What is silence?

More precisely, is silence the absence of sound? (What is sound, anyway?) In this episode, we get a little philosophical, talking with Bill Yost (Arizona State University) about what sound is and the role perception plays in our understanding of sound and silence.

Associated paper: William A. Yost. "What is silence? Therefore, what is sound?" J. Acoust. Soc. Am. 154, 2333–2336 (2023). https://doi.org/10.1121/10.0021872.

Read more from The Journal of the Acoustical Society of America (JASA).
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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 I'm talking to Bill Yost of Arizona State University, who is a fellow of the ASA, a past ASA president, and winner of the ASA's highest honor, the gold medal. He's also the author of a textbook that's been in use for decades, The Fundamentals of Hearing. And we're going to have a bit of a more philosophical discussion than our usual interviews today, as we discussed his forum article, "What is silence? Therefore, what is sound?" which published in the October 2023 issue of JASA. Thanks for taking the time to speak with me today, Bill, how are you?

William Yost  00:52

Good morning. I'm fine, Kat.

Kat Setzer  00:55

Good. So can you tell us a bit about your research background?

William Yost  00:59

Sure. Oh, let's start with my undergraduate and graduate education, both of which were in experimental psychology, but I do have a minor in math and I'd taken a lot of courses at that time, in electrical engineering. But I think today, the courses that I took and the education I received, would be considered neuroscience, maybe behavioral neuroscience, and signal processing, neither of which existed in the 60s when I was in school. In fact, the word, the single word neuroscience didn't even appear in a dictionary until the mid 1970s. So I think today, I would really be considered more of a behavioral neuroscientist. 

William Yost  01:42

In terms of my actual research, I sort of developed a theme in the late 1980s, based on what people today call auditory scene analysis, which is a concept introduced by a guy named Al Bregman at McGill, that what we really do when we say we hear is we hear sources, the cat meow, the car brakes squeak, etc. And that really interests me and I wrote several things about that. And so I sort of organized my research around that kind of theme. And I looked at various aspects pitch, so pitch perception. I did a lot in spatial hearing. And I'll come back to that in a moment, the fact that sound sources can be at different spatial locations. The fact that sounds vary in  their intensity over time, modulation, all those I studied. But when I moved to ASU in 2007, because I had some other responsibilities like being department chair, I concentrated my research on how do human beings localize the source of the sound based on only the sound that's produced. And I was particularly interested in what happens in more real world situations than are often studied in the laboratory, when the sound sources themselves can move around. And when the listener can move around, which obviously happens all the time. And I built, which at that time was a pretty unique, facility, in which I can move listeners, in which I can move sounds around. And that's pretty much what I had done in terms of my research.

Kat Setzer  03:21

Okay, cool. So what motivated you to write this article?

William Yost  03:25

Well, it's sort of strange. A few months ago, I was reading Science. And there was a little bit  in Science has these pieces where they'll talk about articles in other publications that they think may have broad interest, and this one was on auditory perception, which immediately caught my attention. Science almost never has articles on perception, and even more rarely on auditory perception. And the article was about the perception of silence. And the article, the little piece in Science, the Science writer, talked about this as the absence of sound having an effect on perception. And I was sort of bothered by that fact. And also the fact that this was very unique, that also bothered me, and we'll get into why in a minute. But I didn't do a lot except go and read the article. 

William Yost  04:19

And when I read the article, it was pretty straightforward. No need to go into details. And indeed, the authors of the article sort of implied frequently that silence was the absence of sound, although what they actually did was operationally defined silence as when they turned off a sound that they had been presenting to a listener and when the sound, that sound was off, they said that was silence. 

William Yost  04:49

So I didn't do much for a couple days. And then on Tuesday, I'm a New York Times reader. The Tuesday edition of The New York Times that I received has this special section called Tuesday Science, are science articles written by various science writers for the New York Times. And there was this article again, on this Proceedings of the National Academy of Sciences article about sound and its perception. And this article went into more detail about how silence is the absence of sound, and silence affects perception, which is a radically new idea. 

William Yost  05:29

Now I was irritated. So I decided I'm going to write a letter trying to express why I thought this was perhaps overstated. And as I started to write the letter, I realized that what I was going to write was much longer than a letter. And I wasn't exactly sure who I was writing to. Was I writing to Science magazine? Was I writing to New York Times, was I writing to the journal, The Proceedings of National Academy of Sciences, PNAS? And I did, I noticed that PNAS has a type of article called an Opinion Piece. So I said, "Okay, I'll write an opinion piece." 

