Acoustic effects of medical, cloth, and transparent face masks on speech signals
The Journal of the Acoustical Society of America (JASA)
Authors: Ryan M. Corey, Uriah Jones, and Andrew C. Singer
In this episode, we interview Ryan Corey about how face masks muffle speech and make communication more difficult, especially for people with hearing loss.
Read more from The Journal of the Acoustical Society of America (JASA).
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Ambri Phillips 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, The Journal of the Acoustical Society of America, also known as JASA, JASA Express Letters, Proceedings of Meetings on Acoustics, also known as POMA, and Acoustics Today. I'm your host, Ambri Phillips, Business Administrator for the ASA.
Joining me today is Ryan Corey of the University of Illinois Urbana-Champaign. We will be discussing his article “Acoustic effects of medical, cloth, and transparent face masks on speech signals,” which appeared in the October 2020 issue of JASA. Thank you for taking the time to speak with us today, Ryan, how are you doing?
Ryan Corey 00:56
Thank you so much. I'm doing well. It's a pleasure to be here. And thank you for having me.
Ambri Phillips 01:01
Thanks for coming. Okay, so, tell us a little bit about yourself.
Ryan Corey 01:07
Yes, so I am a postdoc at the University of Illinois Urbana-Champaign in Electrical Engineering. And I'm also a person with hearing loss. I've had hearing loss my whole life and worn hearing aids since I was a teenager. I've always been unhappy with how well they work, especially in noisy environments. And so my research focuses on audio signal processing for hearing aids and other listening devices. My goal is to help people hear better in challenging situations like a restaurant or poster session at an ASA conference, for example.
Ambri Phillips 01:45
You gave us a little bit, but is there any more of a motivation behind the study?
Ryan Corey 01:50
Absolutely. So in the summer of 2020, I was doing some interviews with people with hearing loss, to find out about their needs and how they experience hearing technology. And something that kept coming up again and again, was the challenges they face with face masks. And there was one interaction in particular that prompted this study. There's a friend of mine who is an administrator at a school that serves a lot of children with hearing loss or children who are deaf. And she texted me one day to ask, what kind of face masks should my teachers wear when we go back to teaching in person? And I wasn't sure. And it turns out, there wasn't that much research on the subject, especially around cloth masks, which were popular at the time. And so I decided to do some experiments in the laboratory to find out what masks work best and what other interventions can be used to communicate clearly while wearing face masks.
Ambri Phillips 02:59
Okay, what other impacts did the pandemic have on people with hearing impairments?
Ryan Corey 03:05
So like with everything in the pandemic, it depends a lot on who you are and where you work. So for people like you and me who are able to work from home, the biggest effect on communication is that we're now having all these virtual meetings, phone calls, Zoom meetings. And when that's done well, I've found it can actually be helpful. If everyone has a great microphone setup and a quiet room, I find it easier to communicate than in a kind of glass-walled conference room, but that's not always the case. And if there's a poor connection, it can be really difficult. There's also a loss of visual cues when people aren't wearing cameras or aren't well lit. And especially in the earlier days of the pandemic, there was very spotty support for captioning, and for sign language interpreters, especially for remote learning. So that was a big issue. And then for in person interactions, there were a few changes. So with social distancing, people stand farther apart, which makes hearing more difficult. You want to lean in, but that would be against the rules. And then of course, there are face masks, which muffle sound, especially at high frequencies, which are the hardest to hear already for most people with hearing loss. They also block visual cues, which make lip reading more difficult. And it's kind of funny, the pandemic sort of simulates hearing loss for people with normal hearing. So you can no longer hear high frequencies; you want to lean in and ask people to repeat themselves; you have to rely on technology to communicate. And all of those are very familiar experiences for people with hearing loss.
Ambri Phillips 04:59
Okay, What has prior research told us about masked speech?
