Tag Archives: acoustics

Moths with sound absorption stealth technology

The cabbage tree emperor moth (Thomas Neil) [downloaded from https://www.cbc.ca/radio/quirks/nov-17-2018-greenland-asteroid-impact-short-people-in-the-rain-forest-reef-islands-and-sea-level-and-more-1.4906857/how-moths-evolved-a-kind-of-stealth-jet-technology-to-sneak-past-bats-1.4906866]

I don’t think I’ve ever seen a more gorgeous moth and it seems a perfect way to enter 2019, from a November 16, 2018 news item on CBC (Canadian Broadcasting Corporation),

A species of silk moth has evolved special sound absorbing scales on its wings to absorb the sonar pulses from hunting bats. This is analogous to the special coatings on stealth aircraft that allow them to be nearly invisible to radar.

“It’s a battle out there every night, insects flying for their lives trying to avoid becoming a bat’s next dinner,” said Dr. Marc Holderied, the senior author on the paper and an associate professor in the School of Biological Sciences at the University of Bristol.

“If you manage to absorb some of these sound energies, it would make you look smaller and let you be detectable over a shorter distance because echoe isn’t strong enough outside the detection bubble.”

Many moths have ears that warn them when a bat is nearby. But not the big and juicy cabbage tree emperor moths which would ordinarily make the perfect meal for bats.

The researchers prepared a brief animated feature illustrating the research,

Prior to publication of the study, the scientists made a presentation at the Acoustical Society of America’s 176th Meeting, held in conjunction with the Canadian Acoustical Association’s 2018 Acoustics Week, Nov. 5-9 at the Victoria Conference Centre in Victoria, Canada according to a November 7, 2018 University of Bristol press release (also on EurekAlert but submitted by the Acoustical Society of America on November 6, 2018),

Moths are a mainstay food source for bats, which use echolocation (biological sonar) to hunt their prey. Scientists such as Thomas Neil, from the University of Bristol in the U.K., are studying how moths have evolved passive defenses over millions of years to resist their primary predators.

While some moths have evolved ears that detect the ultrasonic calls of bats, many types of moths remain deaf. In those moths, Neil has found that the insects developed types of “stealth coating” that serve as acoustic camouflage to evade hungry bats.

Neil will describe his work during the Acoustical Society of America’s 176th Meeting, held in conjunction with the Canadian Acoustical Association’s 2018 Acoustics Week, Nov. 5-9 at the Victoria Conference Centre in Victoria, Canada.

In his presentation, Neil will focus on how fur on a moth’s thorax and wing joints provide acoustic stealth by reducing the echoes of these body parts from bat calls.

“Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasonic frequencies,” said Neil, a researcher at Bristol University. “The thorax fur of moths acts as a lightweight porous sound absorber, facilitating acoustic camouflage and offering a significant survival advantage against bats.” Removing the fur from the moth’s thorax increased its detection risk by as much as 38 percent.

Neil used acoustic tomography to quantify echo strength in the spatial and frequency domains of two deaf moth species that are subject to bat predation and two butterfly species that are not.

In comparing the effects of removing thorax fur from insects that serve as food for bats to those that don’t, Neil’s research team found that thoracic fur determines acoustic camouflage of moths but not butterflies.

“We found that the fur on moths was both thicker and denser than that of the butterflies, and these parameters seem to be linked with the absorptive performance of their respective furs,” Neil said. “The thorax fur of the moths was able to absorb up to 85 percent of the impinging sound energy. The maximum absorption we found in butterflies was just 20 percent.”

Neil’s research could contribute to the development of biomimetic materials for ultrathin sound absorbers and other noise-control devices.

“Moth fur is thin and lightweight,” said Neil, “and acts as a broadband and multidirectional ultrasound absorber that is on par with the performance of current porous sound-absorbing foams.”

