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Basic sound theory and synthesis.

What is sound?

Sound is what you hear. You hear what your ears capture, and your ears capture subtle and rapid changes in air pressure. These changes in pressure, or pressure waves travelling through the air, typically come from moving objects or parts of objects. If something moves, it pushes on the air in front of it and that causes a wave of pressure that spreads out and grows weaker the further it goes. It's essentially the same phenomenon that can be seen by playing around with your hands on a water surface.

These pressure waves can be of different intensity (experienced as volume, very loud sounds can even be felt in the rest of your body), and they fluctuate back and forth at the same speed as the object that caused them (heard as frequency or pitch). Our ears can interpret sound frequencies ranging between approximately 20 Hz (Hertz) to 20000 Hz (or 20 kHz, for kilo-hertz). One Hertz corresponds to one back-and-forth motion in a second.

A single Hertz can easily be produced by waving your hand back and forth, and it doesn't appear as sound since our ears can't "go that low". It does however generate a movement of air that you can easily feel. If you were able to wave your hand faster, at more than 20 back-and-forths per second, it would start sounding like a very deep bass tone.

As an example of what you actually can hear, a Swedish dial-tone is usually 440 Hz, or the musical note 'A'. That means that it is generated by an object swinging back and forth 440 times each second, which is pretty fast as far as everyday motion goes. Apparently the American equivalent is a mix of two tones at 350 and 440 Hz, but the result is still in the same range.

When sound needs to be illustrated graphically, it's common to use a horizontal time axis and show the position of an imagined moving object on the vertical axis. This means that the graph can be interpreted as a "movie reel" showing how an object would have to move to recreate the sound. For a simple tone, the graph looks like a wavy string, and if you travel to the right in the image you see that the object would move up and down at a regular interval which will correspond to the frequency of the tone.