Sound Absorption and Diffusion

Anybody who's been in a noisy restaurant can tell you how the acoustics of a built environment can impact individuals in the space. In particular, absorption and diffusion of sounds can greatly affect the acoustics. This episode, the editors of the Special Issue on Sound Absorption and Diffusion: Modeling, Measurement, and Application, Mélanie Nolan (Universidad Politecnica de Madrid), Peter D’Antonio (RPG Acoustical Systems), and Ning Xiang (Rensselaer Polytechnic Institute), share current research in the field. 

Find all the articles in the special issue here!

Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

Kat Setzer (00:48)

Anybody who's been in a noisy restaurant can tell you how the acoustics of a built environment can impact individuals in the space. In particular, absorption and diffusion of sounds can greatly affect the acoustics of a space. Today I'm talking to the editors of the recent JASA and JASA-EL Special Issue on Sound Absorption and Diffusion: Modeling, Measurement, and Application. With me are Melanie Nolan, Peter D’Antonio, and Ning Xiang. Thank you for taking the time to speak with me today. How are you?

Ning Xiang (01:17)

Fine, thank you. Great.

Peter D’Antonio (01:17)

Just great.

Melanie Nolan (01:19)

Very well. Thank you for having us.

Kat Setzer (01:21)

Thanks for being here. First tell us a bit about your research backgrounds.

Melanie Nolan (01:24)

Sure. So my name is Melanie Nolan. So I'm a research fellow at the Polytechnic University of Madrid in Spain and an adjunct professor at the Laboratoire d'Acoustique de l'Université du Mans in France. I'm also currently chair of the Technical Committee on Room and Building Acoustics within the European Acoustics Association, the EAA. My background is in engineering acoustics. So I first studied general engineering at the Ecole Centrale de Lyon in France, and then I specialized in acoustics with a master's degree from the Technical University of Denmark, where I then also did my PhD. Since then I've been very fortunate to work  inseveral research environments, including Saint-Gobain-Ecofond in Sweden and RPI in the US through the Hunt Fellowship.

So my research has always been about sound absorption and acoustic materials. I study how materials interact with sound waves and how we can measure and predict their absorption properties more accurately. So I use a combination of advanced measurement techniques, wave field analysis, and acoustic modeling to better understand what's happening when sound meets a material and how this knowledge can be applied to designing better acoustics in rooms.

Ning Xiang (02:38)

My name is Ning Xiang. I'm now at the Renssalaer Polytechnic Institute as a professor, and I have nearly 40 years of experience in studying and doing research in acoustics, especially in architectural acoustics. So my research in the direction, including also room acoustics, noise control, and signal processing, my recent focus, especially, in many of the topics, the recent topic, including room acoustic simulations, characterization of sound absorbers, and then sound diffusers, bending wave parameter characterization of physical elastic panels. So I'm also very active in the Acoustical Society of America, and I'm also part of the European Acoustic Association because I'm also the long-term member of the German Acoustical Society.

My recent involvement with Acoustical Society, actually along many decades of the including also the associate editor for the Journal of Acoustical Society of America and I'm now serving as coordinating editor for the section of architectural acoustics. And I also authored and co-authored a couple of books. I don't want to actually name all of them, and thank you for your question.

Peter D’Antonio (04:12)

My acoustic career began in 1983 when I founded RPG Diffuser Systems to develop and experimentally validate novel diffusive and absorptive surfaces that would expand the acoustical palate. This work pioneered the modern sound diffuser industry and led to the creation and implementation of a wide range of innovative acoustical surfaces. Over the course of my career, I've lectured extensively and published numerous scientific papers in peer-reviewed journals. Together with Professor Trevor Cox, I co-authored three editions of the reference textbook, Acoustic Absorbers and Diffusers.

I also contributed chapters to the Master Handbook of Acoustics and the Memorial Volume in honor of Manfred Schroeder. I've contributed to international standards for measuring the scattering and diffusion coefficients. I'm a fellow of both the Acoustical Society of America and the Audio Engineering Society, and in 2012 I was inducted into the music industry's Technology Hall of Fame. In 2023, the Acoustical Society of America awarded me the Wallace Clement Sabine Medal for contributing to the theory, design, and application of acoustic diffusers. I'm also president and founder of the Chesapeake Acoustic Research Institute and Director of Research for RPG Acoustical Systems and Ready Acoustics.

Kat Setzer (05:30)

Man, you all have such impressive backgrounds. So how did this special issue come about?

