Threshold, Ratio, Attack, Release and More
The settings depend upon both the music and the playing style of the bassist. It's necessary to listen to the part and adjust the compression settings to obtain the sound you want. If the compressor has an input level, adjust it to get a nice hot signal, but not loud enough to clip.
The threshold level determines the maximum level—signals louder than the threshold are reduced in volume. At 0dB, only the loudest signals (peaks) are compressed, retaining most of the natural dynamics of the player. Settings between -2dB and -5dB are often used; when even more compression is desired, a threshold of as much as -10dB or even -15dB might be chosen.
The ratio setting determines how much the signal is reduced. As an example, a 2:1 ratio cuts the output of any signal above the threshold in half. A ratio of 3:1 is a good place to begin in your search for the optimum ratio. 4:1 is also widely used, but occasionally a ratio as high as 10:1 will suit the sound. Since the electric bass has a wider dynamic range than many instruments, compression is often applied in order to maintain a more even volume level and to match the dynamics of the kick drum. When peaks are reduced, the entire bass part can be boosted in the mix without adding distortion. Remember, the settings depend upon both the music and the playing style of the bassist. It's necessary to listen to the part and adjust the compression settings to obtain the sound you want. The higher the ratio, the more the signal is compressed.
The attack setting will determine how quickly the peak is reduced. Slower attack times allow the initial transient of the note to come through, for a punchier sound. If the attack is fast, all sharp peaks will be cut, and the part will be smoother. A medium attack time of 20ms to 40ms is a good starting place.
The release time (also known as decay time) determines how quickly the compression goes away. If it's set too low, it may compress a quieter note that rapidly follows an above-the-threshold note. A medium setting is good—fast enough to be ready for a quiet note, not so fast that it boosts noise that occurs between notes. Between 125ms to 250ms is usually appropriate for bass.
The Empirical Labs Distressor imparts a warm, vintage sound inspired by classics like the 1176 and LA-2A.
Depending on which compressor you use, you may be able to select between hard-knee and soft-knee. Set on hard-knee, the compressor waits until the signal crosses the threshold, then it reduces the signal at the specified ratio for a punchy sound. With soft-knee compression, the ratio gradually increases as the signal approaches the threshold, resulting in a more natural feel and a wider dynamic range.
In addition to all of the standard controls, Waves' fantastic plug-in recreation of the classic API 2500 Compressor includes 3 different Knee settings.
If the output level is equal to the level of the peaks of the uncompressed signal, the overall loudness will be higher. However, cheap compressors may add noise when the output level is raised, so it may be preferable to boost the volume at the mixing board. Adjusting one setting affects other settings. For example, an adjustment to the threshold may require an additional adjustment to the ratio. Keep at it until you get the sound that fits.
Additional Thoughts on Compressors
Now that you've got a handle on how compressors work, your next step is to find the right one for you. Consider your budget, sound and workflow. If you're an in-the-box type, there are a variety of affordable, great sounding compressor and limiter plug-ins to choose from, based on classic and new designs. If you're looking for something that can pull double duty in the studio and on-stage, you should probably consider a compressor pedal, whether designed for guitar or bass. Finally, if you're not the type of player that's scared by a rack and patch bays, we've got plenty of rack-based compressor and limiter options for you to choose from. You might even want to consider a channel strip, which typically includes EQ and a compressor.