Master your MP3s with Audacity

Step 4: Setting hard limits

While compression affects sound that rises above a certain level, limiting keeps your signal from crossing any given amplitude. In other words, limiting is the hard cap to compression's soft ceiling. In the Effects menu, select "Hard limiter" from the LAPSDA plug-in package. "Wet" and "dry" refer to the strength of an effect, with 1 being full effect and 0 representing no effect. In most cases, you'll probably want to keep the dB at 0 and the wetness at 1, a setting that means any signal spiking above 0 dB will be completely attenuated back down to 0. In other words, the signal is restricted. For example, if you were to set compression to -12 and a hard limit at 0, Audacity will taper your spikes after surpassing the -12 mark, and will shave any peaks over 0. If you're making a ringtone, you'll want to lower the hard limit, starting with -1dB.

So far, our steps have been useful for editing individual tracks to mix them with others. The Envelope tool is a handy shortcut for mastering a pre-recorded MP3 that's free of encryption or Digital Rights Management technology (DRM). After you compress the MP3, you can use the Envelope button (the inverted triangles with the horizontal blue line between them, located in the top left toolbar) to differentiate the gain in various sections of the song. Clicking the newly created envelope creates handles, or "control points" that you can then drag to raise or lower the volume between nodes. You can create as many control points as you'd like for maximum volume flexibility.

Once you've learned how to master a song or track, your MP3 file is ready to feature in a podcast or personal Web site, like your MySpace page, for example. First, install the open source MP3 encoder LAME. In Preferences, set the bit rate to 80kbps or 96kbps for spoken word, 32kbps if you're making a ring tone (more on this later,) or 128kbps for a music file. If you've got bandwidth to kill, a 160kbps or 192kbps bit rate is closer to CD quality, but your sound won't suffer at 128 kbps and will be faster to load. In the File menu, choose "Export as an MP3".

Making ringtones has its own set of rules, since most phones won't support high-quality tunes. Trim your clip carefully; you'll probably want it to last a minimum of 5 seconds and a maximum of 20 seconds before the tone loops. Most phones will automatically loop your ringtone file, which you can hear by selecting a range in Audacity and simultaneously pressing Shift and clicking Play.

Click the "Project rate" option on the bottom left bar of the Audacity screen, and set it to 11Hz. Depending on your phone, you may want to hike this up. However, a higher frequency will mean a larger file size. Compress the track as in Step 2, but when you equalize the tracks, you'll want to remove the bass completely (slide the bar to -24dB on the Y-axis) from 30-300Hz (on the X-axis), as your phone's speaker cannot produce these frequencies. Since your phone is more sensitive to peaks, set the limit to -3dB as you would if you were exporting your MP3 for your Web site. Following instructions from Step 4, open the Hard Limiter plug-in and set the limit to -3dB.

Now you'll want to reduce your output from default stereo to a single channel. In Audacity, you can do this by selecting "Split stereo track" from the drop-down menu on the track title. Cancelling out the bottom track will delete it, leaving you with a mono output. In Preferences, set your bit rate to 32kbps (or higher if your phone supports it and you have enough free space). Before exporting the MP3, you'll need to confirm the frequency that your phone will accept and adjust the project rate accordingly.

Special thanks to Matt Stone for expert assistance.