Melanie Nolan (05:35)

This special issue really came from the recognition that sound absorption and sound diffusion are fundamental topics, and not just in room acoustics, but really across almost every area of acoustics. So whether you're working on noise control, virtual acoustics, acoustic metamaterials, or soundscapes, understanding how sound interacts with materials is essential. So in room acoustics, these materials are key to controlling noise and creating comfortable acoustic spaces, but accurately measuring and predicting their behavior is still a real challenge, and that's exactly what makes this such an active and exciting area of research. Over the years, we've seen many different approaches that have been developed to measure and model sound absorption and diffusion from classical laboratory methods to more advanced techniques. And at the same time, we are seeing rapid developments in new materials and modeling tools. And this has really created a need to bring together the latest research and highlight current challenges and opportunities in this field. So the idea behind the special issue was to sort of gather contributions covering the full chain from understanding the physical mechanisms behind absorption, scattering, diffusion, to improving measurement methods, developing better models, and designing new acoustic materials and solutions.

Kat Setzer (06:56)

Okay, okay. So to take like a larger view, what are absorption and diffusion and how are they related?

Peter D’Antonio (07:02)

So absorbers are designed to essentially soak up energy and prevent reflections from entering and returning into the room. High mid-frequency absorbers are generally porous, fibrous open-cell materials, like fiberglass, mineral wool, felt, and fabric wrap panels. Porous absorbers dissipate the energy through internal friction. And mid-low frequency absorbers, like Helmholtz resonators, dissipate energy through resonant motion, and they're called reactive as opposed to passive absorbers. Absorbers are intended to improve speech clarity, control echo, reverberation reduction, and flutter echo elimination. And they're characterized by an absorption coefficient. 

Diffusers, on the other hand, preserve the acoustic energy in the room, but redistribute it so that reflections are weaker, softer, and arrive from many different angles. They include geometrical surfaces but also reflection phase grading diffusers, which are a little more scientific. Geometric convex shapes, like cylinders and pyramids, specularly reflect sound, like sound from a mirror. Whereas diffusers are called phase gratings. They consist of a periodic array of wells of varying depth or height respectively, determined from number theory sequences. And they cohere at scattering from optical topologies, and they distribute the sound into many different directions at a lower level. And they improve spaciousness and development and provide a natural ambiance. And they're characterized by a diffusion coefficient. So that absorbing and diffusing surfaces serve fundamentally two different functions. One absorbs sound, the other uniformly scatters it.

Kat Setzer (08:46)

Okay. That reminds me, we had a  podcast earlier last year, I think, with Michael Vorlander about, “Show your scattering coefficients,” and it sounds like that's, kind of ,the diffusion's kind of that same idea. It’s that the surface has kind of different textures to it almost, so to speak, to scatter those sound waves so that you just don't ,they don't reflect back and you don't hear it as much. Is that the idea kind of?

Peter D’Antonio (09:10)

Right.

Kat Setzer (09:11)

So how are absorption and diffusion generally used to shape the acoustic conditions of rooms?

Peter D’Antonio (09:16)

Each room requires a different acoustical design. We can begin with what I call production rooms. Production rooms such as a recording studio or a mix room, they must be acoustically neutral, so that the signature of the room does not imprint itself on the recording. So for example, if the room is boomy, the mixing engineer will compensate by reducing the bass, resulting in a mix that sounds very thin elsewhere. It lacks bass elsewhere. And so to prevent this, porous absorption is used to control early reflections and eliminate interfering echoes. And bass absorbers are used to control low frequency issues. These are called modal issues that resonate depending upon the design of the room, the geometry of the room. And so diffusers are typically placed on the rear wall to break up small reflections and create a uniform enveloping sound.

And then you have reproduction rooms, where someone is listening to what was created. To accurately hear the mix as it was created, a reproduction room, whether an audiophile listening room or a home theater, must also be acoustically neutral. As in production spaces, absorption is used to control early reflections while diffusers provide a sense of spaciousness and immersion without adding coloration. And then low frequency control is achieved through a combination of resonant absorbers to maintain an appropriate reverberation time. 

The performance spaces are quite the opposite because unlike small rooms where the goal is to minimize acoustical distortion, performance spaces rely on architectural shaping to create a spacious, diffuse, and engaging listening environment. The desired reverberation time depends upon the type of performance—shorter for speech, longer for orchestral music. Adjustable elements such as velour curtains are often used to fine tune the reverberation time to suit this specific event.

