Applied Physics Letters - Acoustic metamaterial panels for sound attenuation in the 50–1000 Hz regime
The Hong Kong University of Science and Technology has shown experimentally that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies. The small decay length of the evanescent waves at these frequencies implies that several membrane panels can be stacked to achieve broad-frequency effectiveness. We report the realization of acoustic metamaterial panels with thickness 15mm (0.7 inches) and weight 3 kg/m^2 (about 6 pounds per square yard) demonstrating 19.5 dB of internal sound transmission loss (STL) at around 200 Hz, and stacked panels with thickness 60 mm and weight 15 kg/m^2 demonstrating an average STL of >40 dB over a broad range from 50 to 1000 Hz.
These noise-cancelling panels consist of a latex rubber membrane stretched over a 3-millimetre-thick rigid plastic grid of 1-centimetre-wide squares. In the middle of each square is a small, weighted, plastic button.
When sound waves hit the panel, the membrane and weighted buttons resonate at difference frequencies. "The inner part of the membrane vibrates in opposite phase to the outer region," says Yang. That means the sound waves cancel each other out and no sound gets through.
Each weighted membrane only cancels out sound waves within a small band of frequencies. But changing the weight of the buttons alters the operational frequency, says Yang. By stacking five membranes together, each tuned to a specific band, you can create a soundproof panel that works in the range from 70 to 550 hertz.
Zhiyu Yang webpage at The Hong Kong University of Science and Technology Department of Physics
2009 research publications of the Hong Kong University of Science and Technology Department of Physics
A 2008 paper on earlier version of the research, 4 page pdf - Membrane Type Acoustic Metamaterial With Negative Dynamic Mass
Simple membrane type metamaterial that can exceed the mass density law of sound attenuation by 200 times.