William Yost  06:07

I sort of finished a very, very rough draft and all of a sudden I got an email, and then a phone conversation withmy very good friend, Judy Dubno. Judy is a colleague, a friend, she's also now the treasurer, of the Acoustical Society of America. She too, was a past president of the ASA. She too, is a gold medal winner. And we discussed this event that had nothing to do with what we're talking about. At the very end, I said, "Oh, by the way, Judy, I'm working on this opinion piece for this article in PNAS," and I started to describe it. And she immediately said, "Yeah, I saw reference to that. That was really weird." And she encouraged me to go ahead and do the opinion piece.

William Yost  06:50

Well, I sort of worked a little bit on the rough draft. And then I started to have lots of second thoughts. Do I really want to go through this process? I'm retired and trying to move on to new aspects of my life. Do I really want to go through a review process? Do I really want to do this? Who am I writing to? Etc, etc. So I thought, well, let's just put this on the backburner and let it sit for a while, we'll see what happens. 

William Yost  07:17

Well, then, about three, four weeks later, Judy contacts me again, on a totally different topic. And at the end of that email, she said, "Oh, by the way, what happened to your PNAS article? I was really looking forward to seeing what you have to say." So I wrote her back as well, for various reasons, some of which I explained in my return email, I decided not to go forward. But if you're interested, here's sort of a very rough draft of what I was doing. And I sent it to her. Well, very shortly, she emailed me back and said, "Wow, that's, that's really interesting. But I do understand your concerns about trying to publish it. Have you thought about an ASA podcast?" 

William Yost  08:03

And I said, "Not only have I not thought about it, I didn't know ASA had a podcast. 

Kat Setzer  08:08

Yeah. 

William Yost  08:10

"Well, it will probably be easier if I see what this is all about. So I'll ask Kat what's going on here." And she emails me back the next day and says that, indeed, this might be something that ASA is interested in. And, well, here we are.

Kat Setzer  08:30

Yes, exactly. Here we are. And I have a feeling this will definitely be something a lot of ASA members will probably be interested in, because obviously we all study sound. So how is sound typically described in say, an introductory acoustics or hearing course? 

William Yost  08:46

Well, I think I think that's sort of an important point right now. Almost anyone who studies sound seriously, especially if you're a physicist, which I am not, has probably heard the phrase, "My God, why are you studying sound? That's a dead area of physics."

William Yost  09:02

 And in some sense, that's true that is sort of the definition that we use to define sound has been around maybe for 150 years. And sort of the core of that hasn't changed. It doesn't mean things about the study of sound haven’t changed. But the core definition hasn't changed.  I... The first section of my textbook that you talked about, Fundamentals of Hearing, is all about sound and its definition. And I constantly taught a course on the introduction to hearing, hearing science. And basically, in very simple terms, it's explained that when an object vibrates, it moves air molecules and their atoms around and that causes a change in pressure. Sometimes that pressure change is talked about as a sound wave moving through a medium like air. And so it's this pressure change that generates sort of the idea of sound. And that pressure change exists whether there's an object vibrating or not. The molecules and their atoms constantly vibrate, constantly move around, and produce changing air pressure. So if silence was the absence of sound, it would mean there's no change in air pressure. And that only happens at zero degree Kelvin, where nothing moves, okay, or in a complete vacuum, when there is nothing to move. Okay? So we're not going to be around when those things happen. So that's part of my concern about whether silence is really the absence of sound.

Kat Setzer  10:45

Okay, got it. And really, the question of sound is an old one, right? Philosophers have been discussing it for a long time; scientists have been discussing it for a long time. Can you tell us about the historical discussions about sound and its perception?

William Yost  10:58

Well, you're right sound has really fascinated scholars for I think, probably as long as there's been written history. Ancient Greeks were sort of intrigued by the fact that someone playing a lute over there was perceived as sound by the listener over here. So how does sound get from there to here? Galileo, this October was the 400th anniversary of his famous Assayer letter in 1623. Galileo wrote, "Hence, I think, taste, odors and colors, and so on, are no more than mere names, so far as the object in which we placed them is concerned, they only reside in consciousness." Well, the ancient Greeks and Galileo did not know about changing air pressure, okay? But they knew that sound occurred. They knew that there was this event. So to me, that sort of underlines the point that the definition of sound must involve a perceptual component.