Melanie Nolan (11:09)

Maybe I can just jump in very quickly about ordinary rooms—so everyday spaces like classrooms, offices or hospitals. There, the main goal is usually to create a comfortable environment where people can communicate easily. So sound absorbing materials can help reduce excessive reverberation and unwanted reflections, which then improves speech intelligibility and lowers background noise. Diffusion can also play a role by spreading sound more evenly through the space. And this helps create a more natural and balanced acoustic environment, so making the room feel more comfortable for the people using it. And that's especially important because we know that poor acoustic environments can have real consequences. So, for example, in schools, we know that excessive noise can affect learning and cognitive development in children, or in hospitals, we know that it can slow down patient recovery and make it harder for the staff to do their jobs. So I think it's really in ordinary rooms that we see the, the direct impact that sound absorption and sound diffusion can have on health and well being.

Ning Xiang (12:23)

In addition, so we are talking about absorption or absorbing materials, and the focus so far has been in the rooms, in the space, enclosed space. But even outdoors, even actually for noise control engineering, it is highly critical to have a certain sound absorbing mechanism and deployed in those situations that the noise need not to be actually controlled or mitigated.

Kat Setzer (12:53)

Right, right, that makes sense. I've heard about like in urban soundscapes adding in like rooftop garden spaces or green spaces helping with that and that kind of thing. Is that sort of the idea?

Ning Xiang (13:04)

Right, right. In urban environment on a highway, for example, noise is actually a big issue and to make that actually more comfortable for the human beings and we need to come up with good solutions for that.

Peter D’Antonio (13:21)

Yeah. Also highway barriers, which are important.

Kat Setzer (13:25)

What are the current challenges facing this area of research?

Melanie Nolan (13:28)

Yeah, so one of the biggest challenges is that simply sound absorption, scattering and diffusion are actually very complex phenomena, especially when we move from idealized materials and controlled laboratory environments to real-world applications. So in practice, materials are often heterogeneous and their acoustic behavior can depend on many factors, so their shape, how they're installed and even the sound field around them.

Another major challenge would be measurement. So we have well established standardized measurement methods, but they often rely on simplified assumptions that don't always reflect what happens in real environments. And this becomes particularly important when we're dealing with new or unconventional materials where these traditional approaches may not be enough to fully describe their behavior. 

There is also an increasing need to better connect measurements with acoustic modeling. So today we actually have very powerful numerical tools that can simulate how sound behaves in a room. But making sure that those simulations accurately reflect reality is still a big challenge. And a lot depends on the quality of the input data and especially information about how the surfaces absorb or scatter sound. So if those properties are not characterized accurately, it's very difficult to make reliable predictions about a room's acoustic performance. 

So overall, I would say it's a very active and rapidly evolving field that has made tremendous progress, but bridging the gap between what we measure under control conditions and what actually happens in real spaces remains one of the key challenges, I would say.

Ning Xiang (15:10)

So Melanie talked about also simulations and even room acoustic simulation or sound simulations. there have been well-established tools for room acoustic simulation or sound propagation, particularly relying on geometrical acoustics. Geometrical acoustics is that considering sound traveling propagation as very much similar to the light travel in the direct paths as sound waves. And geometrical acoustics has been around for many, many decades and quite a successful, but the recent trends in our room acoustics research and application focus on the room wave-based simulation. And in that aspect, we really also need to come up with very good tools to characterize the material, for example, or the surfaces, interior, or even outdoors surfaces that interact with the sound waves. 

And Peter and I are recently very keen on the low frequency materials, the low frequency we we are talking about something, you know, 200 hertz below, and the wave-based simulations really also require some reliable data. So in that aspect, the low frequency characterization of the material is very challenging. And also trying to get reliable, accurate measurement is one challenge. Another one is that how you actually incorporate this experimental measured results often is in the complex valued information and incorporate that into the wave-based room acoustic simulation. And in that simulation field there was frequency based, that that means that consider the sound waves in only in the frequency domain, and there were also the approaches also in time domain, such as so-called finite difference time domain method or the SEM method, spectral element method. All these methods are really very powerful and they're very efficient. But there are challenges that how can you get the data, especially in low frequency, because this is a wave-based method, considering the wave propagation in a very actually fundamental theoretical way, and very effective in low frequency particularly. So we need also the measurement data, we need to incorporate those data, even just modeling of those data into simulation is quite challenging.

Kat Setzer (17:58)

Why are low frequency sounds harder to incorporate into simulations than higher frequency sounds?

Ning Xiang (18:05)

I would say the low frequency, the wavelength, the wavelength is that in one circle of the wave, periodic wave traveling within one circle and how long it travels. That wavelength is very long and the characterization of such material becomes very challenging. And so far in the measurement, such as a normal incident measurement, often occurred in a tube, very constrained. The wave guide, so to say, to get the wave going through tube-like structure, but that is very constrained, very, very small size. Even in low frequency, some of the tube can be actually, you know, two-feet-by-two-feet cross section, or that represents only small section of the material. But that small structure in compare with the long wavelengths and there were a lot of issues.

