Some rules of thumb to keep in mind when it comes to bit rates. Here's number one:

1. The higher the bit rate, the better the sound quality (and size) of the resulting file. Lower the bit rate, and your sound quality goes down. But your file sizes go down too.

Depending on your particular needs, you can move off the benchmark of 128 kbps. You can go higher than 128 kbps for less compression and even better sound quality. Or you can lower the bit rate to further shrink the size of your mp3 files. Of course, sound quality will suffer the lower you go, but you can get some amazingly small files in the process.

The table below can help you estimate the kind of mp3 quality/compression and bit rates to use for your own applications. We also give you approximate file sizes to help you compare.

Anything below 128 kbps is going to put the hurt on your audio quality, but you might want to experiment and see how low your file sizes can go. For instance, you'll be surprised at how well "voice only" audio can sound at the lower bit rates.

Constant bit rate vs. Variable bit rate
To further complicate the bit rate game, many encoders allow you to customize the encoding process by selecting either constant or variable bit rates.

Constant Bit Rate encoding (CBR) maintains a steady bit rate stream through the entire mp3 file. This means a consistent level of compression and predictable file sizes. CBR has also been around longer, and is supported by all MP3 players. It's the safe, reliable, consistent route to take when in doubt. Settings range from 16 kbps to 320 kbps depending on the file size you're targeting. Remember that files encoded faster than 128 kbps may have little or no detectable improvement in sound quality.

CBR has its drawbacks, especially when encoding certain kinds of music. Consider this rhetorical question: is your music always consistent in terms of dynamic range, complexity, polyphony, and stereo separation? Many times, a single piece of music will vary some or all of these qualities. Some passages are complex, some are silent with narrow stereo separation; other passages are dense, complex, and busy with wider stereo separation. And yet, Constant Bit Rate encoding pays no mind to all that and keeps its bit rate steady as she goes. You run the risk here of using more bits than is necessary for the music's quiet parts, and not enough bits than you'd optimally want for encoding for the busier, more dense musical sections.

What can happen (experiment with CBR encoding to see if you can hear it) is that the complex/difficult parts don't sound as good as other parts of the music. On the flip side is the impact of things you can't hear: the wasted bits encoded during the simpler, quieter sections.

Which brings us to rule of thumb number two:

2. Constant bit rate encoding = consistent, limited file sizes but variable sound quality.

To address the shortcomings with constant bit rate encoding, Variable bit rate encoding (VBR) was developed. VBR is newer than CBR; most mp3 players don't support VBR…yet. The most recent upgrades to FreeAmp, Winamp, and MusicMatch support it. And the Xing encoding program supports it too.

If you seek consistently high audio quality, and file size isn't as much of a concern, you should consider using it. VBR makes intelligent decisions before allocating its bits to the encoding process. It adapts the bit rate to the complexity of the audio, based on a scale you can set in the preferences of your encoder. The low end of a scale from 1 to 100 would result in the lowest quality/highest compression. The high end of the scale would result in the highest quality/lowest compression. The results? Overall, you'll get better and more consistent sound quality compared to CBR encoding at the same bit rate.

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