We have our sound saved as a digital file that sounds great, but takes up an enormous amount of space. How can we make it smaller? We can remove extra space and extraneous data to "clean up" the file, but that will only take us so far.

It turns out that what we actually hear when we listen to sound is only part of the total soundscape. There is a scientific term for this phenomenon... Psychoacoustics. Through years of study scientists have been able to identify the exact frequencies that our ears hear and the portions of those frequencies that our brains process.

Our example this time takes us to a Hollywood movie set. The director wants to give the impression that our heroes are wandering through a dusty and dangerous frontier town. The cost and effort of moving the production and building an authentic cowboy town would be enormous. So the director has a set designed which looks like the main street of a frontier town. The fronts of the general store, the hotel, the saloon, and the brothel are façades. They are simply walls that lean against posts. Backdrops at either end of the street are painted to look like the empty desert around the town. When the film is finished, moviegoers will not be able to tell that the scene was not filmed in the Wild, Wild West, because it looks to them like it was. It doesn't matter that there were no bags of oats for sale in the general store or no tables in the saloon or no velvet couches behind the doors of the brothel, because the visual effect of the main street is exactly the same with or without the contents of the buildings.

The same is true of digital audio files. Using sophisticated encoding software, it is possible to build a sort of sound "façade". The parts of an audio file that we do not actually hear are removed, leaving only what is necessary to create the desired effect. Since we never heard these frequencies before, their removal does not affect the quality of our listening experience. An oscilloscope would show us the difference between our original file and the newly compressed file, but our ears think they sound the same. The notable difference is the size of the files. If our original file was 50 MB, most encoders could compress it (without noticeable sound quality differences) to 5 MB or less. If we are willing to sacrifice a little sound quality (so that the file sounds like it might if it was recorded from an FM radio station), it can be compressed to 2 MB or less.

This is the innovation that has spurred our current digital audio revolution. It is now possible to store hundreds of songs in the same amount of space that previously would have only held a few. The songs are small enough that they can be downloaded or transferred over the Internet without eating up precious bandwidth and time.

MP3 is a digital audio compression format. Our next section tells you everything you could want to know about MP3.