Moth fur? This has changed my view of moths although I reserve the right to get cranky when local moths chew through my wool sweaters. Here’s a link to and a citation for the paper,

Biomechanics of a moth scale at ultrasonic frequencies by Zhiyuan Shen, Thomas R. Neil, Daniel Robert, Bruce W. Drinkwater, and Marc W. Holderied. PNAS [Proccedings of the National Academy of Sciences of the United States of America] November 27, 2018 115 (48) 12200-12205; published ahead of print November 12, 2018 https://doi.org/10.1073/pnas.1810025115

This paper is behind a paywall.

Unusually I’m going to include the paper’s abstract here,

The wings of moths and butterflies are densely covered in scales that exhibit intricate shapes and sculptured nanostructures. While certain butterfly scales create nanoscale photonic effects [emphasis mine], moth scales show different nanostructures suggesting different functionality. Here we investigate moth-scale vibrodynamics to understand their role in creating acoustic camouflage against bat echolocation, where scales on wings provide ultrasound absorber functionality. For this, individual scales can be considered as building blocks with adapted biomechanical properties at ultrasonic frequencies. The 3D nanostructure of a full Bunaea alcinoe moth forewing scale was characterized using confocal microscopy. Structurally, this scale is double layered and endowed with different perforation rates on the upper and lower laminae, which are interconnected by trabeculae pillars. From these observations a parameterized model of the scale’s nanostructure was formed and its effective elastic stiffness matrix extracted. Macroscale numerical modeling of scale vibrodynamics showed close qualitative and quantitative agreement with scanning laser Doppler vibrometry measurement of this scale’s oscillations, suggesting that the governing biomechanics have been captured accurately. Importantly, this scale of B. alcinoe exhibits its first three resonances in the typical echolocation frequency range of bats, suggesting it has evolved as a resonant absorber. Damping coefficients of the moth-scale resonator and ultrasonic absorption of a scaled wing were estimated using numerical modeling. The calculated absorption coefficient of 0.50 agrees with the published maximum acoustic effect of wing scaling. Understanding scale vibroacoustic behavior helps create macroscopic structures with the capacity for broadband acoustic camouflage.

Those nanoscale photonic effects caused by butterfly scales are something I’d usually describe as optical effects due to the nanoscale structures on some butterfly wings, notably those of the Blue Morpho butterfly. In fact there’s a whole field of study on what’s known as structural colo(u)r. Strictly speaking I’m not sure you could describe the nanostructures on Glasswing butterflies as an example of structure colour since those structures make that butterfly’s wings transparent but they are definitely an optical effect. For the curious, you can use ‘blue morpho butterfly’, ‘glasswing butterfly’ or ‘structural colo(u)r’ to search for more on this blog or pursue bigger fish with an internet search.

Tractor beams for humans?

I got excited for a moment before realizing that, if tractor beams for humans result from this work, it will be many years in the future. Still, one can dream, eh? Here’s more about the current state of tractor beams (the acoustic kind) from a January 21, 2018 news item on ScienceDaily,

Acoustic tractor beams use the power of sound to hold particles in mid-air, and unlike magnetic levitation, they can grab most solids or liquids. For the first time University of Bristol engineers have shown it is possible to stably trap objects larger than the wavelength of sound in an acoustic tractor beam. This discovery opens the door to the manipulation of drug capsules or micro-surgical implements within the body. Container-less transportation of delicate larger samples is now also a possibility and could lead to levitating humans.

A January 22, 2018 University of Bristol press release (also on EurekAlert but dated January 21, 2018), which originated the news item, expands on the theme,

Researchers previously thought that acoustic tractor beams were fundamentally limited to levitating small objects as all the previous attempts to trap particles larger than the wavelength had been unstable, with objects spinning uncontrollably. This is because rotating sound field transfers some of its spinning motion to the objects causing them to orbit faster and faster until they are ejected.

The new approach, published in Physical Review Letters today [Monday 22 January]{2018}], uses rapidly fluctuating acoustic vortices, which are similar to tornadoes of sound, made of a twister-like structure with loud sound surrounding a silent core.

The Bristol researchers discovered that the rate of rotation can be finely controlled by rapidly changing the twisting direction of the vortices, this stabilises the tractor beam. They were then able to increase the size of the silent core allowing it to hold larger objects. Working with ultrasonic waves at a pitch of 40kHz, a similar pitch to that which only bats can hear, the researchers held a two-centimetre polystyrene sphere in the tractor beam. This sphere measures over two acoustic wavelengths in size and is the largest yet trapped in a tractor beam. The research suggests that, in the future much larger objects could be levitated in this way.