Another challenge is that some of the material, especially in low frequency, some of the material act or interact with the sound waves in a so-called local way. And local way means when the sound strikes material on the surface, and that the prediction of the interaction is relatively easily manageable within that range. But once actually the materials go beyond that, it can behave also non-locally, and that prediction becomes very challenging, especially incorporate that into the model. There was some effort already, but way, way beyond actually the reality.

Kat Setzer (19:53)

Let's pivot into the actual special issue and dealing with these challenges. what were some of the overarching themes that showed up in the special issue?

Melanie Nolan (20:01)

Yeah, so we ended up with thirteen papers in the special issue and in the editorial we grouped them into two broad themes, sound absorption and sound scattering and diffusion. But a few key topics showed up across the special issue. So the first was the continued effort to improve how we model and characterize sound absorption, and particularly for porous and fibrous materials. So many of the papers focused on refining material models, estimating transport parameters more accurately, and exploring new manufacturing approaches.

A second major topic was measurement innovation. So there was a strong emphasis on developing more reliable and realistic ways to measure absorption and diffusion, including in situ methods, angle-dependent characterization, and approaches that address long-standing limitations of standardized measurement methods.

Another recurring topic was the growing integration of data-driven methods with traditional physics-based acoustics. So several contributions used machine learning and hybrid approaches to improve either prediction accuracy or extend measurement capabilities or simplify complex material models.

And finally, on the diffusion side, there was a clearer focus on better understanding and simulating scattering processes. So the papers advanced both the measurement of diffuser performance and the incorporation of more realistic scattering behavior into room acoustic simulations.

Kat Setzer (21:31)

So let's go into one of the first themes that you brought up, which is the study of absorption in porous or fibrous materials. So two questions: First, why are porous and fibrous materials used for sound absorption? And then second, tell us about some of the notable work related to these materials that appeared in the special issue.

Peter D’Antonio (21:49)

Why are fibrous materials used so frequently? The answer very simply is that they're inexpensive, and they're easy to obtain ,and they provide broadband absorption in the mid and high frequencies. So they're very useful to control echoes, reverberation time, in the higher frequencies. And they can be hidden behind acoustically transparent fabrics, allowing the designer to achieve the desired visual aesthetic without affecting the performance. And so when they're used in sufficient thickness, which is not always the case, these materials are very effective at reducing reverberation and improving sound quality in a wide range of rooms.

Melanie Nolan (22:29)

Yeah, and regarding your second question, I think that one of the interesting aspects of the special issue was how it showed that porous and fibrous absorbers are becoming much more designable materials rather than just passive materials that are selected and tested after the fact. So several papers focused on understanding the relationship between the microstructure of these materials, things like fiber, geometry, spacing, porosity, airflow properties, and their resulting acoustic performance. So, for example, one study looked at fibrous absorbers produced through extrusion-based 3D printing. And what was particularly interesting about this work was the idea that additive manufacturing can be used to deliberately control the internal geometry of the absorber. So by changing parameters such as fiber, diameter, spacing, the authors could actually tune key properties, like porosity and airflow resistivity, allowing them to optimize the sound absorption performance.

Another contribution focused on the challenge of accurately characterizing fibrous materials. So that study explored how combining measurements with acoustic models can provide more reliable estimates of material parameters. And that really highlighted the value of bringing experiments and predictive modeling together to better understand and predict why a material absorbs the sound the way it does. 

So I think that more broadly, the papers reflect a larger trend in the development of porous and fibrous sound absorbers. So moving from traditional, largely empirical designs toward materials whose microstructure can be deliberately engineered. So the goal there is not just to create a material that absorbs sound well, but to understand how its internal structure controls the acoustic performance so that we can design thinner, lighter, and more efficient sound absorbers.

Ning Xiang (24:26)

So I see that is very good and I'm very happy to see as Melanie already told. The fiber and porous material previously less designable. And then you create a certain density of the fibers, and then after fact you experimentally measure it, or modeling and say, hey, this has behaved like that. But the recent trend is really very promising, that also reflected in this special issue a couple of papers talking about that understanding the fibers, the microstructure, and how we model that in a way, and use 3D printing to validate their findings and then come up with a very, very nice solution down the road, how can we design that much better?

