinal wave, Dr. Wang described. He had actually been exploring if it is possible to understand transverse noise, which needs shear force. He conceived the idea that artificial shear force may occur if the air is discretized into “meta-atoms,” i.e. volumetric air restricted in small resonators with size much smaller sized than the wavelength. The collective movement of these air “meta-atoms” can provide increase to a transverse sound on the macroscopic scale.
Conception and awareness of “micropolar metamaterial”.
He ingeniously developed a type of artificial product called “micropolar metamaterial” to implement this idea, which looks like an intricate network of resonators. Air is restricted inside these mutually linked resonators, forming the “meta-atoms”. The metamaterial is hard enough so that only the air inside can vibrate and support sound proliferation. The theoretical computations showed that the cumulative movement of these air “meta-atoms” indeed produces the shear force, which provides rise to the transverse sound with spin-orbit interactions inside this metamaterial. This theory was confirmed by experiments performed by Dr. Ma Guancongs group in HKBU.
Negative refraction caused by the spin-orbit interaction in momentum space. Credit: Wang, S., Zhang, G., Wang, X. et al./ DOI number: 10.1038/ s41467-021-26375-9.
Moreover, the research study group found that air acts like a flexible product inside the micropolar metamaterial and hence supports transverse noise with both spin and orbital angular momentum. Using this metamaterial, they demonstrated two types of spin-orbit interactions of sound for the very first time. One is the momentum-space spin-orbit interaction which generates unfavorable refraction of the transverse sound, indicating that sound bends in the opposite instructions when passing through a user interface. Another one is the real-space spin-orbit interaction which produces sound vortices under the excitation of the transverse sound.
The findings demonstrated that air-borne noise, or noise in fluids, can be a transverse wave and bring full vector residential or commercial properties such as spin angular momentum the exact same as light does. It supplies brand-new perspectives and performances for sound adjustments beyond the conventional scalar degree of freedom.
Dr Wang Shubo from City University of Hong Kong (initially from right) and his research study team. Ms Tong Qing (2nd from left) is a co-author of the paper. Credit: City University of Hong Kong.
” This is just a precursor. We prepare for more expeditions of the intriguing properties of the transverse noise,” Dr. Wang said. “In future, by controling these extra vector homes, scientists might be able to encode more data into the transverse sound to break the traffic jam of conventional acoustic interaction by typical acoustic waves.”.
The interaction of spin with orbital angular momentum makes it possible for unprecedented noise manipulations by means of its angular momentum. “The discovery may open an avenue to the development of novel applications in acoustic communications, acoustic picking up, and imaging,” he included.
Reference: “Spin-orbit interactions of transverse noise” by Shubo Wang, Guanqing Zhang, Xulong Wang, Qing Tong, Jensen Li and Guancong Ma, 21 October 2021, Nature Communications.DOI: 10.1038/ s41467-021-26375-9.
Dr. Wang is the first author and the matching author of the paper. Dr. Ma is another matching author. Partners consist of Professor Li Jensen from The Hong Kong University of Science and Technology, Ms. Tong Qing, a PhD student from CityU, and other researchers from HKBU.
The work was supported by the Research Grants Council in Hong Kong and the National Natural Science Foundation of China.