Most of the time, the desired result for bass is to provide a uniform, even bottom end that becomes a consistent background for the other instruments. Compression, which reduces dynamic range variations, can help achieve that goal by boosting soft sections and taming peaks.
The following offers suggested starting points for common compressor parameters (Fig. 1), but note that the input level is crucial. For example, if the input is always below the threshold, there won’t be any compression. These settings assume that the bass’s level to the compressor uses up most of the headroom, and comes close to 0 dB.
As you tweak parameters, pay attention to the compressor’s gain reduction meter, which shows how much the compressor is restricting the dynamic range. For example, if the gain reduction meter shows ‑3 dB, that means the gain has been reduced by 3 dB to bring down a peak. Unless you’re going for a heavily compressed sound, you’ll probably want to compress by ‑6 dB or less. Also recognize that because the various parameters interact, it’s often necessary to go back and forth among several parameters until you dial in the exact sound you want.
- Begin by setting the Ratio control to around 3:1. This means that for every 3 dB increase in input level above a specified threshold, the output increases by only 1 dB. Try a higher ratio if the bass signal is extremely uneven, but the higher the ratio, the more it will “squash” signals above the threshold. This can make the sound thinner. Because our ears are not particularly sensitive to level variations, it’s easy to overcompress—be careful.
- Now set the Attack and Release (also called Decay) controls. Attack determines how fast it takes for compression to kick in once the signal exceeds the threshold, while release sets the time required to return to a non-compressed state after the signal returns below the threshold. Many compressors include an “auto” setting for these parameters, which is usually the easiest way to go and often nails the right values. Otherwise, start with a fast attack (0-20 ms) and medium decay (100-150 ms).
- Next comes the Threshold control, which determines the level at which compression begins. Initially set it to the highest available value (usually 0 dB), which means only notes exceeding 0 dB will be subject to compression. Then, while the bass plays, look at the gain reduction indicator. Lower the threshold so that there’s an average of 2 or 3 dB of compression. The peaks from loud notes should indicate around 4-6 dB of compression. This is a common, “industry-standard” type of setting that avoids extremes.
If you need more compression to keep notes sounding even, lower the threshold so that compression begins with lower-level signals, or increase the ratio. But ideally, you don’t want to use too much compression. Hopefully, the bass player will have a sufficiently good “touch” that excessive compression won’t be necessary.
- Compressing the dynamic range lowers the overall level; use the Output or Gain control to compensate. While observing the input levels feeding your mixer or recording software, adjust the output so that the maximum peak (not average) levels of the compressed and uncompressed signals are identical. You can tweak the output control a bit if you need more or less overall level.
Generally, the output gain will approximate the amount of gain reduction. For example, if the gain reduction meter shows 4 dB of gain reduction on peaks, set the output control to give +4 dB of gain.
- Next, adjust the Hard Knee/Soft Knee switch, if present. Hard knee adds more punch to the sound; for a ballad, soft knee may be more appropriate, because the transition into compression is gentler, and more nuanced.
Once the general settings are squared away, you might want to re-visit the attack and decay settings. A fast attack compresses the beginning of the note’s transient, which keeps the attack level even but reduces punch. The release time is a bit of a compromise. It needs to be fast enough so that it doesn’t affect the next bass note’s attack, but not too fast — because of the bass’s very low frequencies, a short decay setting may actually follow individual cycles of the string vibration, and cause a subtle distortion.
Select a very slow release (e.g., 250 ms), play some notes, and listen to what happens. If the note attacks are low in volume because the release time has not yet recovered, shorten the release. Grammy Award-winning guitar player Jay Graydon says that the best way to find a good release time is to start with a fast release setting, and increase the time slowly. When you damp (hand mute) a bass string, the gain reduction meter should move back to 0 at a rate slow enough for you to detect, but not so slow as to take a significant amount of time.
Using EQ with Compression
If adding compression seems to make the sound smaller, try adding some low-end EQ around 100 Hz. Remember that placing EQ before the compressor increases the amount of compression in the frequency range being boosted; because the EQ is increasing the level at those frequencies, they exceed the threshold more often. When adding low-end EQ, you may want to raise the threshold a bit to avoid overcompressing the lows compared to the rest of the signal.