Researchers in Hong Kong have discovered that sound can travel through space in the same way as light. By becoming a transverse sound wave, it enables new applications for sound manipulation.
Sound can travel through space in the same way that light does, research from the City University of Hong Kong (CityU) has discovered, the results of which are published in the journal Nature Communications.
The propagation of the sound wave is not transverse, but longitudinal, while the light wave moves transversely.
That means that light is perpendicular to the direction of the light wave, while sound travels parallel to the direction of the sound wave.
What the new research has discovered is a new type of “transverse sound wave,” which travels in the same way as a light wave.
Scientific challenge
The discovery challenges the scientific consensus on sound waves and establishes a new parameter that could pave the way for new applications of sound beyond the conventional scalar degree of freedom, according to the researchers.
“While sound in air is a longitudinal wave in the usual cases, we show for the first time that it can be a transverse wave under certain conditions,” explains lead author Shubo Wang in a statement.
He adds, “The finding provides new degrees of freedom for manipulations of sound.”
The reason sound is a longitudinal wave “under usual conditions” is the absence of shear force in air or, more generally, in fluids.
Air shear
The understanding of sound as a longitudinal wave stems from the absence of what in physics is called “shear” or “shear force”. In engineering, this term is used to refer to the force that passes through an object without bending.
What the researchers have done is to devise a way in which this shear force can be created in the air or in the fluids surrounding a sound wave, in an artificial but continuous way.
This effect can be achieved if the air or fluid passing through the sound wave can be confined in small resonators: in this way it is possible to obtain an unusual transverse sound on a macroscopic scale.
An artificial material, specially designed for the experiment, contains a complex network of resonators that cause the transverse sound wave to appear.
Elastic wave
When the air passing through the sound is confined in this network of resonators, it forms a sufficiently hard metamaterial capable of vibrating the air surrounding the sound wave and causing the shear force that gives rise to the transverse sound wave.
The researchers found that the metamaterial designed for this research forces the air surrounding the sound wave to behave like an elastic material, capable of supporting the transverse sound.
They experimentally proved that sound in air, or sound in fluids, can be a transverse wave and have full vector properties, just like light, the researchers note.
The discovery provides new insights and functionalities for sound manipulations beyond the conventional scalar degree of freedom.
Promising applications
In the future, by manipulating these additional vector properties, scientists can encode more data in transverse sound to break the bottleneck of traditional acoustic communication using normal sound waves, the researchers note.
What has been achieved so far is only a foretaste of what is to come, and the researchers intend to continue exploring the intriguing properties of this transverse sound wave to leverage this result in new applications that overcome the limitations of current acoustic communications.