Putting The Squeeze On Live Sound

Speaking Of Limiters

To function as limiters compressors use ratios of 10:1 up to infinity:1. Apart from special applications like the `too small’ system, the most common use is an overall system protection.

In its most basic form, this consists of a single limiter placed before the system crossover, or if the system is passive, before the amplifier. If you’re running a stereo drum submix, you could insert two compressors on the submix, rather than on individual drum channels, which you may prefer to gate. You’ve got slightly less control, but compressing the group will keep all the drums under control. Just be mindful of that output gain.

A more sophisticated form is to put limiters on each output of the crossover. This works well, because each frequency band is limited separately and doesn’t interact with any others, making any limiting less noticeable. On the downside, though, you’ll need a lot of limiters on, say, a stereo 4-way rig!

A more musical, less expensive idea is to use a multi band compressor over your main mix. This splits the signal into low and high frequencies, compresses them separately, then recombines them into one and sends it off to the rest of the system. A unit like this—such as the ARX Afterburner (apologies to ZZ Top!) allows the engineer the opportunity to make creative compression changes to either low or high frequencies, for more perceived bottom end, perhaps, or as a unique sound creator when used as a channel insert.

A Few Good Channels

Most engineers own/have fewer compressors than they really need. But some dynamics control is better than none. So let’s assume you’ve only got one compressor. Try using it over the whole mix at about 3:1 ratio. And if you’re running stereo, you’ll need two channels that track together (usually a switch marked stereo link or similar).

Tracking together means one channel usually controls the dynamics of the other. If your compressor(s) are properly adjusted the whole mix will improve because any transient loud parts are automatically pulled back before they get too loud. In real terms it fattens up the bottom end and keeps your stereo image centered.

A mandatory experience for most live music engineers is the inevitable too-big-a-hall with too-small-a-system scenario. By increasing the compression ratio to about 10:1, compressors in limiter mode can establish the impression of big and loud across the whole mix. At this quasi-limiter setting, the compressor soaks up all peaks, so you can push things harder, increase compressor output several dB, and keep the whole mix much louder.

The downside is that only one input channel at a time is going to be up front because whatever is loudest in the mix triggers the compressor, and effectively pushes back the rest. So, when there are vocals, they take over, and when there’s not, the band (usually kick and/or snare) shifts up front instantly.

Also remember that setting the threshold so that the limiter circuit is always operating at max, over an entire program will strip the mix of any dynamic content and turn an otherwise good mix into a monotonous drone. Still, no one can manually move enough faders fast enough to really control a loud mix. So, while imposing single channel limiting over an entire mix does compromise sonic purity, it also gets the gig done.

The key is to make sure the entire mix is not being overly modulated by the kick and snare drum as everything but those drums will drop out momentarily on each beat. It’s a matter of touch.

Bigger Rigs

If you have multiple compressor channels, say four, you can put one on the kick drum, one on the lead vocal, and one each on left and right. That way you can have a big kick sound that’s really up front but not overpowering, while the vocals can be loud but not so loud that they duck everything else on a loud scream. Start at about 3:1 ratio and experiment; too little compression and the levels will be uneven; too much and the sound will be strangled.

In a mid-to-large setup, say a 40-channel console with a dedicated 8-channel monitor mixer, as many as eight 4-channel compressors—plus an Afterburner-like device on the main outs—may be needed. If you’ve never worked at this level you might wonder where to use 32 compression channels? After all, a minute ago we were lucky to have just one.