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Audio: Squeezing Sound

I guess in the minds of some, audio had its day, and they've moved on to video. Not so fast! Streaming audio, MP3s and other compressed audio sources are everywhere on the Internet, bigger and better than ever before. Using a little imagination and tools you already have, it's easy to leverage the power of audio on the Internet. Spend the next few minutes here, and you'll be squeezing sound like a pro.

Uncompressed audio files are big. The average song ripped from a CD is 30-50MB. While it's possible to post a monster file like that to the Internet, it would take quite a while to download, making it completely impractical for most purposes. That's where audio compression comes in. Using a combination of psychoacoustics, math and a little bit of magic, we can easily squeeze large audio files to a tenth of their original size or smaller. Of course, there is a trade off in quality - compressed audio files are "lossy" and don't sound exactly like the original.

There are several types of compressed audio - AAC, MP3, Windows Media, Ogg Vorbis - and each has its own way of squeezing sound. But in general, they all go through similar processes. They analyze incoming audio for content, and compare this profile to models based on human hearing - what we hear and how we hear it. Then, the encoder decides which elements to eliminate. First to go are extreme highs and lows. Next are softer sounds that will be masked by louder ones. This continues until we have a compressed audio file. Another important decision is the bitrate. For near-CD-quality music, a bitrate of 128KB is a standard minimum. For voice, the bitrate can go much lower. Many syndicated talk shows stream their voice broadcasts around 48KB, a compromise that won't seriously degrade audio quality. If you're a complete geek and want to know even more, Wikipedia has an excellent article on audio compression. For the rest of us, the general rule is: the harder you squeeze, the smaller the file and the worse it sou…