Kat Setzer  12:01

Okay. And that's really the underlying argument in your article is that you're asserting that the sound is a perceptual quality rather than a physical quality. Can you expand on what you mean by the statement? 

William Yost  12:13

Yeah, so let me be clear, I'm not saying there isn't a physical aspect to sound. There absolutely is a physical aspect. In my view, it is the changing air pressure, and all the things that go on when that occurs, okay. But for that air pressure to be called a sound really has to involve some aspects of perception, okay. So normally, the normal situation, like you and I talking here, or anyone listening to this podcast, something vibrates, like the speaker,diaphragm, okay, that moves the air around. There are air pressure changes. And those air pressure changes are picked up by the inner ear, okay, and translate into neural impulses, action potentials, in the auditory nerve, which then relays information about that changing air pressure to the brain. Okay? And then the brain interprets those neural changes about the air pressure changes, and decides that it's sound. Okay? And so it's really this neuro-perceptual process that makes up this whole thing we call sound. 

William Yost  13:33

And I think that there are situations in which sound is perceived, and there are no air changes. And there are also situations in which air changes occur and we perceive no sound. For instance, one of the things that sort of reinforces this idea that sound has a perceptual component is that it's very, very common to sort of divide sounds into different classes: audible sounds, infrasound, very-low frequency, ultrasounds, very-high frequency, and infra and ultrasounds are reference to what humans detect, okay? Ultrasound means we don't hear it. Infrasound means we don't hear it. Okay? So it's not about the physics. It's about what humans perceive that allows this division. And as we know, the Acoustical Society of America has sections on ultrasound. Things like wind turbines have a lot to do with infrasound. So there's a lot of interest in these things. And it's not just the physics of air pressure changes that is intriguing. So if 50,000 hertz is a frequency way above anything that we can hear, okay? This is we might actually say that's something that's silent to us. But if you're a bat, or a dolphin, or any other echolocating animal, you damn well want to hear 50,000 hertz or you're not gonna survive. Okay? So while 50,000 hertz may be silence to us, it's not silence to an echolocating animal, okay? Infrasound oftentimes can be detected, okay, not because you hear it, but because you feel it. If the air pressure changes are great enough, there's a sensation of feeling. Okay? So maybe silence is something you feel, but you don't hear. So those are sort of the concerns I have if you just simplistically talk about silence as the absence of sound. Okay.

Kat Setzer  15:47

Okay, so let's get into some examples. What's an example of when air pressure change doesn't result in sound?

William Yost  15:51

Well think about everyday weather. Every morning, you turn on the morning news or the evening news. And the weather person will have a map with a big H and a big L, high-pressure/low-pressure areas, that move through where you live and change the weather conditions. Well, those are changing air pressure. We don't call those sounds, no one would call those sounds, for many reasons, one of them being they move way too slowly. Okay? So there are air pressures that have nothing to do with sound. Now if that air pressure is very, very fast, we might in fact perceive a sound-- that is it becomes wind, and we can perceive wind, even if there are no objects that are vibrating. We're in a wind tunnel, and all that is  happening is we're having a very, very rapid change in air pressure, the auditory receptors will pick that up send a signal to the brain and brain stem.

William Yost  16:20

Okay, so then also on the opposite end, what's an example of when sound occurs without a change in air pressure?

William Yost  16:55

Well, there are lots of examples consider tinnitus or as physicians used to call it tin ni tus. That is defined as someone hearing a sound, when there's absolutely nothing out there producing the sound there is no object vibrating, there are no air pressure changes that are at all related to the sound of one ears, okay. And that tinnitus sound can be quite noticeable. It can be so-called roaring tinnitus. It has to do with things we do not understand. We don't know what causes tinnitus. It's obviously something's happening in the brain that triggers us to believe that the sound occurred when, in fact, there was no object that vibrated. 

William Yost  17:47

Then there are people fit with cochlear implants, a marvelous device. Many of whom have an implant had a severe hearing loss; that's the reason they have the implant. After they've been implanted after some time, in some cases, not a lot of time. And in many contexts they could, quote unquote, hear as if they weren't at all hearing impaired. Okay? The cochlear implant sends an electrical signal that eventually stimulates auditory nerve fibers. Okay, that send neural action potential signals to the brain and the brain says, "Ah! A sound occurred." And so that process doesn't involve, doesn't have to involve any vibrating object, any change in air pressure, I can stimulate a patient's electrode artificially, and they'll say very clearly that a sound occurred. And in fact, I can also stimulate their brainstem and they will perceive the sound (not anywhere near as clearly as if I stimulated their auditory nerve). Therefore, I can completely bypass the entire auditory periphery, you have no outer ear, no inner ear, no auditory nerve, no middle ear, nothing. And yet you perceive the sound. So to me, one of the important aspects of the work on cochlear implants, which I do, is it shows you don't need an ear to hear, but you do need a brain to hear. 

