Which brings us to rule of thumb number three:

3. Variable bit rate = consistently high sound quality but unpredictable file sizes.

If it's important that your mp3 file sizes are consistent, it's best not to use VBR encoding. Variable Bit Rate encoded files will likely be larger than Constant Bit Rate encoded files. The difference can be radical. A song encoded with CBR at 128 kbps may actually be smaller than the same tune encoded with VBR scaled to 50 percent. Let your ears decide what you can get away with, and always keep in mind any bandwidth restrictions on the end-user side of the equation.

The best way to understand bit rates is to start tinkering with them. I encourage you to run some tests of your own. Take your favorite CD (one that you're really familiar with), sample a brief clip, encode at different bit rates, and compare every nuance. Understanding the impact of bit rates should be very useful when you need to deliver audio at a target size, or when a client demands uncompromising quality.

Sample Taste Test
I've included a short audio clip here for the sake of comparison testing. This is a snippet of sequenced music sampled from a Kurzweil 2500 keyboard at 16bit/44.1kHz. I then exported the sample as a .WAV file and loaded it into the MusicMatch mp3 player/recorder. Using MusicMatch, I then encoded the sample at different bit rates and saved the resulting mp3 files. Note (if you can) the differences in sound quality and how bit rate impacts file size. By the way, the original WAV format source file was 2.27 megabytes - pretty large for a mere 13 seconds of music, eh?

NOTE: Click on filename to download. On PCs, choose "Save this file to disk," or "Open this file from its current location" if you wish to launch your music player and play the file without downloading.

Observations:

Variable Bit Rate at the highest level results in the biggest file size, as expected. Theoretically this should also give you superior sound quality.

With the Variable Bit Rate set to the low end of the scale (6%) the file size is kept quite low while maintaining good fidelity.

Note how dropping the bit rate to 96 kbps affects the file size. Further note how dramatically the lower bit rates affect the sound. To my ears, they have a squash and crinkle effect.

Jim Esch is a freelance writer and editor based in West Chester, PA. He has been writing about technology and multimedia since 1987, when he first laid hands on an Amiga computer. When he's not working with words, you can find Jim producing DIY music in his home-based project studio.

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