Spiderweb Thread Inspires Ultrasmall Microphones
Sound recording could take a cue from arachnid acoustics
âHumans, being arrogant animals, fashioned the microphone after their own earsâbut thatâs not necessarily the best way to do it,â says Ron Miles, a mechanical engineer at Binghamton University.
Instead, Miles contends, why not model microphones on a creature without any ears at all? In a presentation at the Acoustical Society of Americaâs meeting in Ottawa on Thursday, Miles described how taking a cue from arachnid acoustics could alter the future of sound recording.
About 150 years ago, a German physician named Hermann von Helmholtz sussed out the first step in how the human ear processes sound: Pressure waves in the air vibrate the eardrum at different frequencies, activating electrical signals that the brain uses to create the experience of hearing. Less than a decade later, inventor Emile Berliner patented a microphone operating on the same principle, with a taut metal diaphragm in place of the eardrumâs tympanic membrane.
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These pressure-based devices have served us well for more than a century, but the microphones we add to myriad gadgets today need to be smaller, more sensitive and clearer than ever before. And when a pressure-based microphone is miniaturized to a certain pointâsay, for a cell phone or smartwatchâit gets ânoisy,â Miles explains. The smaller the diaphragm, the more easily it is rattled by stray molecules floating in the air. In other words, the microphone apparatus itself is so sensitive to background noise that the latter can drown out desired sounds.
Miles says sticking to the pressure-based model may be holding microphone technology back. âIf you want to make something small, you should think about how small animals do it,â he says. They have the advantage of millions of years of evolutionary R&D.
Many arthropods, including mosquitoes and spiders, donât have organs that perceive a soundâs pressure waves at all. Instead they detect the airflow generated by a sound: specialized hairs on their body sense the speed and direction of air particles as those particles are swept up by a sound wave. And as Miles and his team found in 2022, some spiders even fully outsource hearing to their web: soundsâ airflow causes the silky strands to vibrate, which the arachnids can sense through touch.
After this discovery, the researchers set out to determine if an airflow-based detector could actually sense and distinguish between the range of frequencies needed for a human-use microphoneânot just those that interest hungry spiders. The Binghamton team took strands of silk from orb weaver arachnids called bridge spiders (which conveniently live in the universityâs nature preserve) and used a laser vibrometer to record how they responded to different sound frequencies.
The average human can hear sounds from about 20 hertz, or cycles per second, to 20 kilohertzâand the spider silk proved responsive all the way from 1 Hz up to 50 kHz. âItâs a much bigger range, better than any [existing, pressure-based] microphone,â Miles explains. âIts frequency response was basically perfect.â
Because it would be impractical to have little fragments of spider silk in our phones, Miles and his team are working to develop a silicon chip that simulates the substanceâs properties. âInstead of threads, we make cantilever beams,â he explains. âTheyâre just like little diving boards but half a micron thick.â
A test of these chips, published in April in the Journal of the Acoustical Society of America, indicates that tiny versions of flow-based microphones donât suffer the same âperformance penaltyâ that pressure-based microphones do when they are miniaturized. âIt was pleasant surprise,â says Binghamton acoustic engineer Junpeng Lai, the studyâs lead author. âIf the cantilever is thin enough, size doesnât matter. If you build it 10 times smaller, the sound fidelity is the same.â
While weâre still years away from microphones based on arachnid technology, Miles and Laiâs findings are an âelegantâ demonstration of the seemingly endless applications of spider silk, says Fritz Vollrath, an evolutionary biologist at the University of Oxford, who has studied spiders and their webs for nearly 50 years. Over the course of his career, Vollrath has seen spider silk inspire advances across disciplines as disparate as materials science, soft robotics, nerve regeneration, and optical and chemical sensing.
âWeâre so used to this marvel in day-to-day life that we donât question it,â Vollrath says. âWhen you start studying it, you begin to realize how amazingly sophisticated the web really is.â