Kat Setzer  19:31

Interesting, interesting. Okay, so we talked about perception of sound, but the articles that motivated to write this response also discuss the perception of silence. What are your thoughts on the perception of silence?

William Yost  19:43

Well, right, that's what sort of triggered all of this. In the article as I said, a minute ago, contains sort of two definitions. One is silence is the absence of sound. And the other was, or is that silence is when a vibrating object that you know about, that you can identify, doesn't produce air pressure changes anymore. So, consider listening to music on your, on your cell phone, on your iPhone or whatever device you have. When you turn off your iPhone, the iPhone issilent, okay? That's sort of obvious, okay? But you're still surrounded by sound. Okay? That is you haven't turned off sound, you've only made the iPhone itself silent. So all the experimenters had to do in their article was get out a sound level meter or use their iPhone as a sound level meter, and they would have measured sound. It might have been very soft, maybe as low as 25 dBA. But nonetheless, there's sound there. 

William Yost  20:52

In addition to defining silence as the absence of sound, which I don't think is very accurate, the reports in the PNAS, as well as in Science Magazine, as well as in New York Times, suggested that the research that was done demonstrated a unique finding that silence has perceptual auditory consequence. There are several examples of other situations in which the absence of sound, or the absence of a known vibrating object you don't know of anything that could be producing the sound, has perceptual consequences. So one of them is, if you are in a unique room, a room that doesn't have normal reflections, sort of a reflection-reduced room; in the extreme, these are called anechoic chambers, in which there are very, very few echoes or reflections off of the surfaces of the chamber. If you are in such a room, okay, and don't produce any sound at all, you're not aware of any sound being produce, the room is perceived as different. Everyone who's in such a room, everyone, including those who are hearing impaired, say the room sounds different, okay. And what they'll sort of use is a vocabulary along the line... is that the room sounds really dead, it doesn't sound alive. But remember, there's nothing in the room that they can point to that's producing the sound. 

William Yost  22:32

I have such a room, for my lab, and over the last almost two decades, there have been hundreds and hundreds of people, participants in my research projects, the visitors, people interested in what this weird lab is all about, who have been in there. And almost I think in every single case, if you were to come to ASU, we would go to my lab, we would walk in and I would say, "Kat, don't say anything. Don't do anything." I'd wait a second. And then I would say, "Is there any sound in the room?' And you would say something probably along the lines of, "No it's really silent." And then I will say, "Does this room... Are you perceiving this room? Do you hear this room? Does this room sound different than other rooms that you have been in?" And you will say instantly, like all the other people that I've asked that question to, "Absolutely! This is really weird! It really sounds different." 

William Yost  23:36

So another example, a little bit more esoteric, if I were to present a tone, a low-frequency tone, a low-pitch tone, over your headphones to you, along with a noise that's going to both ears that's intended to mask your ability to detect that tone. Okay? What you would find is if I present the tone to one ear, and the noise to both ears, it's easier to detect, I can turn the tone down and you can still detect it related to if I present the tone to both ears and the noise to both ears. Okay, so there's this interaural difference for the tone that doesn't exist for the noise. And that makes it easier to hear the tone when it's in only one ear, when there is  interaural difference versus when it's not. And that difference in your ability to detect the tone in one ear versus two ears persists even when I turn the noise off. If I lower the level of noise, you continue to have a lower threshold for detecting the tone in one ear versus two ears. Okay? So even when the noise is silent, okay, there's a perception, a perceptual effect that is easier to detect, it's easier to perceive the tone in one ear than it is if the tone is in both ears. Okay? 

William Yost  25:10

So in the tone example, as well as in the listening room, anechoic listening room, okay? Unknown to most people, sound is present. When I'm in the listening room with you, your heart and my heart are beating. Your lungs are expanding and contracting. There are other body parts that are moving around, okay? The air conditioning system and heating system is vibrating. The lights are vibrating. The building sways. All those are sources of sound; they produce the sound, that if I had a sound-level meter, I would measure in the room, even though neither of us can say that there's a heart beating, or a lung expanding or contracting, although both of us sure as hell hope there is:  ours! Okay? But all of those produce the sound that we're hearing and the air pressure change from all of those things, reflects off of walls differently in an anechoic chamber than it does in a normal room, and we perceive that. So this so-called absence of sound has perceptual consequence. 

