How It's Done
A three-minute song on a CD requires almost 32 million bytes -- that's a lot of information! Using a 56Kbps modem, it would take about two hours to download a file this big. MP3s are popular because, with their compressed size, they can travel over the Internet quickly and don't take up too much space on your hard drive. A song that takes 32 megabytes on a CD takes 3 megabytes on an MP3.
One of the most interesting ways to compress a music file is called perceptual noise shaping -- it's built around the way human beings hear sound. For example, the human ear:
- cannot hear certain sounds
- hears some sounds better than others
- hears only the louder sound when two sounds are being created at the same time
The human ear hears only the loudest sound when two or more sounds happen at the same time.
Now hear this! The human ear is most sensitive to sounds between 500 Hz and 8,000 Hz, a much narrower range than that of other animals.
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By getting rid of all the parts of the song that the human ear doesn't hear well, MP3 encoders can shrink the size of a file without sacrificing sound quality.
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