So far in our Building Your Personal PA blog series, we’ve looked at why having a good PA is important, as well as examining mixers, amps and speakers, and microphones. In our fifth and final instalment, we’ll cover signal processing for personal PAs.
After reading this article, you’ll be ready to dial in your personal PA mix to perfection.
The Equaliser (EQ) is probably the most important processor in the live sound arsenal. EQ can help you adjust the main speakers to match the room acoustics, eliminate feedback in the mains and the monitors, and sculpt the sound of individual channels.
Whether you’re using a powered analogue mixer, passive analogue mixer, or a digital mixer, you’ll find individual EQ controls of some sort on each individual channel. In more basic mixers, these may consist only of knobs that boost and cut the low, mid, and high bands. Higher-quality mixers often add sweepable frequency knobs to the midrange or even to all three bands. These give you more precise control over which frequencies you’re adjusting. The channel EQs in digital mixers provide more control and more extensive features than you typically find on analogue models.
The graphic EQ is another important Equaliser type for a PA system, especially if you have monitors. Graphic EQs allow you to boost or cutting a range of fixed frequencies. The more frequency sliders on a graphic EQ, the more precision it offers.
Some mixers offer graphic EQs built in, or you can add them in the form of outboard rackmount units, which get connected between the outputs of your mixer (main or monitor) and your powered speakers or amplifiers. Digital mixers provide a variety of EQ types including graphic EQs. For example, the Soundcraft Ui12 features a digitally controlled 31-band graphic EQ as well as a host of other effects.
Tour sound engineers use the graphic EQ for “ringing out” the speakers, which involves boosting gain on the various frequencies of the graphic EQ until feedback is heard, and then cutting them. When properly done, it allows you to get more gain from the speakers before feedback. It’s an especially important task to do with stage monitors, which, because of their proximity to the microphones, are highly prone to feedback. Check out this series of articles to learn how to do it.
Feedback suppressors are commonly found in digital mixers, as part of speaker management processors (see section below), or as standalone outboard units. Most feature digital algorithms that detect and attenuate feedback frequencies before the feedback becomes audible.
While they shouldn’t be thought of as a substitute for properly equalising your system, they function as a secondary line of defence against feedback, and can make operating a PA system a lot easier.
dbx provide an exceptional digital algorithm known as AFS (Anti Feedback System) in their DriveRack signal processors and AFS2 Feedback Eliminator.
Compressors and Limiters
Dynamics processors, such as compressors and limiters, control the dynamic range of your music, which is the difference between the loudest and softest portions. They can be used for applications like smoothing out a lead vocal on an individual mixer channel or preventing loud volume spikes in the main speakers.
Compressors work by lowering the level of any signal that goes above a user selectable threshold. The amount of compression is controlled by the ratio control, which is expressed in decibels, such as 2:1, 3:1, 4:1 and so forth. A 2:1 ratio will reduce the signal that exceeds the threshold to half its original level, a 3:1 ratio to a third of its original level, and a 10:1 to a tenth of its original level. The higher the ratio, the more you’re squashing the dynamic range, which is the difference between the loudest and softest signal.
While you’re likely to find compressors for each channel in a digital mixer, most analogue mixers, except expensive professional models, don’t include individual channel compressors.
A limiter is an extreme form of compressor, which has a very high ratio, and is designed to squash down peaks. Limiters are usually found on the master channels of digital mixers and some analogue mixers. They keep loud peaks from getting through to the speakers, which is important both for speaker protection and for preserving the audience’s listening experience.
Speaker Management Processors
A speaker management processor is typically an outboard processor that offers a range of functions for a PA system. These often include equalisation, feedback suppression, dynamics processing and crossovers (the latter is useful if your system includes a subwoofer).
There are affordable models available, such as the ultra-affordable dbx GoRack Performance Processor. It can be connected to your speakers, and lets you easily control the volume, mute the entire system, adjust the Anti-Feedback circuit, and add compression and EQ. It also has a Sub Synth feature that enhances low end in the speaker system by synthesizing frequencies an octave below the area of 100 Hz. This feature can make your speakers seem to reproduce more bass.
Reverb and delay are two important effects to have in your PA, allowing you to add space to vocals and instruments. You could use an outboard reverb unit or multi-effects unit, connected to your mixer. You probably won’t need to do that, however, because most contemporary analogue PA mixers (and powered mixers) have built-in effects, offering at least reverb. Some give you a range of creative effects including chorus, flanging and more. For example, Soundcraft Signature Series consoles are equipped with a Lexicon digital effects processor built-in.
You’re likely to get the widest selection of effects from a digital mixer. It also has the advantage of letting you save custom settings.
Because mixers often provide some or all the signal processing you’ll need for your PA, it makes sense to wait until you’ve settled on a mixer to decide whether you need additional processors. Depending on your system, you can probably get by with the channel EQs and reverb from your mixer as your only processors.
If you have monitors in your system, you’ll probably find you need a graphic EQ, as well. If you can swing it, speaker management processors with built in limiters and anti-feedback circuitry will make your life a lot easier.
First published in the HARMAN Insights Blog by Mike Levine, former editor of Electronic Musician. Mike has written numerous articles on music technology and recording.