William Yost  26:39

In the case where I presented the sounds over headphones, okay? Well, it turns out that, if you're listening and I'm, we're both listening with headphones, our hearts are beating our lungs are expanding and contracting, our other body parts are moving, and that causes air pressure changes in our ear canal. And despite the fact that we're wearing headphones over our pinneas, over our ears, it doesn't completely block all the external sounds that are surrounding us. They also cause air pressure changes in the air. And it turns out that those air pressure changes in one ear are different than the air pressure changes in the other. So there's an interaural difference in those air pressures. And as I explained a minute ago, that causes a difference in your ability to detect sounds in one ear versus two ears. So there is a perceptual consequence of no sound, even though there actually is sound. So you have to be, in my opinion, very careful about a assuming what sound is and isn't, what silence is and isn't, and whether or not there are consequences of those kinds of assumptions.

Kat Setzer  28:02

So we know that animals can hear things humans can't. Are their perception of sound and silence going to be different from humans'?

William Yost  28:10

Well, yeah, I think we all have our example, like your cat, if you had a dog that seemed to be able to hear sounds that we can't hear. Okay. That's why there are dog whistles. And in fact, you know, you probably know, that you can get an app, okay, that your kids can hear, but you can't hear. 

Kat Setzer  28:31

Oh, yeah, I've heard about that. Right.

William Yost  28:33

The fact that you lose your hearing, as you get older, the kids can have an app that they can play things that you're not aware that they're listening to. So yes, okay, that is the case. And animals can in fact, hear things, quote-unquote, perceive things based on sound that other animals might not be able to. My dear friend and longtime colleague, Dick Fay, who unfortunately recently died, undertook an enormous task back in the 1980s, of compiling a databook, a handbook of perceptual data on hearing from as many vertebrate species as he possibly could get data for. And it's almost the case, I think that he got just about every known data point on vertebrate animals on auditory perception. One major set of data that he got, were these so-called thresholds of hearing, which I just explained a minute ago. That is, how well can an animal detect sounds of different frequencies? What's the sound-pressure level? What's the air pressure change necessary for a sound for an animal-- a rat, a human, a dolphin, a fish, a bird-- to detect sounds of different frequencies? And Dick compiled so-called thresholds-of-hearing curves, that is curves of what's the softest sound that can be detected as a function of frequency for well over 300 vertebrate animals, okay? And most people, when they look at all those curves plotted on the same coordinate system-- which Dick did; he normalized them and plotted them all together-- are sort of struck by the differences, the diversity. 

William Yost  30:28

Dick, on the other hand, was struck by the similarity. You notice, if you look carefully, that every single threshold-of-hearing curve is U-shapedusually there is a frequency or a frequency region in which the animal is most sensitive. The sound pressure changes, the air pressure changes are very, very, very low or low, maybe very low's not the issue right at the moment is low. But if the frequency gets lower than or higher than that region of sensitivity, the animal is less sensitive, okay? So it's U shaped. All right? But the remarkable thing to Dick, and, to me and others, is if you go and you look at this sound-pressure level, the change in air pressure necessary for an animal to detect a sound in its most sensitive region, the frequency to which it is most sensitive, What is the sound pressure necessary to detect the sound? Okay? It's about the same. It's about the same. Plus or minus 10 DB, all animals are equally sensitive to about the same intensity. And that's the same as for humans. 

