What is an acoustic metamaterial?

Metamaterials have been a hot topic in the acoustics community since the late 1990s, but there's no  consensus among researchers as to what a metamaterial actually is or when they first came about. Christina Naify (University of Texas - Austin) took a deep dive into the literature about metamaterials and then posed the question to an audience of researchers in a session at the ASA conference in Chicago. In this episode, we talk to her about what came up during this discussion.

Associated paper: Christina J. Naify,  Alexey Titovich, and  Michael R. Haberman . "What is an acoustic metamaterial?" 51, 065002 (2023). https://doi.org/10.1121/2.0001813.

Read more from Proceedings of Meetings on Acoustics (POMA).

Learn more about Acoustical Society of America Publications.

 
Music Credit: Min 2019 by minwbu from Pixabay. https://pixabay.com/?utm_source=link-attribution&utm_medium=referral&utm_campaign=music&utm_content=1022

Transcript

Kat Setzer  00:06

Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, editorial associate for the ASA.

Kat Setzer  00:25

Today we're talking to Christina Naify, whose article, "What is an acoustic metalmaterial?" appeared recently in Proceedings of Meetings on Acoustics, and is based off a talk she gave at the 184th ASA meeting in Chicago. Thanks for taking the time to speak with me today, Christina. I've been really curious to know more about metamaterials since there really are a hot topic and the acoustics community these days. How are you doing?

Christina Naify  00:46

I'm doing well. Thank you for having me. I'm excited to get to talk about this POMA and the presentation, it was kind of a interesting experience. Unique. 

Kat Setzer  00:56

Yeah, it sounds very unique. So first, just tell us a bit about your research background.

Christina Naify  01:00

Yeah, so I got my bachelor's in mechanical engineering at UC Berkeley and my masters and PhD in material science at the University of Southern California. In general, my research has been on acoustic materials, specifically metamaterials, for the last 15 years or so. I also do a lot of research on 3D printing or additive manufacturing, and how we can use 3D printing to enable new acoustics. During my professional career, I've worked at the Jet Propulsion Lab, the Naval Research Lab, and for the past three years, I've been at the Applied Research Lab at the University of Texas, Austin. In terms of actual metamaterials research, I've done a pretty wide range. I've worked on designs for underwater, for air, I do design building testing with a range of functionalities, from antennas to sound blocking materials, to materials that can steer waves. I've been in ASA members since around 2009, and I'm the current chair of the Structural Acoustics and Vibration Technical Committee.

Kat Setzer  02:02

Awesome. So before we get into what a metamaterial is, can you tell us how this presentation and paper came about?

Christina Naify  02:08

Sure. I'll start off by describing what was kind of unique about this presentation. Instead of a typical ASA, or technical talk where I shared information with the audience, this presentation included a combination of me sharing information with the audience and then asking for feedback directly from the audience. I used a software or website called Mentimeter to engage with the audience and let them provide real time feedback to questions that I posed to them. 

Christina Naify  02:40

There were two main motivations for this. One is that I've had hallway discussions for years with colleagues about what is or is not a metamaterial. Some people have very strong feelings about, you know, this research topic is definitely a metamaterial, and this one is definitely not. Some of those differences are relatively subtle, but some people feel really strongly. 

Christina Naify  03:02

The second motivation is that we have a standing acoustic metamaterials session in the Structural Acoustics and Vibrations TC that's been going on since the Boston meeting in 2017. This was set up in order to always have a session for metamaterials and avoid conflicts where there's no session and then talks get put into a variety of different sessions. And I've been cochairing the standing session since 2017. Because of that, I've seen hundreds of talks on acoustic metamaterials, and the majority of them, even though the topic has been presented at the ASA for a while now, begin with an author-supplied definition of what is an acoustic metamaterial. Again, since those definitions vary, I thought it would be interesting to see if the community could agree on any common themes and what those themes would be. As for the style of presentation, specifically, the audience participation was actually inspired by Lily Wang's keynote lecture at the Denver meeting in 2022 to use Mentimeter to encourage audience participation in her keynote. And I liked the idea of an ASA talk not being a one-way flow of information. And this seemed like an interesting topic to try that out, basically.

