French Researchers Make New Strides in Engineering Acoustic Absorption, Soon to 3D Print Metasurfaces

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logocnrsEN3D printing is being used to help us overcome so many challenges, nearly everywhere we look today. Undeniable, remarkable, and far-reaching, the impacts in so many sectors are helping us make strides only dreamed of previously. And most of us probably did not imagine a 3D printing machine was going to come along and work as the catalyst for so much change.

From the smallest things—for instance, my son couldn’t find his chess set after our recent move, so he just hopped on over to Thingiverse for ideas and then began 3D printing out a new one for the whole family to enjoy—including a new board. And to the bigger things—with NASA using 3D printing repeatedly from crucial rocket thrusters to spacesuits as they work diligently to send a crew to Mars in the next several years. 3D printed models are used in complex surgeries from facial reconstruction and transplants to the removal of kidney tumors. But there are many more offbeat and important uses this new technology is affording as well—many that I’ll bet most of you have never considered. How about acoustics?

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The absorption of sound may not be something you have been worrying about, but researchers at the French National Centre for Scientific Research, and the University of Lorraine are. And recently they’ve published their findings in a paper published in Applied Physics Letters, ‘Acoustic metasurface-based perfect absorber with deep subwavelength thickness.’

It’s hoped that with 3D printing, finally some of the obstacles in absorbing acoustic waves will be overcome, while also offering impact on numerous other applications. The general idea is that previously, shock absorbers have been too ungainly, making it difficult for use in realistic applications. Taking this problem and re-working it, the researchers have now designed a coiled-up acoustic metasurface which they will soon be hoping to fabricate via 3D printing, allowing for even further customization and exploration as they are finding that they can achieve total acoustic absorption in very low-frequency ranges.

“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.

Acoustic absorption systems take sound energy and convert it to heat. The most common way to create them up until now has been to place perforated plates directly in front of concrete objects in order to make air pockets. The continuing problem with this process is in their size, however, and Assouar and his team used previous work in developing coiled channel systems to work past that.

“An acoustic perfect absorber in deep subwavelength is always a challenge due to the fact that the frictional power is linearly proportional to the elastic deformation energy in linear dissipative systems,” state the researchers in their paper. “To enhance the coherent dissipation, the intuitional and common way is to increase the energy density, for example, introducing resonant structures.”

“It is innovative to extend the concept of coiling up space into the perforated system to significantly reduce the thickness of the system. Furthermore, it is apparent that the perfect absorber with deep subwavelength thickness, if can be successfully realized, would have deep implications for acoustic device, applications, and in the field of acoustics in general.”

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(Left) The metasurface composed of a perforated plate (transparent gray region) with a hole and a coiled coplanar air chamber (yellow region). (right) The absorption coefficient, α, of the presented metasurface with a total absorption at 125.8 Hz. Results from the impedance analysis and numerical simulations show excellent agreement. [Image: Assouar/CNRS]

Indeed, what the researchers have done is to allow the sound waves to enter one of their coiled air channels though a center hole that is perforated. They travel through, the length of the waves is increased, and both a low sound velocity and high acoustic refractive index are achieved. The key is that the with this process they can created an absorber that is much thinner. The coiled chamber and its acoustic reactance, somewhat akin to electrical reactance makes up for the resulting reactance of the perforated hole; thus, the acoustic energy itself ends up in the chamber and is absorbed.

“By solving this hard challenge, our designed metasurface-based perfect absorber has intriguing applications and paves the way towards the related devices,” state the researchers in their paper. “Perfect absorption under oblique incidence can be realized after optimizations since the geometrics of the metasurface are much smaller than the working wavelength, indicating the validity of the impedance analysis. Our proposed structure can be easily fabricated with 3D printing technique and takes advantages of the compact size, stable structure, and high efficiency.”

With the benefits of 3D printing in front of them, it will be interesting to watch this concept and project evolve further as the researchers are able to use digital design, infinite customization options, and fabrication techniques that allow for affordable exploring, prototyping, and the eventual use of the metasurfaces in applications such as tunable amplitude and more complex acoustic engineering. Discuss how you think these findings will affect this particular science in the 3D Printed Metasurfaces forum over at 3DPB.com.

[Source: Nanowerk]

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