An interesting set of research: http://phys.org/news/2016-02-absorbing-acoustics-soundless-spirals.html
"Researchers at the French National Centre for Scientific Research, CNRS, and the University of Lorraine have recently developed a design for a coiled-up acoustic metasurface which can achieve total acoustic absorption in very low-frequency ranges."
"Researchers at the French National Centre for Scientific Research, CNRS, and the University of Lorraine have recently developed a design for a coiled-up acoustic metasurface which can achieve total acoustic absorption in very low-frequency ranges."
