Understanding Guitar Resonance | Part 2 | What is Sound?

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Transcript

SPEAKER_00 0:00

Boom! And we are live, ladies and gentlemen. And oh boy, are we gonna get into it today because we are continuing our series on understanding guitar resonance. In the last live stream that we did, I kind of gave you guys what I called the roadmap, which in other words means a 30,000-foot view of kind of the path we're trying to take ultimately in order to understand this. And so, really, in a sense, this video, this live stream, will be more like the first building block of this understanding rather than this comprehensive overview kind of thing. Because I feel like that first video, as it should, leaves more questions than answers. That's kind of the purpose of that video. Now, getting into the building blocks of our understanding of resonance. We're gonna be defining a lot of key terms and things like that. And that's very important. And so if you guys are tuning in here, let me know in the chat. And if you guys have any questions along the way, throw that into the chat. The way this is gonna work is I'm gonna give my spiel over there and explain what I'm gonna explain as best as I can, of course. Afterwards, I will come back and take a look at the discussions going on in the chat. If there's anything good in there, we'll dig into it. Uh, even if you have questions that aren't specifically related to what I'm talking about in this exact episode, as long as they're related to musical instruments and guitars and luthery guitar making, then that's cool. We can uh dive into that as well. I can go in whatever direction uh you guys want to take it. Okay. And um playing with a slinky here because um we're gonna be using this in part of the uh explanation here. So this is gonna be a fun episode. I think we're gonna have a good time. And so before we get started, if you like what I do and you get a little something out of the lessons that I teach, you can go to ericschaeferguitars.com and there you will see a schedule of dates for my eight-day hands-on workshop. There is a workshop in October. That one's full, though. The next available workshop will be in November. I forget the exact dates, but if you go to ericschaeferguitars.com, you can see those dates and sign up for a workshop where we, you know, get into a lot of the stuff that we're talking about here, but obviously in a much more practical hands-on sense, because we're actually building a guitar from start to finish. You can also find at erickshafeferguitars.com, my online course. If you don't want to come out here for a hands-on workshop, you can learn to build guitars at home through that course. And then lastly, there is the tone wood shop, which is now live. So, yeah, you can check out those tone woods. I've got a whole bunch of uh backs and sides and tops of various woods. I'm kind of going fast through this because I'm excited to actually talk about what we're gonna talk about. But uh yeah, there's lots to be that could be said about um those products and services I provide at ericschaferguitars.com. So yeah, you guys start loading up the chat. Oh, Tom Casty says, hi, Eric, stream looks and sounds good. Thanks, Tom. I appreciate that. It's always good to know that uh, you know, we're gonna be talking about audio here in a sense. And um, sometimes for someone who likes to talk about sound waves, uh, sometimes I forget to turn on my lapel mic or something stupid like that. So thank you for that. And we are going to, with all that said, jump over to the bench. So, why do I have a slinky? We'll get into that. Um, I actually just bought this today from the dollar store for this specific explanation. I thought that would be fun. I do have uh two very young kids as well, so I kind of justified it in the sense that uh I'll just give them this after the live stream is over, and they will um that will be just another win for dad. So that's good as well. All right, so we're gonna be basically answering the question of what is sound. The simplest definition of sound, I would say, is that sound is simply energy produced by vibrations. And then, of course, getting into the meaningful part for us as luthers, we know that stringed instruments, in fact, all instruments, are tools for producing vibrations in some form or another. And that's how ultimately it's going to lead back this discussion of the properties of sound to what we really are interested in, which is making quality instruments. So, you know, guitars produce vibrations, a specific type of vibration through the string, which the string then in turn excites the top. Later on in this series, not today, we'll be talking about how all of that energy propagates across the soundboard itself and produces a variety of modes, which vibrates the air that is trapped within the soundbox. Essentially, that air gets pumped out the sound hole, goes into your ear, all of that. So, sound is energy produced by vibrations. It's important to think of it as energy. There's no particle of sound, there's no like packet, there's no physical manifestation of sound, because sound is actually just the propagation of energy. In fact, so you have to be very careful with your words. In fact, I'll probably slip into this every once in a while. It's easy to say that sound travels, but it actually doesn't travel, it propagates, and there's a difference there, as we're going to see. And so the study of sound, the physics of sound, is really the propagation of that energy through a medium. We'll get into the medium part of it later. That's going to be very important when we're studying musical instruments and guitars. By medium, what I'm talking about, of course, is liquids, solids, and gases. So most of the time when we're talking