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Dynamics, Demystified

June 26, 2012
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Grace Potter and the Nocturnals at Coachella 2012.
Dynamics are essential in adding expressiveness to music, but excessive or inconsistent dynamics can be the bane of live performance. Vocals, bass, kick, and snare are the usual suspects for dynamics control in live applications, but any instrument producing inconsistent levels can benefit from dynamics compression.

Compression usually patches into a channel insert, and by dedicating it to one signal you can optimize its settings for that signal. In the analog world, that means you’ll need a separate hardware compressor for every channel, although stereo compressors can usually work as dual-mono processors (Figure 1). In the digital or software-based console world you’ll often find dedicated dynamics per channel (compressor and gate) or you can use multiple instances of a single plug-in (Figure 2). Regardless, insert any compressor pre-fader. Software-based mixers may provide the option to place the compressor either pre- or post-fader; inserts on most hardware mixers are pre-fader. If you insert the compressor post-fader, every time you vary the channel fader the dynamics (pun intended) of the compressor change. Not good. With the compressor patched pre-fader you can set a threshold based on the sound arriving at the mic—and not your manipulation of the fader.

 
Fig. 1. The JDK Audio R22 compressor is stereo, but like most stereo dynamics processors, offers separate I/O for the two channels so it can be used as a dual-mono unit. The R22 has both XLR and 1/4" I/O, making it easy to patch into an insert, between a signal source and mixer with XLR ins, or to compress the overall output on a mixer with XLR outs.
Certain hardware compressors (and gates) feature a rear-panel operating level switch (+4/–10), ensuring proper level-matching with your console. Consult the specs to determine the operating level of the mixer’s inserts. If you set the compressor to –10 and the console insert operates at +4 you’ll get too much compression (even when the compressor’s threshold is all the way up) and the compressor’s input will overload easily. If the comp is at +4 and the console insert is at –10, you’ll need to bring the threshold way down to get any compression and you’ll have to crank the output gain, adding noise.

Don’t Squash the Lead Singer When compressing a lead vocal, start with a ratio of 3:1 or 4:1, medium-fast attack time (30 ms), and medium release time (100 ms). Set the threshold for 1 or 2dB of gain reduction whenever the vocalist sings. When the singer gets louder, the compressor should start reducing gain and smooth out the variations. If not, then lower the threshold, make the attack time faster, and/ or increase the ratio. Slow attack times work well with singers who have mellow voices, or voices that don’t cut through the mix easily. That’s because a fast attack grabs transients, resulting in a loss of high frequencies. Slow the attack time to reduce this. On the other hand, a fast attack can tame a bright or shrill voice, allowing you to make it louder without it becoming overbearing. High compression ratios (e.g., 6:1, 8:1, or even higher) combined with overly fast attack and release times will emphasize lip smacks and breath noise, resulting in a compression artifact called breathing. Longer release times help reduce breathing.

 
Fig. 2. WavesLive MultiRack Buy MultiRack Native is a plug-in platform designed specifically for live sound. Although used by a lot of the major tours, the concept is equally applicable to smaller bands—if
not more so, due to the compact and predictable nature of setup.
What happens when a vocalist does not sing into their vocal microphone is almost as important as what happens when they do. Suppose you have 6 to 8dB of gain reduction on the vocal. When the singing stops, the signal drops below threshold and as there’s no longer any gain reduction, the gain comes back up to normal. This is akin to someone turning up the channel fader. Now envision a drum kit behind the lead singer—the vocal mic has turned into a mic for the drum kit. To avoid this issue, eyeball the gain reduction meter while the singer is not singing. You should see little or no gain reduction. If you see more than a few dB of gain reduction, raise the threshold to avoid compressing the background sounds. Beware of applying excessive compression to a vocal mic in small, lively rooms because when the vocal stops and the compressor “lets go,” the mic gain comes up—inviting feedback.

Slappin’ the Bass DI’d bass guitar is arguably the instrument most in need of compression. Attack and release times relate to the style of playing, and (to a lesser extent) song tempo. A fast attack (5 ms) ensures that the compressor does not miss the transients on slapping, but this setting can reduce some of the definition from finger picks, and when combined with very fast release times can cause distortion. Lengthening attack and release times avoids distortion as well as pumping, a compression artifact that occurs when the release is too short for the note’s decay. Pumping causes the note to get louder as it decays instead of fading out—a very unnatural sound. Try to set the release as long as the sound lasts and no longer (as you’ll hear in the online sound clips). Apply compression conservatively to a mic’ed bass amp because it might create a feedback loop between the P.A. and this mic, for reasons similar to those discussed earlier regarding vocals.

Compressing kick and snare helps the drum kit maintain a solid place in the mix (Figure 3). As gates are also typically used on drums, the question arises, “which comes first, the comp or the gate?” Answer: the gate. Compressing first reduces the dynamic range of the signal entering the microphone—making the (unwanted) leakage closer in level to the desired signal, and therefore more difficult to remove with a gate. Gating first ensures that only the signal you really want reaches the compressor, providing you with a huge amount of sonic control.

