Xin Zhang on WSJ’s The Future of Everything Podcast
WSJ podcast explores the most advanced research on sound reduction. BU Professor Xin Zhang is one of three researchers interviewed. She discusses practical applications and real-life solutions.
The following was originally published on the Wall Street Journal website. Click to listen.
No More Noise 2: Metamaterials Can Make the World a Quieter Place
Materials scientists are getting creative in the quest to quiet our increasingly noisy world. Using metamaterials – man-made materials with special properties not found in nature – researchers could soon reduce or eliminate unwanted industrial sounds.
Full Transcript: This transcript was prepared by a transcription service. This version may not be in its final form and may be updated.
Janet Babin: There’s this sound that’s happening around the world. Kind of like this. It can be hard to detect with traditional acoustic instruments, and some people feel it more than they hear it. It can happen in the city or the country. And it’s slightly different depending on where in the world you are. Some describe it like a low rumble and others call it a constant droning sound. But people have come to call this sound, The Hum.
Trevor Cox: Yes, we have The Hum, The Bristol Hum is probably the famous one, Bristol being a city down in the south, which is actually where I was born.
Janet Babin: This is Trevor Cox. He’s a professor of acoustic engineering at the University of Salford in England, near Manchester. He started studying acoustics in the 1990s. He says The Hum sometimes registers in the low frequencies, around the edge of what humans are capable of hearing.
Trevor Cox: I get quite a lot of regular emails from people saying, “I’ve got this terrible low frequency sound. Can you help me here?”
Janet Babin: These are sounds that once you hear them, people often describe them as near impossible to unhear. There’s heated debate about where the sounds come from. Professor Cox suspects they come from a variety of sources, many that have to do with the reality of 21st century living.
Trevor Cox: Some of the weather events that create hum, we don’t get such extreme weather in Britain. But wind farms, distant Sonic booms, if we have aircraft flying very fast, or industrial processes that turn on and off and create this sound. Yeah, we have all of those sound effects as well.
Janet Babin: Another confounding quirk about The Hum is that not everyone hears it.
Trevor Cox: If we stick a couple of people in a room, one person might be hearing it, one person might not, because we seem to have quite different sensitivity to these low frequency signs. And if we have this position where someone is hearing something and their friends say, “Well, it isn’t a problem. I can’t hear anything.” And then that starts to make you doubt whether it’s really there or not.
Janet Babin: More and more research connects the body’s reaction to unwanted noise with physical symptoms like hypertension, cardiovascular disease, even mental illness. This research has increased the work being done to combat noise pollution. And the problem is garnering more attention from material scientists and engineers. They’ve begun playing with new types of substances, meta and nano materials, to tackle unwanted sound at its source. And they’re starting to get results.
Janet Babin: From the Wall Street Journal, this is The Future of Everything. I’m Janet Babin. Today on the podcast, part two of our exploration of our ever noisier world, and new solutions to make it a quieter place.
Janet Babin: Researchers have been thinking about how to reduce noise for a while. In the past, the way to soundproof something was to create thick acoustic barriers made of heavy sound trapping materials, but those are impractical for addressing some of our modern day noise emitters. Transportation, machines, HVAC systems, that kind of heavy barrier is not going to work for these things. Barriers that block sound often also prevent air from getting through. The world needed to find a way to create sound barriers that were light as air instead of thick and heavy. So researchers began playing around with alternative materials in their quest to make sound disappear.
Nick Fang: This is Nick Fang calling from MIT. I’m currently a professor of mechanical engineering and we have been working on capturing light and sound waves using nanoscale materials and structures.
Janet Babin: Nick Fang, now at MIT, is one of the researchers who’s been defining the cutting edge of acoustic engineering. Back in the early two thousands, Fang was working on a doctorate in mechanical engineering at UCLA. He and a partner were trained to create a material that would make objects invisible, like an invisibility cloak. Yeah, very Harry Potter.
Nick Fang: We were collaborating on the concept of invisible cloak and more fundamentally materials that lead to transparency or the exotic effect of refraction of light.
Janet Babin: This might sound crazy, but you’ve actually seen it happen in real life. Nature has its own way of making an invisibility cloak. Like when you see a car on a highway in the distance, sometimes it seems as if the car is floating above the road, like a mirage.
Nick Fang: This is actually a very important experience that shows us light can bend over a curved pathway.
Janet Babin: Fang says this happens when the ground is hot, but the air is cool. The layer just above the ground is warm and it’s so warm that it can bend or refract the light back upwards.
Nick Fang: And this is because the index of refraction of air, because of heating, is changing gradually from the bottom to close to the ground and to the coat side.
Janet Babin: And it appears that this light is coming from the ground. Viewed at the right angle, the highway seems to disappear.
