Become an MP3 master with Audacity

A guide to mastering MP3 files in Audacity for use in podcasts, Web pages, and ringtones.

Audacity audio editor (Credit: CNET Networks)

With professional music software selling for hundreds of dollars, finding a comparable freeware program is no small potatoes. Audacity is an impressive open-source audio editor that has upped its own ante in the new beta version, Audacity 1.3. If you're attuned to the basics of fading and trimming, this guide urges you on the next step of your journey, mastering MP3 files for Web publishing, cell phone ringtones, and podcasts. Here are a few pointers.

If you haven't yet, download the suite of Windows plug-ins from the Linux Audio Developer's Simple Plugin API (LADSPA). When using plug-ins, remember to select the section of track you wish to affect before opening the tool. Also, if you're planning to make ringtones, check out the special tips at the end of this article.

Clipping

If the meter shows your levels dipping into the red, you'll experience clipping.

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Step 1: Adjust the gain
Axing clipping, or buzzy distortion, is a main goal of mastering audio tracks. With Audacity, you can see if too-loud signals will cause your track to clip by watching the green bars in the Meter toolbar. If they bounce into red while playing, you've got a problem. To clip the clipping, you'll want to ease the output level below 0 decibels (dB), which is the loudest volume you can attain without sacrificing quality.

Reducing the track's gain, or volume, is the most obvious fix. Unfortunately, Audacity doesn't reset its red clipping indicators until you stop and replay the track, so you'll need to adjust the sound by trial and error. Note that if your track exceeds 0 dB in its original recording, you'll never completely shake the distortion.

In Audacity 1.3, the repair effect, used before adjusting the gain, can also help lessen minor clipping by fixing selected spikes and other flaws in a track.

Audacity's compressor tool

Experiment with the compressor tool to find a decibel threshold that gives your track a polished sound.

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Step 2: Compress your tracks
Reducing an audio track's gain, or volume, will curb its clipping, but if that's all you do, you're also turning down the song's presence and force. Counter with compression. The Compressor tool (in the Effects menu) will automatically attenuate the track's volume, dramatically diminishing the gain after it reaches a specified level. It's not a hard cap--the gain can still surpass your dB setting--but tapering the sound produces a more natural, polished finish than abruptly cutting off the track.

Like adjusting the gain, the Compressor may require several passes until you find a sweet spot. As a general guideline, slide the threshold between -10 to -16 dB. If you set it too low (e.g., -18) then you forfeit audio definition. Set it too high (e.g., -8) and your sonic information will sprawl rather than coalesce.

After setting the threshold, experiment with the compression ratio to determine where you set the attenuation. The higher the compression ratio, the less your track will rise above the threshold.

Next, set the attack time. If you want the whole signal to compress immediately, select 0. Set the slider in the opposite direction, toward 1, for a softer attack that compresses the track after pausing a few moments. After each change to the compression tool, be prepared to wait about 15 seconds on a midrange PC for Audacity to apply the modifications.

Audacity's equalizer

Lowering the volume on the left of the equalizer will tone down the bass.

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Step 3: Equalize!
Equalizing can selectively increase or lower gain for any specific frequency, not just those that pass a given decibel threshold. By controlling and blending the frequencies between multiple tracks, you can tailor conflicting bass lines, or let a specific section of a track shine.

Select "Equalization" from the Effects menu, and you'll notice a horizontal line that keeps a steady 0 dB across all frequencies. Using the mouse, you'll be able to click a series of points along the scale to create a curve that associates volume--measured in decibels--with high, mid, and bass frequencies, measured in hertz (Hz). On the equalizer, low volume falls below the 0 line, and high volume rises above it. Low frequency lies between 30 to 100Hz, while high frequency ranges from 1,000 to 10,000Hz.

Clicking and dragging to curve the decibels below 0 dB on the vertical Y-axis and between 30-100 Hz on the horizontal X-axis will silence the bass line of that track, and allow the bass of another track to dominate.

Audacity 1.3 drastically improves its equalizer with enhancements that make for a far more roomy and navigable interface, and add controls that include extending dynamic range.

Audacity's hard limiter

Set your dB limit down to -1 for ringtones.

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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 -1 dB.

Audacity's envelope tool

Think of the envelope as the body of a track, and the handles as volume curves.

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Step 5: Create an envelope
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 prerecorded 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.

Optimizing for ringtones

The 'Split stereo track' feature reduces your output from default stereo to a single channel.

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Step 6: Export your finished product
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 ringtone (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 -24 dB 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, lower the limit to -3 dB rather than to 0 dB 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 -3 dB.

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. Canceling 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.

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