 
Fig. 3. Universal Audio’s 1176LN compressor has been a staple of drum sounds since the 1970s; the 2-1176 builds on UA’s 6176 Channel Strip and combines two 1176 channels from that unit. The 2-1176 can be used as a stereo or dual mono compressor.
Compression can kill a kick or snare sound, so use your ears. Super-fast attack times (0 to 10 ms) can actually remove a snare hit’s attack, while slow attack times may miss the hit entirely. Fast attack combined with a fast release time can cause distortion (though you might like it!) and emphasize room ambience and the ring of the drum. Make the release longer to quiet the room “noise.” The latter settings with a harddriven input, combined with a high ratio, can turn your snare into an industrial clang (check out the snare examples online). For a more subtle approach, set the ratio to 2:1 or 3:1, set the release to about 120 ms, and attack to around 15 ms. If you’re not getting enough gain reduction, either make the attack a bit faster or lower the threshold. When the attack is just right you’ll get a nice “thwap” (that’s a technical term that indicates the amount of thwapness).

 
Fig. 4. Radial Engineering’s Komit brings a compressor/limiter to the compact 500 Series format. In this device the limiter is designed to offer some “character” via diode clipping.
A kick drum’s low-frequency content adds power to a mix, but also makes it challenging to wrestle into a mix. This is especially problematic with inconsistent drummers. A small amount of compression goes a long way to even out variations: try a ratio of 2:1 or 3:1, with medium attack and release times, when you’re getting 4–6dB of gain reduction. Too much compression causes a kick drum to 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 sound sloppy (though it could be useful if you’re trying to recreate a TR-808-type “hum drum” sound). As with a snare, excessively long attack and short release times will cause the compressor to miss the sound of the drum entirely.

You Got Protection? A limiter is a specialpurpose compressor with a high ratio (10:1 or higher) and a very fast attack time. Limiting puts a ceiling on signal level, keeping it from exceeding a maximum level (Figure 4). If that’s your goal, set a compression ratio of 10:1 or 15:1, attack as fast as possible, and a medium release. Set the threshold so that gain reduction starts where you want the signal to stop getting louder. Some compressors offer a “peak stop” feature in addition to the usual compression controls, in which case you can set the peak stop level to your desired ceiling and still apply subtle compression to even out minor level variations.

Perhaps more importantly, limiting can help protect your gear, or simply ensure that a P.A. system never exceeds a desired SPL. Setting a threshold slightly below the level at which a mixer will drive power amps into clipping enables the limiter to “clamp down” on signals even when an engineer is pushing the console hard. For this purpose set a high ratio (20:1 or higher if possible), fast attack, and medium release. One tip I learned from an engineer who performed club installs for DJ systems was to set the limiter as mentioned, then put a security cover over the limiter 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.” Equipment health aside, you can 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 mixer masters.

Expanders and Gates An expander is a dynamics processor that increases a signal’s dynamic range by making soft signals softer. Expanders aren’t used much in live sound applications; the noise gate, a severe form of expander, is much more common. Rather than just reduce level of quiet sounds, a noise gate actually mutes sound below the threshold. The most popular use for a noise gate is hiding bleed on drum mics (note the key word is hide; a gate does not eliminate leakage), enabling a higher degree of control in the mix (e.g., when you raise the snare mic, you’re not also raising the tom). Cymbal mics are generally not gated because most overhead/cymbal mics capture the toms and snare, making it impossible to gate them.

A gate works like an audio crowbar: You have to be able to wedge the threshold in between a loud and soft signal or the gate can’t work. That means you need to start with at least some measure of isolation from unwanted sounds. Placing a mic inside a kick drum often provides sufficient isolation; adding a gate to the kick channel improves it. In situations where a stage floor is resonating you may get feedback on kick hits, and a gate can help control that. If you gate too hard you’ll lose some of the drummer’s finesse and grace notes. When Blue Öyster Cult plays the song “Bux Boogie,” there’s a section where the band plays very softly; for that section I’ll bypass the gates on (drummer) Jules Radino’s kick and snare mics so that he can play as softly as he wants without worrying that the gates will cut off his playing. Ditto for his drum solo.

When adjusting the gate’s threshold, try to set it so that the desired drum always opens the gate but the other drums do not. For example a tom gate should always open when its respective tom is struck, but should not open on snare hits. Your ability to do this depends upon how hard the drummer hits and how carefully you placed the microphones. I usually set a gate’s range or depth for maximum attenuation so that when the gate closes, no sound gets through. Decay and hold parameters need to be set by ear. During sound check have someone play each drum one by one, and set the hold so that the gate stays open only as long as the drum’s sustain—and no longer. Then add a bit of decay so that you can’t hear the gate click closed. I usually set the attack time as fast as possible but sometimes this produces an audible click (much like a lip smack) every time the gate opens. Slowing the attack time a bit often cures this, and also makes the gate less likely to open when another drum hit occurs. Clicking noises tend to be more common in inexpensive hardware units but if you’re careless, any gate can produce unwanted noises.

In future articles we’ll examine how to use filters, keying, and sidechain techniques for dynamics processing—which makes the whole subject of dynamics control even more interesting!

Steve La Cerra is an independent audio engineer based in NY. In addition to being an Electronic Musician contributor, he mixes front-of-house for Blue Öyster Cult and teaches audio at Mercy College White Plains campus.

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