The Demonstration: Someone speaks into a microphone that is connected to an oscilloscope and you can see the patterns creates by their voice.

Quick Physics: Sound is a wave that caused by vibrations in the air. A pure musical sound, such as a whistle, will produce a sine wave like the one shown above. Someone just talking with produce an irregular wave because it is a combination of sounds and the sounds waves add together to get the composite shown on the oscilloscope.

The Details:
You can see sound waves when a microphone is connected to an oscilloscope. A microphone changes the sound waves into an electrical signal. The oscilloscope then shows what these electrical waves look like. For a pure sound of only one frequency – like a tuning fork or whistling – the wave looks smooth and regular (as in the picture below). These are called sine waves. High notes have a high frequency and the waves are very close together. Low notes have a low frequency and the waves are spread out. Other pretty sounds, like singing or a violin, are also regular. They make waves that repeat themselves, but they are not as smooth. This is because many frequencies mix together to make the sound. Ugly sounds, like noise or talking, make jagged lines. There seems to be no pattern at all. Too many frequencies are mixed up together.