Janet Babin: For his doctorate, Fang was looking at how to create materials that would replicate this effect without heat, a way to make light bend so that objects underneath the material seemed to vanish. In 2004, Fang was almost done. The semester was ending. He was finishing up his thesis.
Nick Fang: You can imagine this was kind of late spring, beginning of summer.
Janet Babin: It was the end of his last semester.
Nick Fang: I was tired of writing papers.
Janet Babin: To clear his head, Fang goes to see an Oregon pipe recital at UCLA’s Royce hall.
Nick Fang: I went there by myself. I just wanted to have a random walk on campus and take some kind of relaxing moment.
Janet Babin: Sitting in the theater, Fang had what he described as a Eureka moment.
Nick Fang: Yeah, it was during the concept that we had this connection.
Janet Babin: He started to think about the similarities between the light particles he’d been researching and the sound he was hearing.
Nick Fang: I start to realize the beauty of comparing the optical materials as well as the acoustic materials. At first glance, it was just a mathematical beauty that I found, but later I realized that there are very similar challenges and needs that drive the advancement of both fields.
Janet Babin: Fang took his idea back to his lab mates, but the response initially was a bit flat. The team had been focused on optics, not acoustics, but Fang kept at it and eventually he coauthored a paper showing the similarities between the principles of an invisibility cloak and an acoustical cloak. He demonstrated that in the same way that you can refract light, you can also bend and focus sound waves in a prescribed fashion to go around objects.
Janet Babin: In 2011, he and a team successfully demonstrated proof of this concept. They used ultrasound or sonar to detect a steel rod in a water tank, very much as a submarine would do to detect objects in the water. Then they covered it with their acoustic material, like a cloak. The material was able to bend the specific pathway of that ultrasound so that it didn’t bounce off the post, essentially making invisible for sonar.
Janet Babin: Now Fang’s at MIT and he’s designed a lightweight soundproofing material that’s meant to keep sound from escaping from its source. The material looks like a thin sheet of rubber, like a membrane. Two of these rubber sheets or membrane things are kind of suspended between each other, creating a grid of rubber sheets placed on top of a stiff honeycomb like structure, kind of like corrugated cardboard, but super thin.
Nick Fang: Imagine that now we can coat layer materials onto our surface and such layered material can be tailored such that the speed of sound next to the surface will be higher than the speed of sound over the outer layers.
Janet Babin: Fang says when a sound wave reaches one surface, part of its energy is reflected back to where it came from. How much of the sound is reflected depends on how stiff and dense the material is. The sound absorbing material is lightweight but stiff, so it’s able to reflect airborne noise.
Janet Babin: In the lab, with the special material folded to create a covering of 10 millimeters or just over three eighths of an inch thick, Fang was able to reduce noise from a loudspeaker by 35 decibels. The difference between walking next to a noisy vacuum cleaner or walking into a quiet room at the library. Fang thought about the difference that could make with vehicle noise. He’s since patented this material and it’s on the cusp of becoming commercially available. He’s working with Nissan motor company to build the soundproofing technology into the engine cover of its vehicles, and to add it as a sound insulating shield under the motor. The process is expected to be completed within three to five years. It’ll make life quieter for people riding in the vehicle, and for people walking outside of it as well.
Janet Babin: While Fang’s technology is headed for commercialization, other materials scientists are already working on the next generation of noise control. That’s coming up next.
Janet Babin: The soundproofing technology that Nick Fang created uses man-made materials with properties that aren’t found in the natural world. There are known as metamaterials.
Steven Cummer: If you ask 10 different people, what metamaterials are to them, you’ll probably get 10 different answers.
Janet Babin: Dr. Steven Cummer is a professor of electrical and computer engineering at Duke University. Now you might know that sound gets bounced off of hard surfaces like metal or glass, and it gets absorbed by softer materials like wood or fabric. That’s because of those material’s natural cellular structure, but Cummer says metamaterials can be designed to do either.
Steven Cummer: So for me, the idea behind metamaterials is the idea of using intentionally designed structure in a material to control wave propagation through the material, whether it’s sound waves for acoustic metamaterials or electromagnetic properties for light or radio waves. But the idea is really just not taking what nature gives you in terms of material properties that can control wave propagation, but designing small-scale structure inside a material to give you new or better or different properties.
Janet Babin: These building block structures can be very tiny, but for sound control, they’re not as tiny as you might think.
Steven Cummer: For sound waves, like audio frequencies, the frequencies that we’re listening to and using to converse right now, the wavelength of those waves are like tens of centimeters. And so the size of that small scale structure, what qualifies as a small scale, is totally macroscopic, like a handful of millimeters or even a centimeter.
Janet Babin: But you do have to be specific about what you want. To configure a structure so that it interacts with a specific frequency, Cummer says researchers will spend hours on numerical simulations.
