SOUND IN MOTION
The way spiders and insects hear inspires design of a new, bio-inspired, flow microphone.
Written by Nancy Kristof
Spider webs move in response to the wind. A new microphone works the same way.
MICROPHONES PICK UP SOUND the way humans and other mammals do. We have eardrums buried within our skulls, which sense tiny changes in the atmospheric pressure. The changes in pressure cause the eardrum and a lot of tiny bones to move. The inner ear detects the motion, and those signals are interpreted in the brain.
Ron Miles, a professor of mechanical engineering at Binghamton University in New York, explains that there’s another way to pick up sounds, also derived from how animals without eardrums hear. Those animals sense changes in the velocity of air, rather than changes in pressure, using the tiny hairs on their bodies or antennas.
“It’s basically sensing the motion of the air that goes back and forth in a sound field,” Miles said when explaining how arthropods such as crickets, midges, or spiders hear.
A graduate student of Miles’s, Jian Zhou, told him about seeing a spiderweb in a nature preserve near campus. He noticed how a spider’s web moves in response to wind and noted the strength of its silk. “Since then, we've been obsessed with okay, how do you, as an engineer, how do you make something that can move well with the air and detect the sound,” said Miles.
The result of their collaboration is the basis of a new type of microphone, one that relies on airflow rather than pressure waves. Their patent of a bio-inspired flow microphone is now being used by the Canadian venture firm TandemLaunch and its spin-off company Soundskrit to develop a next generation of microphone.
Conventional microphones work like human ears. They convert the motion of a membrane into an electronic signal.
“When it’s very small, it turns out it’s rather challenging to minimize the noise in the microphone. This is an engineering challenge, and it’s a mechanical engineering challenge.”
– Ron Miles, Professor of mechanical engineering at Binghamton University
MINIMIZING NOISE
Microphones are surprisingly ubiquitous. “There are billions of them made every month and found in practically every handheld commercial product.” Miles said.
Essentially all microphones built ever since they were first developed by Alexander Graham Bell and Thomas Edison nearly 150 years ago work by sensing pressure.
“They’re all basically designed to work like human ears. They have a membrane and convert the motion of the membrane into an electronic signal,” Miles said. This setup is found even in miniature microphones that rely on micro-electromechanical systems (MEMS) rather than old-fashioned coils.
But miniaturized microphones generate thermal noise.
“When it’s very small, it turns out it’s rather challenging to minimize the noise in the microphone,” Miles said, adding, “This is an engineering challenge, and it’s a mechanical engineering challenge.”
Inspired by the spiderweb seen moving in a breeze, Miles and Zhou borrowed a spider to see if they could test its silk in the University’s anechoic chamber. They began to consider if a microphone using the same structural characteristics and velocity-sensing ability of insects could reproduce audio with the same or better fidelity at high and low frequencies.
“What we have found is, that the size of the sensing element, when it’s sensing flow, the size doesn’t matter. You can make them small without having it affect the noise,” Miles said. In theory, a flow-sensing microphone on a cell phone could provide as good a result as a large (and expensive) mic in a recording studio.
The experimental result, discovered by a Binghamton research team, has been submitted to a journal.
To turn experimental results into an actual product, engineers need to explore viable and appropriate materials (spider silk is not likely the most practical), in addition to designing the structure of the mic itself and a system to transduce motion into an electronic signal.
“Maybe it’s hard or different,” Miles said, adding, “but it’s worth a try. It’s worth looking at.”
In addition to noise reduction, flow-sensing microphones are inherently directional. A typical pressure-sensing microphone must be modified to achieve directionality. Determining the direction of sound is important for software that cancels out extraneous noise.
“You can determine the amount of sound power that is flowing past a certain location,” Miles said. “If you can measure the sound power radiated by different surfaces, it tells you a lot.”
Miles said to fully characterize a sound field, one would not use either pressure or flow but both, much the way light sensors measure both color and intensity. “Both pressure and velocity are really important things in a soundwave,” Miles said.
Miniaturized microphones, such as those found in hearing aids, generate thermal noise that is challenging to eliminate.
New research from Binghamton University, in partnership with Cornell University, suggests that spiders are able to use their webs to extend their hearing. The findings come from experiments conducted by Distinguished Professor Ron Miles and doctoral student Junpeng Lai in the Binghamton University anechoic chamber.
ACOUSTIC EMISSIONS
Beyond everyday consumer gadgetry, flow-sensing noise measurement promises a way to improve the understanding of human hearing. Miles is working with a faculty member from the University of Southern California on a separate NIH-funded project to see if flow-sensing sound measurement of the ear may lead to new insights for treating hearing loss issues.
“The ear hears sound, but because of the way the stuff inside your ear works, it also radiates a little bit of sound in response,” called acoustic emissions, Miles explained. “This is an application that I never would have thought of, but it's actually very useful. These kinds of measurements help us understand what’s going on.”
Nancy Kristof is a technology writer in Denver, Colo.
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