Absorbing low frequency sound is a big job, or at least, a job for big things. Acoustic absorption systems require big resonant cavities with large amounts of heavy damping material and significant surface areas to work efficiently. Consequently, the sound-deadening systems used in music studios and anechoic chambers take up a lot of room. Now scientists have flipped this notion on its head by designing coiled metasurfaces that not only completely absorb low frequency sounds, but are a tiny fraction of the size of traditional sound-absorbing systems.
Many standard resonant acoustic absorption systems work by deadening sound energy using an inner diaphragm behind a perforated plate that effectively reduces the wave energy on the way in, then funnels it into a resonant frequency chamber where it is allowed to dissipate. The wavelength of sound at low frequencies – around 200 Hertz and below – however, is in the order of meters in size, so traditional acoustic absorbers need to be physically large to resonate and cushion noise at those frequencies.
To address this, researchers working at the French National Centre for Scientific Research (CNRS) and the University of Lorraine designed a metamaterial sub-miniature acoustic absorber with a perforated center plate that directs the incoming acoustic waves to travel through an internal spiral channel. Despite having a total thickness just 1/223 of the sound wavelength being absorbed, the device effectively manages to increase the total propagation length of the incoming soundwaves, lowering the sound velocity, and creating a high acoustic refractive index to effectively disperse sound energy.
Such an exceptionally small device being able to absorb such low frequencies is made possible because the coiled chamber's acoustic reactance – essentially, the opposition to the flow of sound through a material – matches the impedance of the inlet hole. As a result, all of the acoustic energy is transferred to the chamber without reflection, thereby completely absorbing the wave energy.
"The main advantage is the deep-subwavelength thickness of our absorber, which means that we can deal with very low-frequencies – meaning very large wavelengths – with extremely reduced size structure," said Badreddine Assouar, a principal research scientist at CNRS in Nancy, France.
The researchers suggest that applications of their metasurface may result in tunable amplitude (effectively the sound's "loudness") and phase-changing devices suitable for use in specialized acoustic
engineering, such as using sound to move and manipulate particles. Assouar
and his group intend to develop the sample fabrication process
with 3D printing and conduct subsequent performance analysis to help determine any further practical applications of the system.