Kat Setzer  04:21

Yeah, that sounds like a really fun presentation to be involved in, or session to be involved in, especially since it sounds like the concept of the metalmaterial is so nebulous in a way. So I'm going to steal a question directly from your article. Why do we use the word metamaterial?

Christina Naify  04:36

The word metamaterial, as far as I can tell, to describe a class of materials, was first coined around 1999 by the Defensce Advanced Research Project Agency, in a report. That report aimed to gather information about artificially constructed materials which possess qualitatively new responses that do not occur in nature. So that's the first sort of reference to that word. But I've seen the origin of the word itself, "meta" coming from Greek, and meaning "beyond" or "after." So metamaterials are materials that are beyond materials, essentially.

Kat Setzer  05:11

Okay, interesting. So how did metamaterials initially come about?

Christina Naify  05:17

That's a little bit hard to pinpoint. I would say the theoretical framework that most people consider to be the origin of sort of designing materials with exotic properties, most people would point to a paper in 1967 by Victor Vesalago. That paper had to do with electromagnetics, not acoustics, and published a theoretical analysis of materials that would have negative permittivity and permeability-- so permittivity relating to, like, an electrical response, and permeability relating to a magnetic response. 

Kat Setzer  05:50

Okay.

Christina Naify  05:51

Those unique properties weren't demonstrated until 30 years later, around 1999, and the field has grown since then. It's important, a little bit, to make distinction about effective properties. Because when we say negative properties, those are properties that are based on a dynamic response to a stimulus, not a static response. And that becomes important when we move to the realm of acoustics, where we're talking about things like density and modulus and negative density  tends to get some people's feathers riled. So. 

Kat Setzer  06:27

Okay. Okay. So let's actually make that transition. You said metamaterials started with a focus on electromagnetic waves, but how did they end up getting adopted for acoustic purposes?

Christina Naify  06:39

Acoustics and electromagnetics are very similar. They're both waves, and instead of permittivity and permeability... so electric fields and magnetic fields... In acoustics, we can think about the fact that propagation of a wave is controlled by designing effective density and bulk modulus. So if we think about those negative effective properties that we saw demonstrated in electromagnetics, the analogues to those would be negative effective density and bulk modulus. Rulk modulus is the measure of resistance to a substance to bulk compression. So when you squeeze on something from all directions, how much it compresses. And it's important to remember that negative density is an effective property, like I said, and so if we think about density in terms of Newton's second law, which is like force equals mass times acceleration, if the force on an object and the acceleration on an object are going in opposite directions, then that equation says that the density or effective density or mass has to be effectively negative. And then similarly, for bulk modulus, if you squeeze on something and it expands, again, as an effective property, that would be negative effective bulk modulus.

Kat Setzer  07:56

Interesting. Okay. So for this study, you reviewed the literature to see the different ways researchers define metamaterials. How did you go about doing this? And what did you find?

Christina Naify  08:06

So as I said, for the article, the first place I looked for this presentation in particular was Wikipedia. For someone who doesn't know anything about this field, and hears a buzzword, that's going to be the first place they look, and I wanted to just see what that said. I then surveyed review articles, of which there are many. This field has been around for 20 plus years, and so there have been a lot of review articles over the years. In the interest of time, I stuck to review articles because there are thousands of papers on metamaterials. But I figured by quoting or investigating review articles, I could cover definitions supplied by leaders in the field. So people who are experienced enough to write a review article would likely have a unique definition. In this presentation, we highlighted some of the recurring themes across those articles. But the review article definitions vary widely. So in one paper, the author said that, in order to be a metamaterial, the material must be artificial, typically, but not necessarily periodic, composed of small meta atoms, and in their bulk response have unconventional properties. Another review article says that the material must have properties found in nature. And that's it. There's a really wide definition of, like, how specific people are about like, it has to have these six things or just this one thing. 

Kat Setzer  09:31

Interesting.

Christina Naify  09:31

And that kind of feeds into this dialogue. 

Kat Setzer  09:34

They even sound a little bit contradictory at times. 