about sound waves, we're listening to music through a speaker or something, we're talking about it traveling through the medium of air in order to get to our ears. The physics of sound is basically the propagation of energy through a medium. That's what we're studying. And so most people, I think, are familiar with this image here. I'm going to draw this the best they can. That's pretty good, actually. When people think about sound, they often think of this wave right here, which is called a sine wave. Essentially, you've got amplitude in this direction. I'm just going to write that as amp, and time in this direction. The only reason I'm showing this right now, we're going to get back to this in a moment, is because many people tend to think of this shape, this sine wave, the way that this looks, as the way that sound propagates through the air. And actually, it's not like this at all. This is really just a graphical representation of the properties of sound, which we'll get into in a little bit, which are frequency and amplitude and wavelength. To understand the way that sound propagates, we first have to understand the difference between transverse waves and longitudinal waves. And so enter the slinky. If I disturb this slinky by moving it like this, I am creating transverse waves. Looks kind of like a you know a wave in the ocean. It looks like actually that graphical representation of the sine wave. By the way, a transverse wave is how a guitar string functions, but it is not how sound waves propagate through a medium. And I think understanding this critical difference between transverse waves and longitudinal waves becomes very helpful later on when we're trying to understand how this energy propagates from the string to the saddle, to the bridge, and the soundboard and the air within the sound box and ultimately into your ears. So transverse wave is where the disturbance of the particles is perpendicular to the direction that the wave is propagating. So let me show you that again. Notice how the crests and troughs of this is perpendicular. There, that's better. That's how you show it. There we go. Okay. Now for a longitudinal wave, the vibrations of the particles are parallel to the direction of wave propagation. So it looks like this. So rather than the disturbance creating crests and troughs, like you see right here, the disturbance is actually creating areas of compression and rarefaction. Rarefaction is really just the opposite of compression. You can think of these longitudinal waves as pressure waves. I mean, that's what they are. So you have rarefaction and compression. Um, I'm really glad that I when I bought this slink, the slinky, I was expecting the at the store them to just have a regular silver-looking slinky, but I'm actually really glad that it's this uh rainbow color because I think it's going to help me make my point a little bit better. Remember how I said uh what we're studying is the propagation of energy and that sound is not actually traveling so much as uh it's just energy propagating across the medium. Well, if you think of every strand or coil of this slinky, uh, let's just say as a column of air particles, when I now excite the slinky like that with a longitudinal wave, if you focus on any individual color of these uh columns of air particles, you'll notice that they're not actually getting anywhere. They're not traveling from one end to the other end of the slinky, they're actually just compressing and then decompressing again. And that energy is getting passed along to each uh column of air particles until that energy reaches the other side. But none of the particles reach the other side. It's a just a transfer of energy, and that's it right there. Again, I'm just laying a lot of groundwork here. Rarifaction and compression is going to come into play a lot more later on when we're talking about propagation, not just through the air, but through solids as well, those solids being guitar tops and backs. The guitar itself is solid. Now that we understand what type of wave sound is, longitudinal pressure waves, and that it has to propagate through a medium, and that that medium can be liquid, solid, or gas. That energy propagation, what it does when it experiences an interface between one medium and another. And there's not just interfaces between solids and gases as categories, but also interfaces between one wood compared to another wood, or one material compared to a different material. Which, by the way, when we talk about as luthier is not just about selecting your materials carefully, but also about fitting your joints and everything very carefully. The reason that's so important, and actually, I think the fit I put more attention into than the selection of various tone woods. I find that just being a good, competent woodworker who can fit joints tightly is more valuable than having access to high-quality tone woods because those interfaces, actually, here's another key term, can attenuate or or dampen the propagation of that energy significantly if the interface is more gappy, is not as tight. All stuff we will get into in future episodes. Let's draw real quick that sine wave again just to talk about wave properties. Again, you've got your amplitude, we'll just call that A, and time is going in this direction. So, first of all, what is amplitude? Amplitude, very simple. Most of you guys know this. Amplitude is your volume, that's your loudness, it's how much energy the wave has. That creates your amplitude, and then graphically, what we're looking at it is the height of the wave. So if you're looking at waveforms on the computer or something, it's the height that defines your amplitude. And then frequency, so again, we got time going in this direction. There's a T. Frequency is the number of waves per second, and