Welcome to Audio and Music Engineering. I hope you've been looking forward to this first lesson as much as I have. Mark Bocko and I will be teaching this together and, I thought I would start today with an example or two. That really conveys some of the fundamental principles of all the lessons we're going to have, du, during the course. one of the things that you'll find is that a loudspeaker actually provides a great example of electrical mechanical. All of the kinds of components that really are part of an audio system. And it'll serve kind of as our our, our fundamental example here today. as you saw in the prerequisites we don't require that you have calculus or differential equations fundamentals in mechanics, things of that nature. I will tell you that many of the design equations, that we'll represent in the course were derived using math at that level. I would encourage you to pursue that if you're interested in being able to develop the design equations yourself. However, I'll tell you up front that you'll still be able to design a loud speaker or understand the basic physics of sound and such. Even if you don't understand how to derive the equations yet. So, we're going to bring those to you as if you had gone through the derivations. And then talk a bit about the application of it. So, I think that sets the stage for, for where we're headed. And I thought before I actually got into my core lecture today, that we'd spend a little time just demonstrating some basics with the loud speaker. That I think will help really convey some of the physics that are, that are important to the course. I have some very simple components here on the bench top. this is a simple breadboard that's used frequently to prototype circuits. all I have in the breadboard right now are a couple of capacitors. And I've got them there because I just want to give a demonstration of, of how, you know. Of how crossover networks effect the way audio is, is, is represented in, in particularly in the design of speakers. I have a little, simple amplifier here just to we need power actually to drive a transducer. As you know, most of you are familiar with digital media, and you know, I have an iPad here. You could equally use your iPhone or an iPod or whatever technology you have, maybe a portable CD player to generate your signal. But the signal itself doesn't carry any current. current it's what, is what's needed to actually drive the loudspeaker itself. So that's why the power amplifier's in the loop. So for us today, we go from the signal output of our, of our iPad here to a power amplifier. And the output of the power amplifier, we have two leads that are ultimately going to go drive this speaker. And, this is a two way speaker. you'll see two drivers, this is the mid flash low range driver and what's typically called the, the tweeter, here. this is a port, alright. And then, during the course we're going to talk about two fundamental speaker designs. One is a closed box design, which will have no port at all. And the second is a ported driver and there are benefits and trade-offs associated with both of them. And we'll, we'll cover that in the, in the course. So let's, let's just take a little quick listen to music. And I'm going to show you the impact of of a capacitor, particularly in a circuit. in the, in the circuit associated with the speaker itself. Capacitors and inductors are used as crossovers and basically, they're used to filter out part of the sound. you'll he, hear audio and we'll cover this, the audible range is from 20 hertz to 20 kilohertz for human hearing. the capacitors are really good at creating high-pass filters, which basically allow you to hear the high frequency, but not the low. Inductors do the opposite, they allow you to hear the low frequency, but not the high. And so when you put two, two drivers in, you don't want all of the audio content to go to both drivers. In fact, the low frequency might cause you to melt the tweeter and, and the high frequency would be very directional in this larger driver's speaker. So you wouldn't want to do that either. So you have crossovers that are built in to the design. We won't get into the specific of the design, but I want to at least demonstrate it. So let's, let's play a little clip of a song. And I've just picked one arbitrarily here off my, iPad. [MUSIC]. So you hear the full range audio now from this speaker. And now I'm going to put one of the capacitors in, in series. [MUSIC]. And you here some of the low frequencies gone, so that's with the capacitor. [MUSIC]. And now we've taken the capacitor back out. So you hear how we filter the sound. Now I'm going to go to a one microfarad, a one microfarad capacitor. And we've taken even more of the audio content out, so this is bandwidth. Remember, I said we've heard, we can hear from 20 hertz to 20 kilohertz. So we're hearing the high-end part of the signal here but not the low frequency. And [MUSIC], that's full bandwidth. That's just a simple demonstration of a capacitor, okay. what I'd like to do now is convey something that's very important about why we have transducers in boxes. And I've intentionally with this particular speaker placed it so that I can remove it from the box itself. And let me pop these off, alright. So what I'm going to do now, is I want you to be able to hear the sound radiated when the speaker is in the enclosure. And then I'm going to pull the driver out, and we're going to play it. So I want you to hear the difference between those two. So let's start with it in the, in the enclosure [MUSIC]. And that's how, hear the low frequency coming out? So, we lose the low frequency when we pull the driver out of the box. [MUSIC]. Why, why does that happen? Well, let's think about it. a loud speaker is kind of like a piston. And so this surface is pushing outward, it basically oscillates. And the way we generate sound is this, this piston will basically oscillate, there's a spring that's called a spider here, little hard to see, but it's right, right in here. And that serves as the suspension system for it. There's a magnet back here, and a coil. And the coil's what's being driving by this power amplifier. So it causes the speaker to oscillate. And Mark, later in the course, will talk to you about signals. And when we talk about signals, you're going to understand how they oscillate and how those end up driving the transducer themselves. So this vibrates back and forth. But when it's out of the box, a friend of mine always told me a transducer, a speaker out of the box is like a fish out of water. and the reason is, is because the front side of the driver, actually is pushing sound, or creating a positive pressure wave. But the back side of the driver is actually creating rarefaction, or creating a negative pressure wave. And so, there's cancellation, because there's nothing to separate the front and the back of the speaker. Now, this gets into wavelength, because, you know, at 20 kilohertz, the wavelength of sound is about 1.7 centimeters. It's very small, all right? So, this dimension is really large, compared to 1.7 centimeters, so the higher frequency, you didn't really hear much deterioration in the sound. But at lower frequency, and in fact at 20 hertz, it's like 17 meters. So the wavelength is huge. It's, it's larger than the width of the room that I'm standing in. So because of that the front of the speaker and the back of the speaker are almost coupled to each other. So the low frequency sound appears when you pull it out of the box. So it turns out box design is really important particularly for isolating low frequency. So I just thought we'd take the, opportunity, since we've been talking about speakers. To just give an example of on that was built in a class that I taught here at the University of Rochester, a few years back. I taught a freshman introductory course on loud speaker design. basically it's a two way Speaker like the the box that I showed you earlier. We've got the tweeter here, our mid-range and bass here. And I just wanted to show you a few of the construction pieces. We used PVC pipe. So if you decide to build a speaker yourself, this makes a good port. It's already a nice prefabbed component all you have to do is cut the port to length. So we used a PVC pipe in the design. You'll see screws here and you know we didn't, this wasn't built for aesthetics. when the speaker was built, the students actually were just trying to get it together and sealed nicely. But they used wood screws. And you'll, you know, I'll talk to you a little bit about cabinet design, but you really want to seal the cabinet up. You want to basically make sure you have tight connections that bring the, the surfaces together and you want to use glue and caulk and things. To really seal the cabinet up, because you don't want, what are the equivalent of extra ports, or ways where you can get air whistling or wheezing if you will. Because of the, when the drivers are going. And just so you know that it actually works, [MUSIC], so, another speaker example. So you could build this by the end of the semester.
