Jian ZhouGraduate student Jian Zhou, testing equipment in an anechoic chamber
Researchers get some enviable sound quality using some wispy fibers coated with gold

P Rajendran

Prof. Ron Miles
Ron Miles, distinguished professor and
associated dean of research at the
Watson School of Engineering

Most microphones cannot provide as much sound fidelity as the wiggly hair deep in our ear. Now spider silk may do what earlier boom boxes could not.

“Since many animals hear by the movement of fine hair, that raises a question: Why not acoustic sensors that also move due to viscous forces [in this case, from air],” Ron Miles, professor of mechanical energy at Binghamton University, asked of Truly Curious.

Vibrating strings have often been used to generate sound, as every stringed instrument can mellifluously testify. But before we get to all the fascinating tech that replicates a lot of animal hearing, it may be a good idea to know how we hear.

Sound, which is just compressed waves of air as far as human hearing is concerned, comes up against the funnel that is the outer ear and dives in. It slams into a drum – the eardrum, of course – which sends a set of three connected bones beyond it all aquiver. Like in any good Rube Goldberg machine, these bones – the incus, malleus and stapes – shake a membrane on a spiral coiled tube, the cochlea, sending waves through the fluid within. That is where we come upon the ‘hair,’ or stereocilia.

When the agitated fluid bend these stereocilia one way it finally results in electrical activity in the auditory nerve. The mechanical movements open small channels, thus allowing for the exchange of charged particles – essentially creating an electric current to travel up the auditory nerve into the brain. Depending on which hair vibrate and where in the cochlea, we hear different pitches of sound.

Stereocilia
Stereochilia in a frog’s ear
Photo: Wikimedia

It is no easy task to recreate in microphones the kind of fidelity found in stereocilia.

For raw material, Miles knew he could rely on arthropods – animals with external skeletons and jointed feet – including animals we quickly recognize, such as shrimp, scorpions spiders and insects.

“Many people have been interested in small animals that detect air flow,” Miles said, pointing out that even “mosquito antennae are useful for hearing.”

An engineer, Miles relies on other researchers who dabble in biology, among them Ronald Hoy, a neurobiologist who studies hearing in insects and spiders at Cornell University.

Spiders have hair so responsive to motion that the human eye’s photoreceptors, which can detect even a single proton, cannot compare for sensitivity.

Part of the problem these fine hair address is inertia.

As Miles pointed out, a thick fiber will vibrate more reluctantly than a fine one, just because it will be slower to get moving at all, and once going harder to stop and swing back. That’s inertia. But a shove from a sound wave could break a fiber – or hair – that is too thin.

The solution is to find something thin enough to react almost immediately to changes in air pressure – which is what sound is – but strong enough to survive the push.

Miles and graduate student Jiang Zhou tested a variety of fibers but settled on what, finally, is arthropod cabling.

Jian Zhou
Graduate student Jian Zhou testing the equipment in a echo-free chamber

“We did find that manmade fibers were harder to work with, and easier to break,” he said. “Spider silk is strong and easy to handle,” he said. “And it’s so easy to get.”

Indeed, cross orb weaver spiders (Araneus diadematus) can be found hanging around just outside the university’s Watson School of Engineering in Vestal, NY, where Zhou and Miles work. Zhou found a way to harvest silk from young female spiders since strands from their webs happen to be thin enough.

“Spider silk comes in different thickness. Baby spiders make theirs less than a micron,” Miles said.

Zhou and Miles snipped out a strand of spider silk 3.8 centimeters (1.5 inches) long and 0.5 microns (0.00002 inches) wide. At those dimensions, the silk vibrates with hardly any noticeable inertia, Miles said, noting that the average human hair is 20 times thicker than that.

They then coated the silk with a thin layer of gold and strung it horizontally between two ends of a holder.

As Miles put it, “Grab both ends so it doesn’t have to hold itself up so it can be thin. It does no fall over. At 0.5 microns, the motion dominated by fluid. At 10 microns, it’s not going to move the way you want.”

On one side of the silk was a speaker; on the other there were two important elements: a laser pointer of sorts sending out light at a wavelength of 633 nanometers (0.6 microns or visible red) to reflect off the string, and a high-speed camera. The light bouncing off the the silk headed back into the camera.

The laser-camera unit, called the laser vibrometer, relied on what is called the Doppler effect – the compressing of waves when something is coming your way, and their expansion when they go away.

A simple example is the way a car with blaring horn goes wheeee when it comes your way and whoooo when past you. Do check it out without getting in the way of the car – or, well, you just might miss the whoooo bit.

In the Miles experiment, the air from the speaker pushed the silk back and forth, toward and away from the camera. As the silk stretched and moved toward the camera, it sent more compressed light waves, essentially making it a little more blue. When it arched away from the camera, the light waves from it became longer, essentially more red. Again, the Doppler effect.

The silk was also placed in a magnetic field, and sensors captured the change in the electrical field produced by the gold-covered strand. That relies on Faraday’s Law, which says that voltage equals the product of the strength of the magnetic field, the length of the object, and velocity of its motion.

A spider web near Hinman College at Binghamton University.

The detected change in voltage corresponded well with how the frequency, since the thin silk moved at the same speed as the air vibrating around it.

While he has a proof of concept, Miles concedes there is some way to go before you can buy orb spider microphones on Amazon,

“Whenever you want to sell commercially, you have a huge number of practical considerations to address,” he said. “This idea is really at the very beginning [of the process]. We just had this discovery. That doesn’t mean we produce a commercial product next week.”

While his microphone idea is gestating, Miles is working on other research, too.

“We know spider silk moves within a sound field,” Miles said. “Can spiders hear using spider web since it responds to sound? Can the web be a kind of ear?”

With that interesting evidence-backed question, it’s almost unfair that he gets paid to have fun.

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