Christina Naify  09:36

Yes. 

Kat Setzer  09:37

Yeah. 

Christina Naify  09:39

So one of the things that I thought was interesting, and to poll the audience, I asked what people thought the first acoustic metamaterial was. In many review articles that include some sort of history or background as to the field, a specific paper that was published in Science in 2000 is called out as the first acoustic metamaterial. But in careful reading of that article, they actually don't use the word metamaterial at all. And so I wanted to kind of ask the question about whether that material was a metamaterial, to gauge whether, you know, if the author doesn't say it's a metamaterial, is it? 

Kat Setzer  10:17

Right. 

Christina Naify  10:18

The overwhelming majority of the audience said that it was. So I thought that was kind of interesting. That's a very early paper. So I don't know, you know, how much stock to put in that. But I thought it was an interesting assessment that even though the author of that paper didn't specifically say that that was a metamaterial, the audience said that it was.

Kat Setzer  10:39

Right. Well, if the author didn't know about the concept of metamaterial yet, like, would they today apply that term to their work?

Christina Naify  10:47

I think it's likely. 

Kat Setzer  10:48

Yeah.

Christina Naify  10:48

Yeah, I think it's likely. Subsequent articles by the same authors on similar types of structures... And I'll point out that something that's interesting about metamaterials is that they're not materials, the way that we might conventionally think of a material like aluminum or, you know, plastic or ceramic, things like that. The metamaterials are aggregates of those materials arranged in specific order and/or disorder, arranged in a specific way to elicit a specific response. So, you can make an aluminum metamaterial, you can make a plastic metamaterial, you can make materials out of other base materials. So, when you see a metamaterial paper, the picture of the material itself may not look like a material the way that you may think, but the order in which things are arranged. So, what we refer to as a microstructure, which is structure or the way that the material is cut or assembled, elicits the unique response. 

Kat Setzer  11:57

Okay. Okay. So let's talk a bit about that polling that you did for this presentation. You actually polled your audience about metamaterials to crowdsource some answers to various questions. How did you decide on the questions that you used?

Christina Naify  12:10

Ultimately, the goal of the questions was to feed into a common definition, if we could come to one. So when we set out to compile this presentation, we knew from the start that we wanted to turn it into a POMA. It's a good way to capture this sort of unique presentation style and the result of the polls. And a POMA is a perfect way to do that. So in order to set the stage for the more technical questions that were later in the presentation, we actually started by asking the room, which had about fifty audience members, how long they had researched metamaterials. This is useful in the context, that the rest of the questions, you know, rely on basically some knowledge of the state of the field. And so if people were not familiar at all with the topic, then the responses to those questions would be, you know, certainly skewed by that. Because this was an acoustic metamaterials session, most of the audience, I think only a handful of people said that they don't study metamaterials at all. And the most common response was about zero to two years. And that was just yeah, like I said, to kind of give context for the responses to the answers to the question. 

Kat Setzer  13:27

Right, right. Okay, that makes sense. So you also asked the audience a number of questions regarding whether or not various materials are actually metamaterials. What were some of the examples that you used? And how did folks respond? 

Christina Naify  13:38

We structured a series of questions, titled, "Is it an acoustic metamaterial?", centered around specific themes that tend to reoccur in the acoustic metamaterials literature. The themes that we picked were porous media, so structures or materials that have holes in them, Helmholtz resonators, perforated panels, and bubbles. And then we picked a range of examples for each of those topic areas and asked the audience whether each example was a metamaterial. The examples were also chosen so that one or two of those examples, we were pretty sure people would say were metamaterials, because, for example, the author specifically called them that, or for people in the field those were relatively well known examples, and then picked similar types of examples from the literature that are sort of related to that metamaterial design but in a different field. So for Helmholtz resonators...

Kat Setzer  14:39

Can you define what a Helmholtz resonator is?

Christina Naify  14:42

Oh, sure. It's like a bottle. So you know how when you blow across the top of a bottle and you get like a sound from that?

Kat Setzer  14:49

Yes.

Christina Naify  14:49

And if you change the amount of you know, water in the bottle, you get a different sound. 

