[MUSIC] Okay so let's start at the very beginning. What is sound? Now, I know this sounds like a bit of a philosophical question, but it's very important because unless you understand how sound is made you can't really understand how it's represented in a computer. And you certainly can't think of good ways of using it. So just briefly to give you an introduction. One way of thinking about sound is what is sometimes described as simple harmonic motion. And the idea of simple harmonic motion is that you have a pressure wave in space. Let's say that the space is occupied by a medium, like air. When I clap my hands, I am squashing air molecules together and they are banging into each other, and then they start to move. They move away from my hand and they move towards you, and they hit other air molecules. So they're compressed together, and then they expand. And then they get pressed together again. And this compression and what we call rarefaction is a sound wave, a pressure wave, propagating through space. When it reaches your ears you hear it as sound. So the sounds that we hear are what we call longitudinal pressure waves because they originate from a point source such as my hands or my mouth. And they go towards a receiver, a listener, if there's anyone there. So these waves all cycle through compression and rarefaction, and that process is called oscillation. You can see this process on the diagram that I've got here. There's a speaker generating a sound. This is generating a wave form. When the wave form is at high amplitude here, the molecules are compressed, and then they expand and compress again. This process continues until it reaches the ear. So what sort of characteristics do sounds have? Well firstly, sounds have a frequency. The frequency of a sound is how many cycles per second of oscillation we get from a sound, how many times it compresses and expels. That's the frequency. And sounds can have lots of different frequency content. It doesn't necessarily just have to be one squeezing and expansion process, it can be lots of them all interacting. Another characteristic of sound is amplitude. Now, amplitude is how much air is actually moving. So if I clap my hands really loud [SOUND] more air is being squashed together than if I clap my hands like this [SOUND]. Now, that's how we hear it. We hear those changes in pressure in our ear drum because it makes our ear drum move. It makes our whole mechanism in our ears move more and then we hear it as a louder sound. Now, sounds also have direction, and direction is very, very important in virtual reality. Because the sound direction is how we know where things are coming from. Now, this sounds obvious, but the mechanisms that control this are quite complicated. And there's a lot of great technology inside VR which makes this work really well. And I'm going to go into some detail about that in a moment. Finally, a characteristic of sound which is very important is speed. Now, the speed of sound depends on the medium it's travelling in. So the speed of sound is different with respect to different gases. Also, sound can travel at different speeds through different materials including metal or wood or glass. But normally, we hear sound and we work with sound in the way that it travels through air and we think of sound as something which travels through air most of the time. Now, when sound propagates through space, when it squeezes and expands it also hits surfaces, different surfaces. And those surfaces have properties that cause reflections or absorption. Now, when we are in a cathedral for example, and you start shouting or talking or clapping your hands or singing. You will often notice that there's an awful lot of muddy sound that reflects off the kind of shiny walls of the cathedral or the church. And this is a kind of awe-inspiring sound. We call this reverb. The reverberation, it tells us something about the sound, but it also tells us something about the room that we're in. Now this reflection, the reverberation, is something which we model all the time in computers. So it's important to know about. It can tell us a lot about the quality of the environment and also the distance that something is away from us. It may be that in some environments, if something is far away, it's more reverberant. You can think about the way this works similarly to the way that light waves work. You are used to looking at a mirror and seeing reflections. That's because of a number of things, including the fact that it's a hard, shiny surface. And it's the same sort of thing with reverberation. So the way we measure reverberation is through something called an RT60 measurement. An RT60 tells us how long it takes for the reverb from a particular material to die down below a certain level, in this case, 60 dBs. Every single material which is used in buildings or in manufacture often has an RT60 measurement. Which is used to tell people who want to build buildings or to make environments what the reverb is going to be like if they use that material. So it's quite an important thing to know about. So as I've already explained, sounds travel at a certain speed in air, and this is roughly 330 meters per second. So what that means is if you're standing 330 meters away from me and I speak, you might see my lips moving, particularly if you're using binoculars, but you won't hear it for a whole second. There's going to be a time delay. This may or may not be important in virtual reality environments, and there are all kinds of reasons why and we're going to talk about them a little bit later. But primarily the reason why it's important to think about sound traveling at particular speeds and in particular ways is because it can affect how it sounds as well. You can get more reverberation as I've already said, but also it can contribute to something called the Doppler effect. Now, when the source of a sound, such as a vehicle, is moving towards us at significant speeds, we can hear the frequency of the sound changing. And what's actually happening is that as the sound approaches us it gets higher in pitch, and then when it gets to where we are it's at the right pitch. And as it moves away from us it gets lower. This Doppler effect is used all the time in VR to give the impression of fast moving objects. And you will have heard it in the streets, just with police sirens and ambulance sirens, all the time. It happens all the time. And even ice cream vans, or that just may be broken ice cream vans, I don't know. I'm not sure they go fast enough. Generally speaking, you need to be going about 30 miles an hour for it to really work. So the reason why this happens is that each compression is being moved forward in space, which increases the frequency. In a way, it's an illusion brought about by the fact that the wave fronts are being pushed closer and closer together as the vehicle moves towards you, and then getting further and further apart as it goes away from you. You can see this in the diagram that I've got here. As the object moves, the wave fronts become compressed and the frequency increases. As the object is approaching, the increase will make the sound higher in pitch. And then as it goes away, it will reduce the pitch. So sounds have very specific characteristics that we use to measure sound. And this is very important when we're using audio in virtual reality environments. [MUSIC]
