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Manic Compression, Part 2

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Last month, we visited the nuts and bolts of compression. This time, let’s take a look at compression settings for a few specific instruments. As with many things audio-related, these suggestions should be treated as a starting point for your own techniques.

The Voice

Vocals are typically an extremely important factor in a mix, and part of our job as engineers is to make sure that they are heard at all times. This can be a challenge with singers who sing softly or don’t stay on-mic. If your talent is open-minded, you might suggest that it will help both of you if they remain as close to the microphone as possible. Doing so gives you a high “signal-to-noise” ratio — i.e., it helps keep the vocal louder than the background stage noise that inevitably leaks into a vocal mic. When adding compression to a vocal mic — especially severe compression — keep in mind that when the vocalist stops singing, the compressor “lets go” of gain control, causing the background noise (leakage) to get louder and thus screwing up your mix — perhaps by causing the cymbals to get louder, for example.

Adding a lot of compression to a vocal (or acoustic guitar) in a wedge mix can be a recipe for feedback because when the singer stops, the compressor allows the gain to return to the normal level. When mixing monitors from front of house, you’ll need to be aware of this issue as the effects chain will apply to both house and monitors, and the monitor send could be post-insert (check the manual). That’s one of many reasons why touring pros always have separate desks for monitors and front of house.

As a starting point for lead vocal, try a ratio of 3:1 or 4:1, a medium-fast attack time (20 mS) and a medium release time (100 mS). Watch the gain reduction meter and set the threshold so that you get 1 to 2 dB of gain reduction at all times. When the singer stops, you should get little or no compression, and when the singer gets loud, the compressor should increase its action and tame the peaks. If that is not the case, you can either make the attack time faster or increase the ratio. Keep in mind that fast attack times generally don’t work well with crooners, singers who have mellow voices, or voices that don’t cut through the mix easily. A fast attack time will jump on transients (fast spikes in the audio waveform), creating a loss of high-frequency content. Slow the attack time to reduce this side effect. On the other hand, a bright, shrill voice often benefits from a fast attack, allowing you to place the voice louder in the mix while preventing it from becoming overbearing.

Quick attack and release times coupled with high compression ratios (e.g. 6:1 or 8:1 or higher) emphasize mouth and breath noises, resulting in a compression artifact called breathing. Longer release times help reduce breathing. In some cases you may need more than one compressor to tame an extremely dynamic voice. You can set one compressor set as mentioned above and another set to a fast attack time, high ratio (10:1 or higher) and a higher threshold than the first, whereby comp number two acts as a limiter to put a ceiling on the vocal level. Note: a compression ratio of 10:1 or greater is generally considered “limiting.”

Drums

I like to compress kick and snare drums, because it helps them hold a solid place in the mix — especially when the drummer is not hitting consistently. A little compression goes a long way on kick and snare: try a ratio of 2:1 or 3:1, medium release, attack around 15 or 20 mS, and set the threshold for a few dB of gain reduction. If you’re not getting enough gain reduction, try either making the attack a bit faster or lowering the threshold. Excessive compression on a kick drum will make it sound small by reducing the apparent amount of low-end. Fast attack and fast release emphasizes the drum’s resonance, causing the low-end of an undamped kick drum to sustain and possibly sound sloppy, though this can actually be useful when trying to create a TR-808-type of sound.

On the other hand, setting the attack time too long will cause the compressor to miss the kick transient entirely, and the same goes for snare drum. Extremely fast attack times (0 to 10 mS) can mutilate the attack from a snare hit (probably not something you want to do). Long release times on a snare drum can make grace notes and ruffs disappear, while a fast attack combined with a fast release brings out grace notes, ring and can cause distortion (you might like the effect).

There’s a good chance that you’re also gating drum channels. Which comes first, the comp or the gate? This is an important decision. If you compress first, the dynamic range of the signal is reduced — making the leakage closer in level to that of the drum, and making it more difficult for a gate to do its job. If you gate first, only the signal you really want reaches the compressor, giving you a lot of control since you’ll be compressing the sound of the drum, and not the background noise leaking into the microphone.

Bass

Compression settings for bass relate to the style of playing, and (to a lesser extent) tempo of songs. A fast attack (5 mS) is probably best for slapping bass, ensuring that the compressor catches transients. Faster attack times will remove some of the click from string plucks, while longer attack and release times make the compression more subtle and help reduce “pumping,” an artifact that occurs when the release time is too short for the decay of the bass note. Suppose the bass is playing a note that fades smoothly over several seconds, and the compressor’s release time is set to 250 mS. At some point as the note fades, the signal level will cross the threshold and gain will come back to normal. Depending upon the duration of the note, this may cause the note to get louder instead of fade out — a very unnatural sound. When this concept is applied judiciously to electric guitar, it adds sustain. Try to make the release as long as the sound lasts and no longer.

You can compress a bass DI fairly aggressively (10:1) without penalty, but take care in using a lot of gain reduction on a microphone placed on the bass amp. It’s possible to induce feedback between the P.A. and the bass amp mic for reasons similar to those discussed earlier regarding vocal compression in the monitors. Sometimes with bass, it’s nice to split the DI to two channels, apply compression to one of them and mix the two together —which provides consistency without sucking the life out of the performance.

 

Help Stop P.A. Abuse

A very useful application for compression is protecting gear from abuse or ensuring that a P.A. system is never able to exceed a specific SPL. By setting the threshold slightly below the level at which a mixer will push power amps into clipping, a compressor can maintain (or limit) signal level, even when the mixing console’s master fader is raised past the “0” mark. In situations like this, a high ratio (20:1 or higher if possible), fast attack and medium release can “govern” the system.

Make sure that the compressor is patched between the output of the console and the input to the next stage (usually system processing). If you insert the compressor, it will likely be before the master fader, so raising the master will still drive the next stage to clipping. I learned this from an engineer who installed club DJ systems. He’d set the limiter as mentioned and then cover the front of the rack with tamper-proof screws. Even when a DJ pushed the mixer output as high as possible, the system would stop getting loud, protecting the gear from damage and avoiding a call from the club owner screaming about how the system “broke.” You can also use this technique to make sure that the volume of any P.A. system remains below a prescribed volume, regardless of how meddling fingers might set input channels or the master fader.

—Steve La Cerra