Ryan Corey 05:05
So there had been a few studies before the pandemic, mostly focusing on medical settings. So we knew that N95 masks, for example, had a muffling effect on high frequencies. And they could make it more difficult for, say, surgeons to talk to each other in the operating room. But what hadn't been studied were cloth masks, which really weren't around before April of 2020. But then, after public health authorities recommended wearing masks, it was hard to get medical grade masks. And so a lot of people were getting cloth masks, companies started making them, and a lot of people were making them at home out of old T shirts or something. And no one really knew what effects those had acoustically. And there were also some transparent masks that were starting to become popular, and those had been studied a little bit, especially by Sam Atcherson at the University of Arkansas. But there hadn't been the sort of acoustic measurements to understand what effect they had on sound specifically.
Ambri Phillips 06:21
What steps did you take to conduct your study?
Ryan Corey 06:24
So I started by gathering different types of masks, and I tried to get a representative sample of the masks people were using. So I got surgical masks, I was able to get some N95 masks, and I also gathered some different fabric masks. I got a few that were made commercially, and a few that were homemade by volunteers in our local community, which was very helpful. People really were excited about this work and eager to help. I also found some transparent face masks that had plastic windows, some commercial, some homemade again, as well as a face shield, which were being used in some places at the time. And it was a sort of tricky study to do, because on the one hand, I wanted repeatable measurements, so that I could make a fair comparison between all the masks, but I also wanted it to be realistic. And so I ended up doing two different types of measurements. First, I used a loudspeaker that’s shaped like a human head. And it just so happened that we had one around in the lab; it was built by my coauthor, Uriah Jones, who was a design student at the University. And we had built that to do other studies. And it turned out to be perfect for testing masks, because it can produce the same sound every time. I played a frequency sweep and that allows us to measure the acoustic transfer function, so what effect the mask has on sound, compared to no mask. And the microphone was set up six feet away to simulate social distancing. And the head was placed on a turntable because I wanted to measure what directional effects the masks have, if it was attenuating sound uniformly, or reflecting it off to the sides. But for the second experiment, I wanted more realism, so I used a human subject. And for safety reasons, we could only have one person in the lab, so that subject was me. So I wore the different masks and recorded my own speech. And in that experiment, it's more realistic because it accounts for the fit on the face, lip movements, jaw movements. But of course, it's hard to say the same thing exactly the same way every time, so there's more experimental error. Fortunately, the two experiments were pretty consistent with each other, which suggests the results are reliable.
Ambri Phillips 09:15
What were the results from the study? Did they support your hypothesis?
Ryan Corey 09:19
So the results were that all of the masks for the most part muffled sound above around two kilohertz. And for those who aren't very familiar with speech, in that range, you get a lot of consonant sounds, especially sibilants like “S” and “F”. And those are very important for speech intelligibility, so masks definitely do have an effect there. But not all masks are the same. Some masks work better than others. Even very similar looking masks can have different effects. So I think I can actually do a demo right now. I have some different masks here that I'll try on. So this first one will be a surgical mask, the blue kind that you see a lot of people wearing. So now I've put on the surgical mask. And my data showed that the surgical masks are the best, they only attenuate sound by a couple decibels at high frequency, so you probably don't notice much difference in hearing me. Next, I'm going to put on a fabric mask. I think it's made by an underwear company, so it's nice and soft and comfortable, but it's very densely woven. So now I'm wearing that mask, and you'll probably notice that my voice is a lot more muffled. And especially if I make “S” and "F” sounds, those will not come through as strongly. So for the cloth masks, I had thought that the number of layers would be very important. But it turns out, it's really about the weave of the fabric. So loosely woven cotton, like a T shirt, is pretty acoustically transparent. But a densely woven fabric, like denim, blocks a lot of sound, even if it's only one layer. So you really want to pay attention to the weave of the fabric. As for the transparent masks, the ones I tested had the worst attenuation out of all the masks. So they let you see lip movements, but they do block sound quite badly. The worst overall was the face shield. And face shields are interesting because they actually do amplify sound at lower frequencies around 1000 Hertz. And this is a result other people have found, but that's not really a benefit. They sort of reflect sound backward. They have weird echoey sound. So they're really quite awful sounding.