Steven Cummer: Once you have the idea of the kind of structure you want to put together, you always have to fine tune the properties. What’s the grid size? How small does it have to be? All of those details influence the resulting properties. So everybody in this business does a ton of computer modeling to fine tune the properties of that structure before going to experiment.
Janet Babin: Cummer says a decade ago, researchers were hyper-focused on delivering exotic physical properties that the theory said you could technically make, if you found the right building blocks. There was less concern about targeting a specific application for the materials.
Steven Cummer: Now the field is, I think, much more focused on trying to find practical applications or practical devices that can be improved by using acoustic metamaterials.
Janet Babin: And that is the approach of Dr. Xin Zhang’s team. She’s a professor at the College of Engineering at Boston University. The team went into its research with a specific goal.
Xin Zhang: The question though for whether we can silence sound waves while maintaining airflow has inspired the research community for decades.
Janet Babin: In many situations, it can be impossible to use soundproofing because it blocks air flow. We don’t think about this much, but it’s pretty obvious. Adequate air flow is vital to mechanical systems. If there’s a lack of air components can overheat, they can fail, function inefficiently, or break down altogether. And some of these are machines or applications that get blamed for that humming sound like wind turbines, propellers, engines, cooling fans, pipes. So there are all kinds of mechanical applications that would benefit from lightweight, see-through soundproofing that can stop noise, but still allow air to flow freely. And that’s what Zhang’s team did. They created a soundproofing device that Zhang calls an ultra-open acoustic silencer. It is shaped like a ring. It’s completely open in the center, like a donut. In the lab, it was able to stop 94% of sound energy or sound waves coming out of a loud speaker. Professor Zhang says the size of the device is flexible. Again, it’s a mathematical design of the metamaterials on the inside that give it soundproofing abilities.
Xin Zhang: It’s silencing performance arises from a structure’s shape rather than constitutive materials.
Janet Babin: That structure is made up of six tiny helical or a corkscrew-shaped channels, wrapped around a central porch. Professor Cummer at Duke reviewed the work, but was not involved in its design.
Steven Cummer: It turns out then that as sound tries to propagate through the channel, it interacts with those cavities that you’ve created that are off to the side. And with that, you can actually create frequency ranges that will not travel through those open channels, but instead are stopped.
Janet Babin: The sound is canceled out after its interaction with those open channels. In the lab, professor Zhang’s team attached the ring shaped silencer to a pipe on the outside of a loudspeaker. And the noise went from this. To this. Zhang says the acoustic silencer is flexible not just in shape, but in the range of frequencies it can stop. So it can be tailored to various sizes, uses, tones, and types of sounds.
Xin Zhang: We developed low cost, 3D printable acoustic metamaterials, capable of canceling noise at a specific frequency without blocking airflow.
Janet Babin: 3D printable materials can reduce costs and ease scalability. Zhang says she’s still getting inquiries from companies regarding the acoustic silencer. By being so open, the silencer is able to uniquely address some of the key problems with many prior iterations of noise counselors. Again, that they don’t allow for that important airflow.
Janet Babin: Of course, every sound, each kind of noise has its own signature, its own frequencies, and it will probably require its own solution. So The Hum, if we’re ever able to definitively identify it, will likely have to be dismantled piecemeal. Professor Cummer at Duke says wide scale commercialization of metamaterials could change society. By some estimates for market research firms, the metamaterials market is valued at more than $300 million this year. But the research and development can also be a slog.
Steven Cummer: I know this firsthand from my own research, when you’re coming up with these new structures, you maybe have to make 10 or 20 versions of it before you get one that works, because of all those fine details. And one out of 20 is just fine for scientific publication because your goal is just to show that you can do it. But then once you start talking to a company, you need to be able to hit 20 out of 20. And so there are a ton of problems that need to be solved on the design and manufacturing side.
Janet Babin: It took Nick Fang more than a decade to get his metamaterial from concept to cars. That’s not to say that all commercialization projects would take that long, but the world is not getting quieter, and new sounds, like the were of wind turbines or the buzz of delivery drones, will likely add to the din. All the more reason for us to start planning for the future of noise now.
Janet Babin: The Future of Everything is a production of The Wall Street Journal. Stefanie Ilgenfritz is the editorial director of The Future of Everything. Leigh Kamping-Carder is deputy editor of The Future of Everything. Our fact checker is Maddie Bender. Thanks to our intern, Ava Sasani. Our sound designer is Sarah Gibb Alaska. Kateri Jochum is The Wall Street Journal’s executive producer of audio. And I’m Janet Babin. Thanks for listening.
Professor Zhang is a professor of mechanical engineering, electrical and computer engineering, biomedical engineering and materials science and engineering