Dr Asier Marzo, lead author on the paper from Bristol’s Department of Mechanical Engineering, said: “Acoustic researchers had been frustrated by the size limit for years, so its satisfying to find a way to overcome it. I think it opens the door to many new applications.”

Dr Mihai Caleap, Senior Research Associate, who developed the simulations, explained: “In the future, with more acoustic power it will be possible to hold even larger objects. This was only thought to be possible using lower pitches making the experiment audible and dangerous for humans.”

Bruce Drinkwater, Professor of Ultrasonics from the Department of Mechanical Engineering, who supervised the work, added: “Acoustic tractor beams have huge potential in many applications. I’m particularly excited by the idea of contactless production lines where delicate objects are assembled without touching them.”

The researchers have included a video representing their work,

I always liked the tractor beams on Star Trek as they seemed very useful. For those who can dream in more technical language, here’s a link to and a citation for the paper,

Acoustic Virtual Vortices with Tunable Orbital Angular Momentum for Trapping of Mie Particles by Asier Marzo, Mihai Caleap, and Bruce W. Drinkwater. Phys. Rev. Lett. Vol. 120, Iss. 4 — 26 January 2018 DOI:https://doi.org/10.1103/PhysRevLett.120.044301 Published 22 January 2018

This paper is open access.

A Moebius strip of moving energy (vibrations)

This research extends a theorem which posits that waves will adapt to slowly changing conditions and return to their original vibration to note that the waves can be manipulated to a new state. A July 25, 2016 news item on ScienceDaily makes the announcement,

Yale physicists have created something similar to a Moebius strip of moving energy between two vibrating objects, opening the door to novel forms of control over waves in acoustics, laser optics, and quantum mechanics.

The discovery also demonstrates that a century-old physics theorem offers much greater freedom than had long been believed. …

A July 25, 2016 Yale University news release (also on EurekAlert) by Jim Shelton, which originated the news item, expands on the theme,

Yale’s experiment is deceptively simple in concept. The researchers set up a pair of connected, vibrating springs and studied the acoustic waves that traveled between them as they manipulated the shape of the springs. Vibrations — as well as other types of energy waves — are able to move, or oscillate, at different frequencies. In this instance, the springs vibrate at frequencies that merge, similar to a Moebius strip that folds in on itself.

The precise spot where the vibrations merge is called an “exceptional point.”

“It’s like a guitar string,” said Jack Harris, a Yale associate professor of physics and applied physics, and the study’s principal investigator. “When you pluck it, it may vibrate in the horizontal plane or the vertical plane. As it vibrates, we turn the tuning peg in a way that reliably converts the horizontal motion into vertical motion, regardless of the details of how the peg is turned.”

Unlike a guitar, however, the experiment required an intricate laser system to precisely control the vibrations, and a cryogenic refrigeration chamber in which the vibrations could be isolated from any unwanted disturbance.

The Yale experiment is significant for two reasons, the researchers said. First, it suggests a very dependable way to control wave signals. Second, it demonstrates an important — and surprising — extension to a long-established theorem of physics, the adiabatic theorem.

The adiabatic theorem says that waves will readily adapt to changing conditions if those changes take place slowly. As a result, if the conditions are gradually returned to their initial configuration, any waves in the system should likewise return to their initial state of vibration. In the Yale experiment, this does not happen; in fact, the waves can be manipulated into a new state.

“This is a very robust and general way to control waves and vibrations that was predicted theoretically in the last decade, but which had never been demonstrated before,” Harris said. “We’ve only scratched the surface here.”

In the same edition of Nature, a team from the Vienna University of Technology also presented research on a system for wave control via exceptional points.

Here’s a link to and a citation for the paper,

Topological energy transfer in an optomechanical system with exceptional points by H. Xu, D. Mason, Luyao Jiang, & J. G. E. Harris. Nature (2016) doi:10.1038/nature18604 Published online 25 July 2016

This paper is behind a paywall.