Ambri Phillips 12:11
Okay, I was just going to ask are transparent masks useful? But I guess they're not?
Ryan Corey 12:18
Well, so. So that's a controversial question. There's a lot of debate within the hearing loss community about transparent masks, because there is this trade off, where you can see lip movements, you can see facial expressions and emotions. But you get much less sound. And now I should mention that there have been new masks developed since I did this study, I have not tested all of the new kinds that have come out. And it's possible that some of the newer products can give you the best of both worlds. But there is sort of this tradeoff between sound and vision. And there might be a different answer for different people. Personally, I don't like the transparent masks, I understand better when I can get more sound, but someone who relies more on visual cues might prefer them. But part of the challenge here is that as the listener, I don't generally have any say, in what mask a conversation partner is using unless, you know, it's a close friend or colleague or something. And I've seen some workplaces or schools where they'll provide transparent masks as an accommodation. So if you have hearing loss, you can request a transparent mask to wear. And it's kind of frustrating because it doesn't help me if I'm wearing a transparent mask, it matters what the other person is wearing.
Ambri Phillips 13:54
Makes sense. What is the best technology to use while wearing a mask?
Ryan Corey 14:00
Great question. So one goal of this research was not just to understand the effects that masks have, but to find something that we can do about it. So sometimes we don't have any choice but to have a conversation while wearing a mask. So is there anything we can do to make that easier when we have to? And one interesting result of the turntable experiment was that masks don't block sound uniformly in all directions. They attenuate it in the front, so if you're standing face to face with someone, they'll block the sound a lot, but they seem to reflect it off to the sides and up and down. So with the human subject, we tried different microphone placements to see if the talker could wear a microphone, and that would help. And that was a good result. So the most common place that people might wear a microphone is on the lapel. So you would see those in broadcasts or in lectures in an auditorium. And it turns out that lapel microphones aren't very affected by face masks. There's a little bit of attenuation, but a lot less than someone who's standing six feet in front of the talker. And so amplification can really help. So in a classroom setting, if you have a sound reinforcement system, that can be very helpful. Those are pretty ubiquitous in higher education: An auditorium will usually have a sound system. In K-12 classrooms, they generally aren't available, and they were hard to find, especially in 2020. So I heard from some teachers who were ordering home karaoke systems to use in their classrooms, which I'm sure had to be frustrating for them. There are also, for hearing aid users, there are accessories that are available from the hearing aid companies that are little wireless microphones that you can clip on to people, and it will transmit their voice right to your ears. And those are great because they essentially bring the talker closer to you, reducing noise and reverberation. So it can get over that social distancing, and those should work reliably with masks as well. Unfortunately, I have not found an easy solution for interacting with many different people. So for a cashier, for example, who has to stand behind a plastic barrier, and interact with a stream of customers. You know, I think that's the most challenging situation. And it's difficult to do anything about that.
Ambri Phillips 16:52
Do you have any other studies that you plan to conduct in the future?
Ryan Corey 16:56
Yes, so my main takeaway from this work was how important these amplification technologies can be, those remote microphones, those broadcast systems, because they can, you know, close that distance gap and cut through the masks. So I think we're going to see a lot of growth in those sorts of technologies in the future. And some of my more recent research has been on ways to improve some of those wireless microphone technologies, to make them work more seamlessly with hearing devices, to make them sound more natural to listeners. One other impact that the pandemic has had is an expansion in conferencing systems. So a lot of classrooms and workplaces have been leaning into remote learning and hybrid work and things like that. And so they've been installing these high tech conferencing systems that include microphone arrays that can enhance sound from a distance. And those are mostly used now for conferencing, but I think they could also be used to improve accessibility for people in the room. And so I'm very interested in ways to use that new tech for accessibility as well.
Ambri Phillips 18:20
Well, thank you for taking the time to educate us on the effects that mask have on those that are hearing impaired. We look forward to seeing your future studies.
Ryan Corey 18:28
Thank you so much, Ambri.
Ambri Phillips 18:32
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