William Yost  31:55

Now I want to take a little diversion. Okay. Back in the 1930s, at Bell Laboratories, a group of applied physicists working in Harvey Fletcher's laboratory, okay, compiled a whole bunch of data on humans ability to detect sounds, and then conducted their own thresholds-of-hearing measures. How well can humans detect sounds at different frequencies, and what is the sound pressure level necessary to do so? Okay. And they discovered that in the 1000-to-4000 hertz region of frequency is where we are best, where humans are best, at detecting the sound. And the sound pressure level at that frequency region is 20 microPascals, Pascal is a measure of pressure, so a millionth of a unit of pressure. 20 millionths of a unit of pressure is what is required, whether you're listening over headphones, or whether you listen through a loudspeaker in a sound field. That's what you need to detect sound. Well, that is an enormously small sound pressure. So Sylvain and White asked the question, okay, is it the physiology and the anatomy of the auditory system that's determining the fact that you can hear such a slight level, 20 micropascals? Or is it the fact that air molecules, as I said earlier, produce sounds just by moving around. If there's no object whatsoever, air molecules are running around hitting each other, producing changes in air pressure, producing changes in sound? Is that the limit? So they did a bunch of calculations made some assumptions, and concluded quite definitively that you could not design a more sensitive ear, because if you did, you wouldn't be able to hear anything, because the air molecules have so-called thermal noise-- nowadays called Brownian motion of air molecules just running around-- would mask whatever it was that the auditory system was designed to detect. Okay? So, we literally can hear at the limit, as can almost... no almost, all other vertebrates, that Dick studied. These 300 other vertebrates also are detecting sound at the limit. 

William Yost  34:43

So if you wanted to talk about perception in terms of detection, there is no difference among the animals. We are all as sensitive as we possibly can be. Okay? Which of course, sort of justifies my assumption that silence is not the absence of sound. Silence doesn't exist, unless you're at zero degree Kelvin, or in a vacuum, and then you don't exist. So that's sort of a part of my argument is, of course. The other part is that silence does affect perception, and there are numerous examples of that.

Kat Setzer  35:26

Okay. So let's get into that idea just a little bit further. So if silence isn't the absence of air pressure, what is it then? And is true silence even possible?

William Yost  35:36

Well, okay. So if silence were to be defined as the absence of sound, as I've said several times now, I just don't think that's possible. Okay? I don't think you can say that sound was absent. Therefore, that's not a good definition of silence. If silence, however, is when an object no longer vibrates and produces air pressure changes, then yes, there can be silence; the cell phone can go silent. Okay? I can turn off the noise in your headphones, okay, so there is no noise. Noise has gone silent, okay? 

William Yost  36:17

So if the auditory system does not respond with a perception of sound, then that's probably not a bad way to think about what silence might be. So if you are completely deaf, by taking out your ears for some unfortunate reason, okay, then you would not be able to hear, you would not be able to experience air pressure changes. And that might be a good definition of silence. But of course, then I can't ask you anything about hearing in that situation. So I'm sort of stymied at studying whether or not that situation has some effect on your auditory perception. So, indeed, I think that it's not inappropriate to talk about silence. But I think you have to be careful, I think you have to sort of say what it is that you are no longer presenting that had produced the sensation, or the perception, of sound, okay? And then use that as your kind of definition of what you mean by silence. And I think it's the examples that I've provided and others that I could also provide, okay, the fact that you can do that, the fact that I can turn off something that is producing a sound that you perceive, has perceptual consequences that have been studied for a very, very long time. So the fact that this article in PNAS is about another consequence of turning off sounds that was perceptible is very informative, well worth knowing about, but it's not necessarily unique.

Kat Setzer  38:12

So it's kind of like silence is really relative to the source rather than relative to all of the setting, right? Like, you're not gonna get rid of all sounds, but silence, like the cell phone is silent. The person has stopped talking is silence? 

William Yost  38:25

Right

Kat Setzer  38:25

Yeah.

William Yost  38:26

Well, it's relative to the source and your perception, right?

Kat Setzer  38:32

 Right. 

William Yost  38:33

Because you could have the perception of sound, even if there's no source that produced it. So I just, I just think that this is the sort of thing that in my early education in psychology, which kind of grew out of philosophy. In fact, when I entered college in 1962, and started studying in the Psychology Department, that department had just separated five years ago from a department that was called Philosophy and Psychology. Colorado College where I got my degree, set up a new department called Psychology, and maintain another department, which was the old Philosophy and Psychology Department, as Philosophy. So my early education was heavily bound up in these issues about what is really perception, what's really sensation, what is the difference? And how does that relate back to issues that are more philosophically based, so I don't think it's necessarily surprising that I might have seen things in these articles that I talked about at the beginning that bothered me a little bit based on my training

Kat Setzer  40:00

Yes. Well, thank you for such a fascinating discussion. I think that folks will appreciate the kind of alternative viewpoints in terms of how we look at or consider the concept of silence, since we have so many physical-based discussions of it and don't talk about perception of it  as much. So thank you for this very thought provoking discussion, Bill, and I'm sure our listeners will find it interesting as I have. And have a great day! 

Kat Setzer  40:26

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.