Kat Setzer  14:54

Yes.

Christina Naify  14:54

That's an example of the Helmholtz resonator. So it's a mass spring system. The spring is is the air inside the bottle and the mass is actually the air inside the sort of neck of the bottle. So you can get like a resonant response from that.

Kat Setzer  15:08

Okay.

Christina Naify  15:09

Or like a resonant response, being like a single frequency response. 

Kat Setzer  15:12

Okay, got it. Thank you. 

Christina Naify  15:13

So for Helmholtz resonators, people have shown that you can arrange a series of Helmholtz resonators, and get negative effective properties. But Helmholtz resonators are also used in architectural acoustics to change the acoustic response of a room, including designs from a POMA that I found dating back to the 15th century when they obviously weren't calling that a metamaterial. And so the question was, is that design from the 15th century a metamaterial if they have Helmholtz resonators, designed to elicit a specific acoustic response. In general, only about a third of the audience said that it was but a paper from 1952, also one tuning Helmholtz resonators, was voted as a metamaterial by half the audience. So I'm not really sure what to make of that. I think some of it is just people have their own idea about what should or should not be. Ultimately, those three designs, the one that we call a metamaterial, or the authors did, the paper from 1952, and the architectural acoustic examples, are all arrangements of Helmholtz resonators. And people had very different opinions about which ones were metamaterials or not.

Kat Setzer  16:27

That's very interesting. So how did the audience end up expanding on or editing the definitions that you found in the literature?

Christina Naify  16:33

So the definition that I collected or we collected from the audience was primarily achieved by asking the audience to populate a word cloud. A word cloud shows words or ideas, phrases, things like that, submitted by the audience. And as more people submit the same exact word, that word grows in size to emphasize its importance. So we asked the audience for words or phrases that describe an acoustic metamaterial and allowed each participant to submit up to four entries. So the word cloud is included in the POMA. And the word with the most submissions was subwavelength. One thing I noticed that was interesting from that word cloud was that the idea of extraordinary properties, which was really important, sort of, in the initial definition of metamaterial did not show up as a very defining feature in the word cloud. It's definitely there. And it's there in several forms. But it's not the most defining feature of what the audience said a metamaterial was. Even less common was the idea that the response that was designed had to go beyond, right? This idea of, you know, really exotic properties. And in general, I would say that the definition was narrower than I expected. This is especially true compared to the Wikipedia definition. That definition includes a phrase like sonic crystal as being equivalent to metamaterial. And whether those things are equivalent, I don't know the answer. I think it would have been interesting to ask the audience whether they think a sonic crystal, which is just a periodic array of individual scatterers--so you imagine a bunch of spheres arranged in a uniform manner that would generally be referred to as a sonic crystal. It could also be a metamaterial. Wikipedia says that they're the same. I don't know that... That didn't come out in the poll, or in the word cloud. 

Kat Setzer  17:45

Right? Did the session end up changing your definition of an acoustic metamaterial?

Christina Naify  18:40

I think the main thing that sort of stood out to me was the emphasis on subwavelength structure. I don't necessarily always include that in my definition. I have called things or thought of things as metamaterials, that do not have a subwavelength structure. And by subwavelength structure, I mean that so if you have a bunch of scatterers, a bunch of spheres, for example, sort of all arranged uniformly in what we would call a lattice, so spaced evenly from each other. A lot of convention in the field indicates that the spacing between two of those scatterers has to be smaller than the wavelength of an impinging wave in order for the bulk arrangement of spheres to be considered a metamaterial. 

Kat Setzer  19:29

Okay.

Christina Naify  19:30

That has to do with sort of this idea of a homogenized property, so that the wave doesn't necessarily see or-- I use the word "see" kind of awkwardly, but that's, that's how we describe it. The wave doesn't necessarily see each individual sphere; it sees the collection of them. That's very similar to kind of the way that we physically see, you know, materials. So if I look at the door, I don't see individual atoms. I see, you have bulk response of the atoms that make up the door, right? 

Kat Setzer  20:03

Got it. 

