Natural sound absorbers? Moth wings can cut 87% of noise energy
Around 65 million years ago, bats first evolved the ability to echolocate by producing high-frequency clicks with their mouth or nose and listening for echoes bouncing off objects.
Since then, bats and moths have been involved in an "acoustic arms race" between predator and prey. Now, while moths are under enormous predation pressure from bats and have evolved several defenses for survival, their choice of weapon - the moth wing - holds the key to transforming noise-canceling technology.
Researchers who recently discovered that moth wings indeed offer acoustic protection from bat echolocation calls have been studying whether their structure could "inform" better-performing sound-absorbing panels, when not moving in free space.
Now, in a study published in Proceedings of the Royal Society A, these experts at the University of Bristol have discovered that the scales on moth wings could be excellent sound absorbers even when placed on an artificial surface.
Excellent sound absorbers
In a statement, Professor Marc Holderied, of Bristol’s School of Biological Sciences said, “What we needed to know first, was how well these moth scales would perform if they were in front of an acoustically highly reflective surface, such as a wall."
“We also needed to find out how the mechanisms of absorption might change when the scales were interacting with this surface.”
Prof Holderied and his team tested this by placing small sections of moth wings on an aluminum disc, then systematically tested how the orientation of the wing concerning the incoming sound and the removal of scale layers affected absorption.
They were right. Moth wings proved to be excellent sound absorbers, even when they were on top of a solid acoustical substrate. The wings absorbed as much as 87 percent of the incoming sound energy.
They also noted that the effect is broadband and omnidirectional, covering a wide range of frequencies and sound incident angles.
"What is even more impressive is that the wings are doing this whilst being incredibly thin, with the scale layer being only 1/50th of the thickness of the wavelength of the sound that they are absorbing," explained lead author Thomas Neil. "This extraordinary performance qualifies the moth wing as a naturally occurring acoustic absorbing metasurface, a material that has unique properties and capabilities, that are not possible to create using conventional materials."
Designing and building prototypes
As cities get louder, the need for such efficient, non-intrusive, sound mitigation solutions grows. These lightweight sound-absorbing panels could have massive impacts on the travel industry, with weight saving in planes, cars, and trains increasing efficiency in these modes of transport, reducing fuel use and CO2 emissions.
The scientists plan to replicate the sound-absorbing performance by designing and building prototypes based on the sound-absorbing mechanisms of the moth. Currently, the absorption that they have characterized in moth wings is in the ultrasound frequency range.
Their main challenge is to design a structure that will work at lower frequencies whilst retaining the same ultrathin architecture employed by the moth.
"Moths are going to inspire the next generation of sound-absorbing materials," said Holderied.
"New research has shown that one day it will be possible to adorn the walls of your house with ultrathin sound absorbing wallpaper, using a design that copies the mechanisms that give moths stealth acoustic camouflage."
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