Anyone who has studied acoustics will know when it comes to low-frequency sound, traditional sound-absorbing materials tend to be bulky, heavy or thick to do the job, but maybe that will no longer be the case.

A group of researchers at the French National Center for Scientific Research (CNRS) in took up the challenge to design subwavelength absorbers specifically for low-frequency sound waves.

Recent advances showed that the speed of sound can be reduced in a structured medium while increasing the material's ability to attenuate or reduce the sound, which hs enabled the group to design subwavelength resonators whose total absorption of the sound energy is controlled through simple geometric parameters.

A quarter-wavelength resonator, as its name suggests, is a tube that is closed at one end that resonates when the wavelength is equal to four times the length of the tube. Noé Jiménez, an acoustician at CNRS explains...

In other words, it resonates at f=4c/L, where c is the speed of sound in the resonator and L is its length. By reducing the speed of sound in the resonator, both resonance frequency and length of the resonator can be drastically reduced.

The group were able to reduce the speed of sound by loading a quarter-wavelength resonator with Helmholtz resonators, Noé....

Our resonator resembles closed flutes with mouthpieces plugged into the holes of the main flute, A decrease in the speed of sound within the resonator lets it resonate at a lower frequency, while the attenuation of the sound wave increases.  These losses are also dependent on the geometry of the loading resonators. The absorption of the whole structure, for a thickness that can be 100 times smaller than the incident wavelength.

What this means is that a very thin structure made of a metamaterial (synthetic, custom-designed material) can completely absorb very low-frequency sound from nearly every direction surpassing traditional acoustic treatments.

Thanks to the resonances and the losses being produced within the air itself, we expect these subwavelength structured materials to have longer lifetimes than structures made of other subwavelength resonant structures.

In practice, this allows a reduction in thickness and weight of acoustic treatment for the absorption of acoustic energy at very low frequencies. However, there is some more work to do before we might see them in our studios...

Broadband and subwavelength absorbers are promising for soundproofing applications, but, on the other hand, the panels we've developed are only designed to work in reflection…so once the transmission is introduced the problem becomes more challenging.

You can read the full research paper here but full access will require the payment of a fee.