Kat Setzer (25:14)

Right, right. So it's not as random where like before it was like, “Here's a random fibrous material! Let's see how it does!” compared to, “Okay, this is what we need; let's create it with 3D printing.” Okay.

So a number of the contributions also related to the challenges of accurate absorption measurement across different experimental environments. What kind of findings came up in these papers?

Melanie Nolan (25:35)

So one of the things that really came through the special issue is that measuring sound absorption is much more complicated than it might seem at first. So we often talk about the absorption coefficient of a material as if it's a fixed property. But in reality, the value that you measure depends a lot on where and how you measure it, the sound field around the sample, and the measurement technique itself. So several papers focused on reverberation chamber measurements.

Which are still the standard way of measuring absorption. So the basic idea there is that the chamber creates what we call a diffuse sound field, where sound arrives at the material equally from all directions. And from that, we can calculate a diffused field or random incidence absorption coefficient, which is basically an absorption value that is averaged over all possible angles of incidence.

And the challenge there is that real chambers are never perfectly diffuse. So it's actually possible for two laboratories measuring the same material to get different results. And that's… Actually, this problem has been debated for nearly a century, and it's one of the reasons why the ASA was established in in 1928. There is an interesting Acoustics Todayarticle about that history.

So in fact, some of the questions that were addressed in this special issue have been with us almost since the beginning of modern acoustics. 

But rather than discussing if our reverberation chambers are diffuse, I think that the more interesting question is whether this single averaged absorption coefficient is always the right quantity to begin with. So for very reverberant spaces, like, I don't know, churches or large concert halls, averaging over all angles makes sense. But in places like classrooms or offices where sound often arrives from specific directions. In those cases, a single average number is not sufficient. And that is why there is a growing interest in more detailed ways of characterizing materials, and that includes angle-dependent absorption and surface impedance. So surface impedance gives a much richer description because it tells us not only how much sound is absorbed, but also how the surface affects the reflected sound wave.

So one example from the special issue is a paper that actually the three of us co-authored, where we showed how a reverberation chamber can be used to estimate angle-dependent surface impedance. And that really turns the chamber from a tool that produces a single absorption coefficient into a tool that can reveal much more detailed information about material behavior.

We also saw innovative approaches to dealing with how the sample itself affects the measurement. So, for example, colleagues from KTH and TUM used machine learning to learn the finite size effects of porous absorber samples and predict the absorption that would be obtained for an idealized infinite sample. And that's a very good example of how data-driven methods can help separate material properties from measurement-specific effects.

So I think that the overall takeaway from the special issue is that the field is moving away from asking, “What's the absorption coefficient of this material?” toward a more realistic question, “How does this material interact with sound in a particular acoustic environment?” And hopefully that's leading to more informative measurements and ultimately better acoustic design.

Ning Xiang (29:07)

I want to add on to this, and namely is so Melanie has very well stated. This is a very good. the random incident sound absorption coefficient, this has a long history, starting with our founding scientists and Wallace Sabine, and later on the renowned professor Luther Kramer subdivided also the room acoustic, or architectural acoustic, into three sub-field, the so-called statistical room acoustics,  geometrical room acoustic, and wave theoretical room acoustics. And for those numbers or values, the so-called random incident sound absorption coefficient, often measured in the reverberant chamber, it is still quite useful in terms of statistical room acoustics for those rooms has actually regular or proportionated dimensions, you can quickly calculate using classical equation like a Sabine or Eyring equation, that's fine, but statistical room acoustics give you very limited information. Geometrical acoustics now is quite powerful, but the statistical numbers ,like random incident absorption coefficient or scattering coefficient, has actually not sufficient information to be incorporated in geometrical acoustics. Because of that, the geometrical acoustics suffers from inaccuracy, but still and quite useful up to this point.

And in the wave-based room acoustic simulation or way of thinking, the theory, we see that how the wave interact with the surfaces. And that is not only in the normal incident or in normal incident, that means perpendicular to the surfaces, or the sound coming from all the directions from the same time. Because of that, those numbers or those values becomes less less useful in the current trend.

Kat Setzer (31:12)

Another area of research related to absorption had to do with the design of absorbers. Can you tell us about some of this research?

Peter D’Antonio (31:18)

This is really the domain of what are called “metamaterials,” and these are engineered materials that behave in ways ordinary materials cannot because of their carefully designed internal structure. In a metamaterial, the geometry of the components matters more than the substance they're made of.