that's measured in hertz. So whenever you hear like A440, 440 Hertz, what they're talking about is 440 waves per second. The human ear actually has a limit to what it can sense. So when we're talking about stuff like this, as we'll get into when we get into psychoacoustics, it's important to understand that there's a bandwidth that actually matters in an audible sense to human beings, and that's uh 20 hertz to 20,000 hertz. So outside of that range doesn't actually matter to a listener. Again, when we get to psychoacoustics, that's gonna be really important to understand how the brain processes the signal it's receiving from those various pressure waves. So we've got amplitude, we've got frequency. We're also gonna have the wavelength is you pick any point on this wave. Let's take the peak and you measure it to the same point. Peak to peak, that's your wavelength. It doesn't matter if you measure from trough to trough or just from the middle or something, it's it's the same length. So those are our key properties. But basically, you have a longitudinal pressure wave traveling through air. And the next thing that it's going to hit, of course, is the human ear. The ear itself, like this fleshy thing on the side of my head, you can see that it's effectively a funnel for the sound. So, and then you think of different animals that have exceptionally good hearing, they'll have more of a funnel, like you know, bunny rabbits have bigger ears, for example. So the ear itself is a funnel, goes into the ear. Uh, there's a part of the ear that actually functions just like a resonator, kind of like a sound box is a resonator. Within a certain part of the ear called the cochlea, cochlea, however you pronounce that. There is a membrane of tiny hairs that actually the way that those hairs vibrate is going to send an electrical nerve signal to your brain, and that allows for pitch discrimination. I think I'm going to pause there. If I keep going further, we're we're going to get into psychoacoustics, which I is a whole nother thing. So let me just encapsulate this discussion by just saying sound is a mechanical disturbance, which creates a longitudinal pressure wave. And then when that energy propagates to your ear, it's going to be funneled towards the tympanic membrane, which is the eardrum. That energy in turn vibrates the eardrum, goes through the various other systems of the ear that I that I just mentioned. You're reaching a point where the physics of sound is going to then switch over to the neurobiology of human perception. In that exact moment, it becomes an electrical signal that goes to the brain. That is the opportunity for a lot of um, pardon my, I'm going to use a bad word here. So cover your ears, kids, but a lot of fuckery. And that's really what the brain is doing. It's processing the reality of a lot of that energy and feeding that information to you, but it's actually also feeding you false information. And um, that's just a little preview for what's going to be in the next episode where we're going to talk about psychoacoustics. Because honestly, if the the brain is there to help you to keep you alive and have you propagate yourself into the future, and if it was just giving you a perfect representation, an exact representation of reality all the time, and specifically within the realm of sound, it was just deciphering all of the sound, all of the pressure waves that are present all around you at equal amplitudes to your brain. You would not be able to make sense of the world like you're able to make sense of the world as the brain is feeding you things now. So I think that's important to understand. Um, let me take a break and uh jump over to the chat. Okay. Um, yes, sometimes bad words are needed. Yeah. You know what? I'm of the mindset that, um, and I have little kids, uh, but I'm of the mindset that there are no bad words. So if you're using a word uh appropriately, and the reason you don't want kids to, when they're very young, to encounter the F word, for example, is because they don't have the maturity to know that you don't just use it all the time. And we've all met you know, that guy who just drops F bombs, F in this, F in that, and it's it's not, it doesn't make any sense, it's not appropriate, doesn't you know, add any spice to the conversation, it's just a manner of speaking, and that's uh that's obnoxious, truly obnoxious. So, yeah, it's all about how you use it. Um, how do harm oh, thank you. How do see this is this is what I'm talking about, this is why I rely on you guys. I did want to talk about um a couple other things, uh, like the envelope of of sound and harmonic content. Thank you, guitars and barbecue. Let's actually go back back to the bench. So we we've discussed what sound is. Now there's some key properties of sound that just as far as defining terms, it's going to help later on, just understanding essentially what audio engineers, people who work in music studios and places like that, what they think about and what they uh talk about and the lingo they use when they're talking about sound waves. So the envelope of sound, um, let's say I hit this table. A nice sharp sound, kind of like a snare drum. So um there's a couple things that define that sound so that you recognize it as a thud or a tapping. Um, if you're in your house in the kitchen cooking something and you hear that sound, everyone recognizes that as a knock on your door. You're not confused about you don't just hear pressure waves randomly coming from a certain direction, you recognize that specific type of sound. Why is that? It's because of the envelope and the harmonic content, which this by the way, right here, that's your overtones, okay? This harmonic content. So we'll get into that in a second. But first, the envelope. So this we'll start