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Visualising electrical signals on a screen is made possible by an oscilloscope. It's a machine which draws a graph of signals on a screen. Originally the shareware icon was the only thing on the site about oscilloscopes, but then later there was How to Convert a TV into an Oscilloscope. The good reason for having an oscilloscope can be seen by the fact that you can't SEE electrical signals. Even to see if a power connector has AC or DC on it isn't obvious by just looking at the power connectors, but an oscilloscope shows entirely different things on the screen. Plus, different electrical signals show different waveforms as graphs. There's a lot more to this, but here is the basic fact of oscilloscope usefulness.
Sound, as electrical signals, shows on an oscilloscope as waves which dance. Oscilloscopes can allow you to see signals faster than sound, with frequencies which are well into the radio spectrum.
More about the electromagnetic frequency spectrum can be seen here.
GlobalTestSupply.com - has the largest selection of new and refurbished Oscilloscopes. - was http://www.globaltestsupply.com/oscilloscope/index.cfm
It's also worth putting a good word in for Rapid Electronics whose speciality is test equipment.