The shape, size, and arrangement of the elements determine the material's overall properties. So you could have an absorber that is made from reflective materials. A classical example is the Helmholtz resonator, which was really one of the earliest acoustic metamaterials, even though we didn't think about it in those terms. Even though it's made from non-absorptive materials like wood or metal, its performance comes entirely from its structure. There's an array of holes that have a specific diameter and a spacing in a faceplate of a given thickness backed by an air cavity. This geometry sets the resonant frequency and determines the bandwidth over which this device absorbs sound. On the diffusion side, another example is the reflection phase grading diffuser. It diffuses sound not because of the materials it's made of, but because of the way they are arranged. It consists of a series of divided wells of differing depth arranged according to a mathematical number theory sequence. When the sound strikes this surface, the varying depths impose different phase delays, causing the energy to scatter evenly in many, many different directions, as opposed to a reflection, like light bouncing off a mirror.

Melanie Nolan (32:51)

If I can just jump in about the design of absorbers. I think there was one contribution that really stood out, that was the by colleagues from LAUM in Le Mans, who proposed a novel absorber based on inclined wire mesh grading structures. And what's particularly interesting is that they showed that it is possible to achieve both broadband absorption and nearly omnidirectional performance using a very thin subwavelength structure. And that's something acousticians have been chasing for a long time because there is usually a trade-off. If you want strong low-frequency absorption, you often need very thick materials, and if you want broadband performance, it's often difficult to maintain it for sound arriving from different directions.

Maybe another small comment because we talked about the importance of angle-dependent measurements for sound absorption. It's also really important in metamaterial design because most of these structures are tested in impedance tubes at normal incidence, and that's mainly because it's cheap. You only need a small 3D printed sample. But that also means that we don't really verify how they behave under more realistic multi-angle sound fields. So actually developing cost-effective ways of doing full angle dependent characterization is still a key challenge for metamaterial design.

Kat Setzer (34:08)

Right, right. So we've been talking a lot about absorption, but there was some work related to diffusion and scattering, both in terms of quantification and simulation. Were there any notable results here?

Ning Xiang (34:19)

I think that this special issue also reflect recent activities in that direction to characterize sound diffusers, so namely diffusion and sound diffusion and scattering. There have been a lot of work in the past. One of the leading team is really Peter D’Antonio with Professor Trevor Cox, and they use large fixture in the laboratory, so-called “acoustic goniometer.” And those measurements are quite amazing in the past to characterize the sound diffusers that namely all the angular dependent scattered sound field can be captured by those measurements. 

In this special issue, there was also very promising work just starting, namely using also kind of microphone arrays, but not that circular or spherical, you know, enclosing the material and the test, rather than near the scattered surface. And the Brandão and his co-authors endeavored to do the scanning using microphone… Actually, just use one microphone scanned over the diffusers. And then they use a theoretical way, namely so-called plane wave decomposition, and to resolve the incident and the scattered component of the sound field near the diffusers. That is a very interesting work and hint at also the future effort to characterize diffusers, so-called polar responses of the diffusers. That is quite amazing.

Another way of incorporating scattering also diffusing information in the simulation, there were also a work by Fatela, and they applied a certain diffusion information, so-called distribution of the diffusing materials into the room simulation in terms of the method so-called “radiosity simulation.” The radiosity simulation and also another route is the diffusion equation simulation, those are starting in the essence with optimal or the highest diffusion surfaces. In the classical simulation, there is no way to control the different degrees of diffusing surfaces. And this effort also shows that they can incorporate the scattered distribution into simulation tools. That is also promote a very interesting effort to do that. They apply those work also in a… to simulate the facade and also urban canyon scenarios and demonstrate that the significant of influence of the scattering models into those prediction tools.

Kat Setzer (37:18)

That's very cool. Where do you see this field of research heading in the future?

Peter D’Antonio (37:21)

From my perspective, I think we've outlined three areas. Metamaterials seem to be the current leading area for material development. There's another topic that I'll discuss later, which is called a virtual goniometer, which we developed almost forty years ago to measure the polar scattering of a device. It's very time consuming and there's recent software that we can actually predict those values. I'll talk about that later. And also what Melanie has really leaded the way on is recent advances in evolving the reverberation chamber, in measuring surfaces, which is really long overdue. Those are the three areas I see. And also to minimize interlab variance.

Melanie Nolan (38:04)

Yeah, so indeed I think that the field is moving toward a much more realistic way of describing and designing acoustic materials. So as Peter said, one direction is the move from single number absorption coefficients toward more complete descriptions, so including angle dependent absorption and surface impedance. But of course this development needs to go hand in hand with improvements in acoustic simulation tools. So nowadays, regardless of which simulation software you use, one of the main limitations is still the accuracy of the input data. So we can have very advanced numerical models, but if the material properties we put into them do not represent the real behavior of the surface, the predictions will remain limited. So being able to use more complete material descriptions, so angle-dependent impedance data and scattering coefficients would make simulations much more reliable.