Carbon nanotubes, acoustics, and heat

I have a longstanding interest in carbon nanotubes and acoustics, which I first encountered in 2008. This latest work comes from the Michigan Technological University according to a July 28, 2015 news item on Nanowerk,

Troy Bouman reaches over, presses play, and the loudspeaker sitting on the desk starts playing the university fight song. But this is no ordinary loudspeaker. This is a carbon nanotube transducer—and it makes sound with heat.

Bouman and Mahsa Asgarisabet, both graduate students at Michigan Technological University, recently won a Best of Show Award at SAE International’s Noise and Vibration Conference and Exhibition 2015 for their acoustic research on carbon nanotube speakers. They work with Andrew Barnard, an assistant professor of mechanical engineering at Michigan Tech, to tease out the fundamental physics of these unusual loudspeakers.

While still a fledgling technology, the potential applications are nearly endless. Everything from de-icing helicopter blades to making lighter loudspeakers to doubling as a car speaker and heating filament for back windshield defrosters.

Here’s a few sound sound files featuring the students and their carbon nanotube speakers,

A July 28, 2015 Michigan Technological University news release, which originated the news item, goes on to describe how these carbon nanotubes are making sound,

The freestanding speaker itself is rather humble. In fact, it’s a bit flimsy. A teflon base props up two copper rods, and what seems like a see-through black cloth stretches between them.

“A little wind gust across them, and they would just blow away,” Barnard says. “But you could shake them as much as you want—since they have such low mass, there is virtually no inertia.”

The material is strong side to side, because what the naked eye can’t see is the collection of black nanotubes that make up that thin film.

The nanotubes are straw-like structures with walls only one carbon atom-thick and they can heat up and cool down up to 100,000 times each second. By comparison, a platinum sheet about 700 nanometers thick can only heat up and cool down about 16 times each second. The heating and cooling of the carbon nanotubes causes the adjacent air to expand and contract. That pushes air molecules around and creates sound waves.

“Traditional speakers use a moving coil, and that’s how they create sound waves,” Bouman says. “There are completely different physics behind carbon nanotube speakers.”

And because of these differences, the nearly weightless carbon nanotube speakers produce sound in a way that isn’t initially understood by our ears. Bouman’s research focuses on processing the sound waves to make them more intelligible. Take a listen.


To date, most research on carbon nanotubes has been on the materials side. Carbon nanotube speakers were discovered accidently in 2008, showing that the idea was viable. As mechanical engineers studying acoustics, Barnard, Bouman and Asgarisabet are refining the technology.

“They are very light weight and have no moving parts,” Asgarisabet says, which is ideal for her work in active noise control, where the carbon nanotube films could cancel out engine noise in airplanes or road noise in cars. But first, she says, “I want to focus first on getting a good thermal model of the speakers.”

Having an accurate model, Bouman adds, is a reflection of understanding the carbon nanotube loudspeakers themselves. The modeling work he and Asgarisabet are doing lays down the foundation to build up new applications for the technology.

While a lot of research remains on sorting out the underlying physics of carbon nanotube speakers, being able to use both the heat and sound properties makes them versatile. The thinness and weightlessness is also appealing.

“They’re basically conformable speakers,” Barnard says. The thin film could be draped over dashboards, windows, walls, seats and maybe even clothing. To get the speakers to that point, Barnard and his students will continue refining the technology’s efficiency and ruggedness, one carbon nanotube thin-film at a time.

As I mentioned earlier I’m quite interested in carbon nanotubes speakers and, for that matter, all other nanomaterial speakers. For example, there was a November 18, 2013 posting titled: World’s* smallest FM radio transmitter made out of graphene which also featured the Zettl Group’s (University of California at Berkeley) carbon nanotube radio (unfortunately those sound files are no longer accessible).