Christina Naify  20:04

And that's just because the wavelength that we see at the visible electromagnetic spectrum is larger than the spacing of those atoms. So we do the same thing in metamaterials, to arrange what would be called metaatoms and get a bulk response. But I've definitely called things metamaterials that don't have that specific property. I also don't know that I've ever really written down a definition for myself of like, what is or what is not a metamaterial. So one of the reasons for doing this presentation was to crowdsource, right? 

Kat Setzer  20:43

Right.

Christina Naify  20:44

To kind of figure out what does the community think, especially because there's so many different definitions, people feel very strongly, like I said, and to try to gauge what are the common themes? 

Kat Setzer  20:56

Okay, yeah. So did you have any other major takeaways from these poll questions with regards to what researchers considered to be metamaterials?

Christina Naify  21:03

I think the major takeaway that I had, which I knew going in, but was definitely reaffirmed via this exercise is that there's no definition. The definition of metamaterial is in the eye of the author, or the researcher, or the person in the audience who's listening to a talk. If someone says that their research topic is a metamaterial, then you know, then it is. If someone says that it's not, then that's fine, too, especially because there isn't an agreed upon definition. So... 

Kat Setzer  21:34

Interesting.

Christina Naify  21:35

That was my major takeaway.

Kat Setzer  21:37

I feel like that could end up with a lot of arguments among the research community.

Christina Naify  21:41

And that kind of speaks back to my original, one of my original motivations is sort of the hallway arguments or discussions I've seen, you know, with my peers, about which boxes you have to check in order to make something a metaamaterial.

Kat Setzer  21:59

Do you have any other closing thoughts?

Christina Naify  22:00

I thought this was a really fun exercise, I think it would be interesting and engaging to see more of this type of sort of dialogue between a presenter and an audience at ASA conferences. It's a really fast way to get feedback on the state of a research topic. And ultimately, specifically from this, you know, this presentation, whether something is called a metamaterial is not really important. It's a word that we use to sort of classify a field of research. Specific examples, whether they fall into that field or not, like I said,  it's in the eye of the author. Yeah... 

Kat Setzer  22:37

Right. Right. 

Christina Naify  22:38

I do hope that people enjoyed the presentation in Chicago.  I got feedback from it, and people seem to like the interaction. And I encourage people to take a look at the POMA and see the full results of this survey. And yeah, check out the word cloud. I think it was an interesting arrangement or aggregation of thoughts on this topic.

Kat Setzer  22:57

Yeah, it's a really interesting POMA. We will link to it in the show notes, so folks can go see it. Iefinitely recommend looking through it. It sounds like a really fun presentation. And hopefully, it will, you know, maybe inspire other people to set up sessions like this to create this dialogue, because it does sound like a lot of fun and maybe helpful for the community to kind of expand how we think about, you know, like you said, the major topics or hot topics or new topics. It's really fascinating to think about how nebulous this concept of a metamaterial is, even if it's so prevalent in research today. 

Christina Naify  23:30

Yeah, I mean, these talks have been around, you know, the ASA for almost 15 years, and we still can't agree. And the field has been around for almost 25 years, and there's no consensus, you know, about what's important. The audience said, subwavelength was really important. Some review article definitions don't even mention that as a requirement. So yeah, I don't know. I thought it was... I thought it was interesting, right. I mean, that was, that was the main goal was to elicit a response and try to see if there was a consensus. 

Kat Setzer  24:05

Yeah, 

Christina Naify  24:06

I don't think there is. That's okay. 

Kat Setzer  24:09

Well, I found this really interesting. And I hope that our listeners have enjoyed this discussion as well. And hopefully it will expand folks' thoughts about how the concept of what metamaterials are and how they can be used. Thank you again, for taking the time to speak with me today. I really appreciate it.

Christina Naify  24:23

Yeah, thank you. This was really fun. And I really enjoyed sort of going back through the POMA and the presentation and getting an opportunity to talk some more about sort of how it came to be.

Kat Setzer  24:36

Thank you for tuning into Across Acoustics. If you'd like to hear more interviews from our authors about their research, please subscribe and find us on your preferred podcast platform.