Here are some links that will allow you to hear what pure sounds sound like. We have created a bunch of mp3 files containing pure tones. When you hear a tone, remember that your computer's speakers are pushing the air molecules that are right next to the speaker. These air molecules push on ones next to them, and so on, until the air near your ear drum gets pushed on, are you hear the sound.

Vibrate the air 100 times per second (produces a 100 Hertz sound)
Vibrate the air 200 times per second (produces a 200 Hertz sound

It turns out that the air is able to support not just one single wave of pushes (pure tones), but many, many waves at once. This means various pure tones can be mixed, and sent through the air at the same time. This is where music, speech, and otherwise "noise" come from. Here are a couple of links containing two mixed sounds each. Can you hear each individual sound at work?

A 100 and a 500 Hertz sound played at the same time
500 and a 1000 Hertz sound

If you click on the sounds below, they should sound familiar to you. It turns out that the phone company uses specific combinations of tones to signal that you have pressed a key on your touch tone phone. In fact, if you hold your phone (the part your speak into) up to your computer speakers when you play these tones, it should trick your phone into thinking that you actually pressed the corresponding digit.

687 and a 1209 Hertz sounds (the "1" key on your phone)

The phone company uses a 687 Hz tone in combination mwith a 1209 Hz tone for the signal that the "1" key has been pressed (remember, Hz=Hertz, which is the number of waves per second).

687 and a 1336 Hertz sounds (the "2" key on your phone)
687 and a 1477 Hertz sounds (the "3" key on your phone)