1. Waves

FACT:
If a tree falls in a forest and nobody and no animals are there to hear it, it does not make a sound. Sound is a perception of air pressure variation that starts in the ears and ends in the brain. Sound does not exist outside of the organism that perceive it. We are all obsessed with rhythmic air pressure fluctuations.

When you see wave forms in Garageband or whatever you're using, its essentially a graph of air pressure vs. time. (It's really a graph of the voltage created by the microphone, but that is directly caused by the air pressure.) The center axis of the sine wave represents normal air pressure with no vibrations affecting it. I'm not sure which way it's oriented, but for the sake of argument let's say the wave going up means increased pressure (aka Compression), and down means decreased (aka Rarefaction). How high or low the wave goes away from the center is called it's amplitude, which correlates to the volume. How frequently it alternates positive and negative is the frequency, which correlates to pitch. Frequency is measured in cycles-per-second, aka: Hertz. That's what A440 is on guitar tuners, the A above middle C is set to 440Hz.

THE SPEED OF SOUND:
Sound, at sea level, travels at 340m/s and some change. The denser the medium a sound wave is going through the faster it will go. This is why you can put your ears to train tracks to hear a train that you can't hear in the air yet. Since the particles are closer together, they pass the vibrations on faster. The speed of sound underwater is also different, and gets faster as you go deeper. That doesn't really matter though. 340m/s is all you need to know at this elevation.

SUPER-HANDY MATH & SUCH:
A secret you can grab from this, is sound travels roughly one foot per millisecond. So you can put an Xms delay on a sound to make it sound Xft. further away. (By adding a Xms delay, I mean a delay that's set to 100% mix and 0% feedback. A delay in its most simplest form, not a repeating echo.) This can be especially useful for making your overheads sound further from the drum kit. You'll also see this used in reverb plug-ins sometimes. If you see a "Pre-Delay" parameter, that's a delay between the source and when the reverb starts.

WAVELENGTH:
The inverse of frequency is wavelength, meaning the distance it takes for one compression and one rarefaction to occur. It is measured in meters, or I guess you could use feet. You can use the speed of sound to find the frequency from wavelength, or vice versa, since it is essentially constant.

(1second/440cycles) x (340meters/1second) = (340meters/440cycles) = .77meters/1cycle

So the wavelength of 440Hz is .77m
The higher the frequency the shorter the wavelength.

THE INVERSE SQUARE LAW:
(Fancy name, really simple concept.) What it means in terms of sound, is if you half the distance between you and a sound source, you square the sound pressure level (SPL). Conversely if you double the distance, you would take the square root of the SPL. I'll get more into how you can use this later. A lot of other things in nature follow this trend as well, including light and magnetic fields.