The EM Poll

P.A. 101

Mar 14, 2008 3:24 PM, By Emile Menasche

           

HOW TO CHOOSE THE RIGHT SOUND SYSTEM FOR YOUR NEEDS WITHOUT TEARS

All Aboard for Outboard

In the recording studio, you use outboard gear such as compressors, gates, effects, and other signal processors to put the finishing touches on mixes. In sound-reinforcement situations, the primary purpose of outboard signal processors is to maintain sound quality in a transparent fashion rather than to add sonic interest. (Of course, a sound engineer who knows the band's material well can still put creative effects to good use.)

Compression. A compressor is often essential for controlling the dynamic range of individual voices and instruments. Use it carefully, however, because it can increase the risk of feedback on some acoustic instruments and can significantly affect the sound's timbre.

Limiting. For practical purposes, a limiter is a compressor with a compression ratio that is high enough to prevent a signal from getting noticeably hotter. A limiter with a very high compression ratio is referred to as a brickwall limiter because it stops the signal's level from increasing as surely as if the signal had hit a brick wall. Most live engineers strap a limiter across the main mix outputs as a means of protecting the speakers (and the audience) from sudden unexpected peaks. Again, you have to be extremely careful about how you set the limiter, because if you limit the signal too severely, your system will be working hard, yet not generating enough power. The result will be inadequate signal coming from the speakers and an increased potential for feedback.

Gating. Noise gates are useful tools for sound reinforcement; when a signal level drops below the user-selected threshold, the gate closes, silencing the channel. Applying a gate to a vocal mic effectively mutes the mic when no one is singing into it, which can reduce the amount of unwanted bleed into the mic from other stage sources (such as guitar amps and drums), as well as reduce the chance of feedback.

Equalization and feedback control. Used properly, an equalizer can shape your sound to near perfection. It can also solve some thorny sonic problems. But overuse of EQ can do more to undermine your mix than that screaming drunk in the first row. There are two main uses for EQ on stage: tone control and feedback control.

To a degree, tone shaping is a matter of personal taste. But if you're a performer doing double-duty as a sound engineer, maintain some perspective on the sound. Have a friend sit in the audience and listen to the clarity of the mix. Many inexperienced sound engineers tend to overboost high- and low-midrange frequencies to get more boom and sizzle in the mix. This can lead to muddiness, distortion, ear fatigue, and an annoyed audience.

EQ can also be extremely useful for clarifying a monitor mix, allowing the musicians to hear the monitors better without the engineer having to jack up the power. That helps keep down the sound-pressure level on stage, which is a key element in obtaining a good mix.

Feedback control is one of the essentials of live sound. Whenever you set up your P.A., you'll need to "ring out" the system. Now, what I am about to say is oversimplified, because you would need another article to properly discuss ringing out a sound system. In general, you need to set up your microphones and then use a signal generator or test-tone CD and CD player to inject a pink-noise signal through the system. Boost specific frequencies with an equalizer (usually a graphic EQ) to see which ones induce feedback. The ones that ring are the ones you'll need to attenuate. Use the narrowest bandwidth possible to do this; the ideal type of EQ configuration has a notch filter, which focuses on very specific frequencies. The narrower the frequency, the less the cuts will affect the overall tonal balance of the mix.

While many live engineers still use the manual method, automatic feedback eliminators can ring out the system for you. They work by sensing which frequencies are feeding back and then quickly applying a very narrow, precise notch filter to attenuate those frequencies. The advantage of an automatic system is that it can detect problems continuously as the performance evolves-and that can save the day if a new feedback source crops up.

Note that mic feedback (ringing) is only one kind of feedback you may encounter on stage. (No, I am not referring to remarks from the audience.) Acoustic instruments also generate resonant feedback, which is best tackled at the source (the instrument itself) rather than at the mixing board. Resonant feedback happens when a certain frequency causes the body of an acoustic instrument to vibrate on its own. Flat-top acoustic guitars are particularly prone to this problem, especially if they're sitting on stage in the path of a speaker. Grand pianos are vulnerable, too.

Ambient effects. Delay and reverb are important tools in sound reinforcement. Use caution: a venue has its own natural ambience, and too much reverb coming from the stage muddies things up considerably. If your singers insist on a wash of reverb, feed more into their monitor mix than you feed into the house. This will keep the singers comfortable without bathing the house mix in mud.

Hit the Road, Jack

If you're serious about your sound, invest in a complete and integrated sound-reinforcement system that can deliver your music with no excuses. Even if most of your venues provide a house system, the more you know about how that system works, the better you can work with the sound engineer to get your music across.