And as I mentioned before, this will also have impact on the design of acoustic treatments. So today most absorbers and diffusers are characterized under standardized laboratory conditions that may not fully represent how they are used in real rooms. So if we can measure and model their behavior under more realistic sound field conditions, we can also design absorbers and diffusers that are optimized for specific applications, for example, where sound arrives predominantly from certain directions. 

And as Ning suggested earlier, another major challenge in sound absorption remains the low frequency range. So traditional porous absorbers work very well at mid and high frequencies but they become very bulky when the wavelengths become large. And this is why we are seeing increasing interest in alternative concepts, such as metamaterials

and other engineered structures. The goal there is to create absorbers that are thinner and more efficient at low frequencies.

Ning Xiang (39:53)

So the near future, we are talking about the future trend. As Peter already highlighted that metamaterial or metastructure, metasurfaces, something like that, are quite promising, that can be actually a very promising direction of the research in the near future. We have been talking about those applications more or like in the rooms. So as I already mentioned previously, so the noise control also highly requires those materials. And for example, not only outdoors, not only actually…  for example, HVAC systems.

And our recent challenges in our team is also the HVAC system, which is actually duct you know ventilating also the cool, fresh air into the space. And for the mid and high frequency, as we already have that, that is a solution is quite actually ready, or we can find the treatment doing that. But for low frequency, low frequency means again, the wavelength is very long. And how can you deal with the noise in low frequency, below 300 hertz, even down to actually 20 Hz? To so-called absorbing sound becomes very challenging in the HVAC system. There are very constrained space, there are very constrained geometry, and then you have to deal with those noise. And the meta-like material, like resonance type, not as Peter previously mentioned, helmholtz resonator type. But this is the recent activity using metasurfaces and cut also the very thin surface with a certain pattern like micro pores or micro slit, and when we design that together with the structure back cavity, one can actually design low frequency absorber in a very limited size, but still functioning, effectively functioning in low frequency and quite actually useful for those applications. That is also one of the near future in the direction of the metamaterial, metasurfaces.

Kat Setzer (42:08)

So funny, I never would have thought about HVAC systems in this context, but that makes a lot of sense.

So do you have any closing thoughts?

Melanie Nolan (42:16)

Well first, I would like to thank all the authors and reviewers who contributed to this special issue, and the editorial office of JASA for the support for the process. So we're very grateful to everyone who helped make this special issue happen. 

And as a closing thought, I'd like to emphasize that this special issue is really just one part a much broader international effort. So beyond the papers that we have here, there are several ongoing initiatives within the technical committees of our professional societies that are looking at these challenges. So, in particular, we have two joint working groups that bring together researchers from the Room and Building Acoustics community of the EAA, the Architectural Acoustics community of the ASA, and the Architectural Acoustics community of the Acoustical Society of Japan. So one is the BAM working group, the boundary admittance measurement method, which Peter will probably say a bit more about. And the other is dedicated to the accuracy and reproducibility of absorption and scattering measurements. So the fact that three major acoustical societies are working together on these topics really speaks to their importance. And they're fundamental questions that affect both research and practice. And they benefit a lot from international collaboration and the sharing of expertise. So if any listeners are working in these areas and are interested in contributing, we would be delighted to have them get involved. Yeah, and finally I'm really looking forward to seeing where this field goes over the next few years. It's evolving very quickly, and hopefully we'll have the opportunity to bring the community together again for another special issue in the future.

Kat Setzer (43:56)

Yeah, that'd be great.

Peter D’Antonio (43:58)

So I can describe two areas of research in my closing thoughts. The first, as Melanie mentioned, is the boundary admittance measurement method, which we call BAMM. In both geometrical and wave-based simulation programs, we routinely assign absorption coefficients, scattering coefficients, and surface impedance to the room surfaces. However, we lack a standardized measurement method for directly measuring and characterizing the room's boundary surfaces, which play an enormously large part in the predictions. And so our proposal for the BAM working group is to incorporate boundary admittance measurements into an existing standard for sound transmission methods to measure how much sound goes through a partition. And the reason I propose that is because I and many other acousticians have spent many years on developing the diffusion and scattering coefficient standard. But those standards have not been adopted by the commercial laboratories because they're very time intensive, they're expensive to do, and they just have simply not been adopted. So I wanted to incorporate the BAM< measurement into an existing standard, in which we can utilize the existing standard. So in the standard you have  a source room,  a partition, and a measurement room. And since the material on the test already exists, it's essentially almost the entire wall between the source room and the measurement room, we can develop a method that measures the impedance or the admittance, whichis just the reciprocal of the impedance, if we can measure that for this partition, we would have a very important characterization parameter for wave-based modeling. And I think our first session there will be in the next ASA meeting in in Belgium.