Dexter Johnson in a July 30, 2015 posting (on his Nanoclast blog on the Institute of Electrical and Electronics Engineers [IEEE] website) provides some additional insights (Note: Links have been removed),

It’s been some time since we covered the use of nanomaterials in audio speakers. While not a hotly pursued research field, there is some tradition for it dating back to the first development of carbon nanotube-based speakers in 2008. While nanomaterial-based speakers are not going to win any audiophile prize anytime soon, they do offer some unusual characteristics that mainly stem from their magnet-less design.

Acoustics and carbon nanotubes

Mikhail Koslov from the University of Texas at Dallas has written a Dec. 18, 2014 Nanowerk Spotlight article about his research into carbon nanotubes and their acoustic properties,

Carbon nanotube assemblies enabled design of a hybrid thermo-electromagnetic sound transducer with unique sound generation features that are not available from conventional diaphragm and thermo-acoustic speakers.

EM image of multi-walled carbon nanotube sheet used for thermo-electromagnetic sound transducer. (Image: Mikhail Kozlov, University of Texas at Dallas)

EM image of multi-walled carbon nanotube sheet used for thermo-electromagnetic sound transducer. (Image: Mikhail Kozlov, University of Texas at Dallas)

Kozlov goes on to explain his work in more detail,

… a hybrid thermo-electromagnetic sound transducer (TEMST) [was] fabricated using highly porous multi-walled carbon nanotube sheet that was placed in the proximity of a permanent magnet. Upon electrical AC excitation, thermal response of the material is combined with diaphragm-like sheet oscillations induced by the electromagnetic action of the Lorentz force.

Unlike conventional diaphragm loudspeaker, acoustic spectrum of the TEMST device consists of a superposition of TA and EM responses that can be altered by applied bias voltage. Variation of bias voltage changes spectral intensity and spatial distribution of generated sound.

In particular, propagation direction of the sound can be reversed by switching bias polarity that somewhat resembles voltage-controlled acoustic reflection. Such uncommon behavior was explained by interference of the two contributions being beneficial for diverse sound management applications.

It was found also that amplitude of first TEMST harmonic changes a lot with applied magnetic field, while the second one remains almost field independent. This unusual feature is convenient for magnetic sensing similar to that enabled by Lorentz force magnetometers. The magnetic field detection in the TEMST device is facilitated by the audio sensing system.

Here’s a link to and a citation for the paper,

Thermo-electromagnetic sound transducer based on carbon nanotube sheet by Mikhail Kozlov and Jiyoung Oh. J. Appl. Phys. 116, 094301 (2014); http://dx.doi.org/10.1063/1.4894143 Published online Sept. 2, 2014

This paper is behind a paywall.

Nano oscillation and music

It’s one of those breakthroughs that sounds exciting but is a little hard to understand if you’re not working in that field … still … Scientists at the National Institute of Nanotechnology in Canada have solved a problem that was preventing more widespread application of nano-electro-mechanical systems (NEMS). They’ve developed a technique to control vibration/oscillation that could be compared to ‘unringing a bell’. The ability to stop the vibration of a nano cantilever in less than a nanosecond opens up new possibilities in information and communications technology (ICT) and other fields.  There’s a more detailed article about the work here at Nanotech Wire or here at Nanowerk. The research is described in the Nov. 2, 2008 online Nature Nanotechnology article, “Time-domain control of ultrahigh-frequency nanomechanical systems,” the abstract is here. The article itself is behind a paywall.

Chinese researchers are investigating ways to exploit the acoustic properties of carbon nanotubes which are usually lauded for their strength and their electrical properties. Shoushan Fan and colleagues from Tsinghua University in Beijing and Beijing Normal University created sheets of carbon nanotubes and sent audio frequency currents through them as if they were music speakers. However, unlike a standard speaker which creates sound by emitting a vibration, the scientists did not detect any vibrations from the ‘carbon nanotube’ speakers. The researchers believe that the carbon nanotube speakers work as thermoacoustic devices using temperature and pressure oscillation in the surrounding air to emit sound. For more including a video clip of the carbon nanotube speakers in action and a brief mention of 19th century thermoacoustic devices, go here.

One more reminder about Visible Verse, the video poetry event on November 6, 2008 at Pacific Cinematheque (1131 Howe St., Vancouver) at 7:30 pm. Tickets and more info. here.