Here is a cool page that Murat Ayfer developed that allows you to use your computer keyboard to play sounds out of a web browser. You can also go through and modify various aspects of the waveform, including timbre, time, frequency, duration, envelope and pitch bends. This is a neat and fairly intuitive way to get into understanding how synthesizers can be used, and how you can make your own sounds. There is also a visual display as to how the virtual “strings” would resonate as the sound is being generated. You can also look up presets that other users have made to help you get more of an idea of what is going on.
*Note – currently this page only works through Google Chrome or Safari web browsers. He is working on adding support for other browsers, such as Firefox.
Timbre, (the texture of sound; the thing that makes you distinguish between a guitar and a saxaphone) is our perception of the particular overtonic content of a sound.
This means that a single note you hear has several frequencies playing at once. The loudest frequency is referred to as the note you hear. For example, 110Hz is an A. The other, quieter multiples of the root are its overtones. 220Hz, 330Hz, etc. would be overtones of A.
Tonehack lets you pick these frequencies yourself, and using your cursor, draw how these frequencies change over time.