Another area that I've been very active in developing is what I call a virtual goniometer. Originally forty years ago when we introduced difusion into the architectural acoustics community, we created a surface which we call a goniometer, in which you have a sample, you have a measurement array of microphones, and then you have a loudspeaker. And you can actively measure the reflected or scattered sound from the surface for different angles of incidence. It was a very powerful method, very, very time consuming, and so we have recently developed virtual goniometer, in which we're using wave acoustics, finite element method, and boundary element method, and we call it VirGo for virtual goniometer. So since the experimental measurements for these diffusive or scattering surfaces are so time consuming and require appropriate rooms measurement conditions, we did an experiment at Salford many years ago in which we verified the fact that we could simulate using a boundary element method, a measured diffusion coefficient. And that was pretty exciting. So what we have now developed is a virtual goniometer. And this dramatically reduces the architectural and acoustical development times. So you have a project. An architect wants to use a surface. The acoustician really has no way of evaluating the effectiveness of that surface since the commercial laboratories haven't taken on the standards. And so this is a real delay in the development of a project. Now if the architect or the acoustician wants to provide a CAD model, we can overnight calculate or predict the scattering coefficient and the diffusion coefficient. And so it very, very quickly improves the development time of a given project.

Ning Xiang (47:36)

I want to join Peter in this BAMM, so-called boundary admittance measurement method, in short BAMM. The BAMM initiative led by Peter, and I'm also part of that. I think that from European Acoustics Association, Melanie also is part of that, co-chair with this initiative as a subcommittee. The admittance, acoustic admittance is the concept that the sound wave as pressure, sound pressure, interact with boundary. We are talking about the room surface, the interior room, right? The boundary, and then the boundary react the wave pressure coming into the surface and the reactor as extra particle velocity, that concept is the admittance. And to characterize that, particularly for low frequencies, this is the concept that is a complex valued and frequency dependent. And again, sometimes it's not local reacting, which actually is hard to predict. And incorporating that into the current effort of so-called wave-based simulation is highly required.

And also it's also challenging to incorporate those data either model predicted or experimentally measured. That is actually particularly we are talking about some kind of lower than 200 hertz or even lower than 100 hertz. Before this initiative, geometrical acoustics quite effective over decades, they are not using those data, and when we move our focus onto the room acoustic simulation in terms of wave-based method, we find out, oh, that is still required. As Peter said there is no standard so far. And how can we actually deal with this? How can we create those reliable information so that we can incorporate and is really a near future? We really have to put our effort into it. 

Another interesting aspect that I believe is the geometrical room acoustic simulation, so far, only taking incident or angular independent absorption coefficient, so to say, on the surface, and also angular independent scattering coefficient of the scattered surfaces. When this special issue reflects the effort starting to characterize absorption, such as Melanie's paper in the reverberant chamber, try to measure the angular-dependent reflection coefficient, so to say, of the absorbing material, or Brandao’s effort try characterize the directional information of scattered surface. So there is no limitations for geometrical room acoustics to take this data into their simulation. Namely, in all days on the surface, we have a couple of numbers for the absorption coefficient, scattering coefficient, which has no direction, no information. Now we have that. And to store those information angular-dependent, even highly angular resolved information as frequency-dependent, broadband frequency-dependent data in the complex numbered is no more issue. That the storage, in 40 years ago, there was a big challenge, all the calculation is so expensive. Nowadays there is no more issue. So we really have to move into that direction. Store all the information, geometrical acoustics simulation already overtake loudspeaker directional information into account. That simulation is already widely available. Why not all the surfaces, all the boundaries, all the scattering.

Kat Setzer (51:30)

I like it.

Ning Xiang (51:33)

Yeah.

Kat Setzer (51:35)

Well, thank you all again for taking the time to speak with me today and for all your work putting this special issue together. It was fun learning about some of the new advances in this field and it's, you know, interesting thinking about how these things are applied in our day-to-day life, even, you know, if you're not in architectural acoustics. So hopefully we'll see some of them implemented in our rooms near us soon and…or in city spaces or wherever. Have a great day.

Peter D’Antonio (52:03)

Thank you so much.

Ning Xiang (52:03)

Thank you.

Melanie Nolan (52:03)

Thank you very much.