with this sharp percussive sound. If you were to see this on a um, I forget what the tool's called, actually, in the moment, but you know, on a uh software for analyzing frequencies, what you would see would maybe be something like this. That is kind of what that sound would look like. And so this whole representation on that graph, from where it starts, which let's just say is right here, to where it completely dies out, which is right there, let's say, that is the envelope. Audio engineers will break this apart into four different sections. It's just better for understanding it. There's the attack, which would be, I'm just gonna put an A, um, I'll put it right up here. There's the attack, which in this percussive tapping sound would just be this initial high amplitude part right here, really sharp, really quick. Then there is the decay, put a D for that. That's this big fall, this big drop. You're coming down the mountain here. The amplitude is severely decreasing. So, probably this section right about there. Then there's the sustain within guitars. When we talk about sustain, this is exactly what we're talking about. Within the envelope, we're talking about that portion of the envelope. By the way, the attack is often also referred to as the starting transient. When if you've read the Trevor Gore books and all the magnificent stuff that Trevor Gore puts out, he's always talking about the starting transient and the importance of it in and how you build your guitars all the time. And uh, that's what we're talking about here with the attack. Okay, you can really affect the overall sound of your instrument just by manipulating how the that first attack sounds. And lastly, there's the release, and so later on when we're studying more. This stuff we are going to be talking about the envelope of a particular sound. Okay, now over here with the harmonic content, another, I'd say equally as important aspect of a sound to look at. First of all, recognizing that a sound is not simply one waveform, okay? Unless it's an extraordinarily simple sound like a tuning fork. Okay. This right here, um, a tuning fork will actually have no harmonic content, it will simply play a fundamental. And I don't know if you guys have yeah, a lot of people don't have tuning forks nowadays, it's kind of an old school thing. But um, if you've never tried this, it is really cool in a weird way to um vibrate this. This is what you do with a tuning fork, and then put it in your teeth. And you guys can't hear what I hear, but um essentially the vibrations of this tuning fork meet the the bone of my teeth, which this is going to explain or you know help you better understand why we like bone, why we use that for nuts and saddles. But um, it transfers the vibrations to your teeth and the rest of your skull for that matter, and uh you actually hear quite loudly an A440 tone in your brain, like nobody else around you can hear it, it's just inside of your head, um, like some crazy Neuralink thing. It's cool, but it's old, it's ancient technology, not new technology. So I can hear that crystal clear in my head. Anyway, the reason I brought out this tuning fork is was just to illustrate that um a simple fundamental, a sound that only captures the fundamental is rare. And uh it's basically a tuning fork and um different keyboard synthesizers you can you know synthesize a note, which will only be the fundamental on with you know software. But sounds in real life that aren't tuning forks and software are gonna be much, much, much more complex. And even this sound right here, me tapping on this, that sound is gonna be loaded with overtones. It's good, it's gonna have a fundamental tone, which defines it. The fundamental is the loudest of all of the uh tones present and it defines the pitch of that sound. So if I play uh a guitar string and it's a G note, then the loudest overtone. Technically it's not an overtone, but I'm gonna use that word because the overtones are all the ones behind it. Uh, the loudest overtone is going to define the fundamental. That's the one you really hear, and it's always significantly louder than all the other ones. And then the overtones themselves are what color the fundamental, they sit much quieter behind it and give it timber or character so that you recognize it for what it is. Okay. Um, that's why if I play uh C sharp on a guitar string, you with blindfolded standing in the other room, you recognize that as a guitar string. If I play a C sharp on a piano, same pitch, um, but still blindfolded, you know that's piano. If I play a wind instrument, you can wind instruments are tough. A lot of people don't know even the names of various ones. I'm not even super hip on wind instruments, but I at least know that it's categorically a wind instrument. I might not know if it's a clarinet or a I don't know, a bassoon or something, but um I'll at least know it's a wind instrument standing in the other room. If I hear uh the AC unit behind me, that has a certain harmonic content that immediately registers in my brain as oh, that's the hum of the AC unit. So anyway, the envelope of a sound gives us four different stages to analyze the sound, and this also helps us recognize you know, a short percussive sound is going to have most of its amplitude in the attack, and that tells us gives us a lot of information to understand. Oh, that's a knock at the door, or in the musical context, that's a snare drum. Um, but then also the harmonic content behind it gives us even more information to differentiate sounds from similar sounds, like you know, a snare drum from one of the toms or something like that. They're both short percussive sounds, uh, but they have very different overtones. So, as a uh starting point, let me come back over here. And um, so yeah, that was uh how do