Propagation

Have you ever heard the expression "Faster than the speed of sound"? It turns out that there is no single "speed of sound". In reality, sound moves at different speeds through different materials. Sound moves through water differently than it moves through the air. And this is different than how it moves through metal, or concrete or wood or whatever. Essentially, the more dense the material, the faster sound travels through it. So sound traveling through solids is faster than in liquids, and sound traveling through liquids goes faster than in gases. (If you want to dig into this topic more, you can start here - http://www.physicsclassroom.com/Class/sound/U11L2c.cfm). Furthermore, since altitude, temperature and humidity impact the density of air, those factors will also affect the speed of sound traveling through the air. 

Wonderful, so what's the point? Nothing. Just an excuse to show this picture of a jet as it transitions from below to above the speed of sound. (Thanks WiseGeek) 

So if you clicked on that link, you learned that the speed of sound is about 761 mph or 1225 kph (at some particular altitude, temperature and humidity). But if you think about it, the altitude, temperature and humidity are not going to change a heck of a lot over the course of a song, so we can ignore those minor differences. Sound speed in these terms is not particularly useful for our music applications.

So how about this? Sound travels about one foot in a millisecond (1 ft/ms). So, if you are in a room with really hard surfaces, and you make a sound (like a hand clap or a finger snap), and you are say about 5 feet from the nearest wall. You would expect to hear the direct sound maybe 1ms after you make the sound (unless you do it right next to your ear - not recommended), and an echo about 10 ms later (5ms to get to the wall, and 5 ms to get back to your ears). Well, you might not hear the echo. (10 ms is a really short time). But what if you're 10 feet from the wall? 20 feet? What if you're 10 feet from one wall and 15 feet from the other? You would now start to hear the room affecting the sound that you hear, even though you never change the way you clap or snap. What if your room is a box with 6 hard surfaces and corners and . . . like a gymnasium. You hear the direct sound, several distinct echos and then a sort of indistinct sound that we call reverberation or reverb for short. The reverb effect arises from the fact that the sound waves keep bouncing around the room, (like waves in a cup, tub or pool of water), reducing the volume with each reflection and trailing off over a long time.

Now what if there was a magical device that allowed you to hear the direct sound, and then a copy of it 20ms later, and then again 40 ms. And another device that repeats the echos closer together while they trail off in volume. That might start to sound like the room we just made up. Those magical devices are called Delay and Reverb.

So now you know why when you watch the National Anthem being performed on TV, the crowd never sounds like they are singing together, and crowds at a rock concert never clap to the beat. If you're at the concert it sounds fine, but the recording never sounds right. It's because of the time it takes for the sound to propagate from the source to the audience's ears (which are all over the venue). Their physical response is delayed, and then the sound has to travel back to the recording devices. What a mess!

Ok one more thing about propagation. I lied about sound working like the waves at a beach or in a pool. In reality, sound waves are more like the waves in a Slinky. (For those too young or otherwise unfamiliar, it's a toy spring - "For fun it's a wonderful toy"). This is easier to see than to explain, so watch this excellent (and brief) demonstration by everyone's favorite science nerd, Dr Bill Nye.

For a touch more geekiness, the waves you see in a liquid like water are called transverse waves. The disturbance is perpendicular to the direction the wave is moving. With sound waves (and slinky and billiard ball waves), the disturbance is parallel to the direction of motion. One more video and then I'll leave you alone for this session

So there you have it. If you've learned nothing else, you've learned that a Slinky actually IS a wonderful toy! I think we'll see some more slinky videos as we discuss the concepts of amplitude and frequency. And if I find the right video, we can do both in one session. Until then, go make some music!