harmonics relate to this? Yes. So I think I answered your question there. And um, and then if you're talking about harmonics in the sense of when you play harmonics on a string, those are partials. Um, and that's different. Where we're gonna, well, it's not different, it's related, but different. We're gonna talk about that when we talk about the um string, the mechanics and physics of the string itself, as opposed to right now, where we are just um defining sound and what a sound wave is. Okay, selective hearing question mark. I don't know. Um I must have been talking about something and uh when he said that. So I'm not sure what I was talking about at that point. And uh Tom Cassidy, love having Tom Cassidy here in the chat, writes sure, humans only hear 20 to 20 20 to 20,000 Hertz, but what if I were building a guitar that my dog will enjoy listening to? Uh, I don't know, man. I don't have dogs, so can't help you there. I have chickens. Um, so um, I would have to study uh the range for chickens, but it is interesting because different animals do have vastly different ranges that they sit in. So it is interesting, even just to think about what they possibly experience. Okay, Tao Murmelstein writes, Welcome back, Tao. I remember uh seeing you here before. Since the human ear is particularly sensitive around three to five kilohertz, can a luthier intentionally control this region? Yeah, thanks for bringing that up because there's yeah, the 20 to 20,000 kilohertz is just the absolute range, but there's a smaller window, which he just pointed out there. Um I actually I um I assume that's correct between three and five. I thought it was like two and four or something, but it doesn't I I just don't remember, it doesn't matter what the exact range is. The concept is there's an even narrower window um where uh we're particularly sensitive. Um whereas everything outside of that window, yes, we can we can hear it, but far less than within the three to five or the two to four, whatever it is, range. Can anyway, he asks can a luthier intentionally control this region during the build, similar to controlling harshness with EQ in mixing? Um, yes, and that's kind of the the basis of everything we're going to be talking about, is or not the basis, but the goal of everything we're gonna be talking about is to as much as we can, you know, control things within that region. I will say, and I've I talk about this a lot in the classes I teach here, but um, and this is a gets a little bit more into just ethos with building and uh philosophy, I guess you could say, but um, I don't like the word control in that regard because uh if you get into voicing, you will find that you don't have as much control as you think, meaning, oh, if I pull on this lever, so to speak, it's going to affect the tone in exactly this way. It's less about direct control and more about um using proper principles. There's a certain element of randomness that practically speaking, we're all just gonna be dealing with because within the system of a guitar, everything is touching everything else. So it's actually like functionally impossible to change one thing and predict how it's going to affect other things exactly. There's always an element of surprise when you string up a guitar with how it sounds, even with the best guitar makers out there. Now, the best ones, they're always going to sound good, very good, but there's an element of surprise exactly what the total um balance is on the guitar, okay? Uh, you know, if you wanted to dial down the treble or the projection or whatever, you've wanted to dial down one thing on one guitar in the next build. I don't believe you have the exactly the control to do that. Anyway, um, let me just back up a little bit and see if there were any other questions here. Okay. And uh it looks like we got one more comment here. Guitars and barbecue again, right? Uh, what are major factors when building guitars that shape the overtones? Um it's your modes. So when we get into, I just mentioned the physics of plates. Uh, and in the last episode we did, we talked a little bit about the physics of plates when uh when I brought up um long dipole, cross dipole, and monopole modes. But those the overtones that you experience when you play a guitar are going to be largely determined by the uh the main top resonance, which is largely determined by the various modes that the top is oscillating in. Okay, whether that be monopole mode, which is a pumping action, cross dipole, which is a side-to-side fluttering, um, transversally speaking, uh, across the access of the bridge, or long dipole, which is uh across the long access of the bridge, and then there's a whole bunch of other modes all in between those, across tripole, quadruple, and all of that, and sort of the soup, the mix of those various modes, which every guitar is gonna be vibrating in not just one of those modes, that's not really possible. They're gonna be they're gonna have some mix of all of those modes, and uh that will create essentially the timber and the character of the guitar and the overtones. So um, yeah, that's basically it. Okay. Well, with that said, if you like what I do, uh again, you go to ericshaferguitars.com. I have workshops starting in the fall, plus, I'm making the schedule right now for the 2027 workshop. So if you'd like to get into one for next year, you can actually email me um and pitch a date to me because I'm creating the calendar right now. And so uh if that date works, I I might actually uh be able to make a class on a date that specifically works for your uh calendar as well. So, yeah, reach out to me and um also go to ericshavencontars.com, check out the online Luthery classes and the tone woods, and um, I will see you guys in the next one right here on Tuesday. Bye for now, guys.