Installing and calibrating public address systems requires a different skill set to what most home cinema installers have. Anthony Grimani explains the fundamental differences and how to start navigating commercial-grade audio systems.

So you’ve just finished installing the perfect home cinema for a client when he comes to you and says, “Hey! The sound at my kid’s school is terrible. There’s all kinds of feedback, nobody can understand what the teachers are saying and the announcer for our basketball games might as well be shouting through a bullhorn. I have a great idea… Why don’t you come fix it for us?”

Of course you say yes, because you’re awesome. But you really have no idea what you’re doing, because, well, sound reinforcement is nothing like what you know how to do.

No worries.

In the May edition of Connected Home, I wrote about some of the nuts and bolts of professional, commercial and PA systems; this time I’m going to touch on the bigger stuff. Take this and run with it.

Let’s say the school has a few lecture halls and a gymnasium.

Lecture halls (maybe 18x12x6m) provide the perfect opportunity to apply my advice from last time about speaker selection. Get a couple of powered units from someone like JBL, Mackie or Behringer that meet the criteria for coverage and SPL and you’ll have the genesis of a good system.

Position them far enough out in front of the podium or stage so that the microphone doesn’t feedback. After you’ve checked for proper coverage of the audience, make sure the extreme edges of the coverage pattern don’t leak sound back toward the mic. Then instruct the presenters where they can and can’t go to stay out of feedback trouble.

Also, realise that the common process of putting two speakers in the room to get ‘stereo’ sound doesn’t apply properly here, and that in fact the interferences of sound waves from the two speakers to the ears of the audience will reduce voice clarity. If possible, find a way to hang just one speaker above the speaker podium area. You may need two speakers splayed out to achieve proper coverage or sufficient SPL, and that is OK to do. I also recommend reducing the sound reverberation in a lecture hall. If the room has all hard walls, the reverberation time can often be one second long, which severely reduces voice clarity. Covering the walls and ceiling with between 20 and 30% of absorptive surfaces will clean all that up, and make for much better sound.

It helps to choose a microphone with good rejection characteristics. You want to pick up the presenter’s voice and nothing else. A dynamic mic with a cardioid pattern is a rugged, affordable choice for a fixed podium, but the presenter must be close to (almost swallowing) it. More expensive condenser cardioids and super or hyper cardioids can work a little farther away, but can be more prone to feedback and breath pops. Avoid omnis and figure-eight patterns.

For mobile presenters, forget about lavaliers and spring for something that gets the pickup element close to the presenter’s mouth. Countryman and others make very discreet mics no bigger than a wire that fits around the ear and extends a short way down the cheek. As a bonus, it’s available in different colours to match skin tone.

Next up is the gymnasium. Where do you start? Treat it with sound absorption! Even the best PA can’t overcome a 3+ second reverb time. A general rule of thumb is to cover 30-35% of the surfaces with dense glass fibre or rock wool absorption at least 5cm (effective to 500Hz) or preferably 10cm (effective to 125Hz) thick.

It’s also a much a bigger room, so it needs bigger sound. Unfortunately, you can’t just stack more of the compact JBL speakers from the lecture hall. Why not? Comb filters. These occur when the same sound arrives at a listener’s ear from two (or more) different sources. It doesn’t just sum and get louder. The timing difference causes narrow phase cancellations that make the frequency response resemble the teeth of a comb. In the case of your JBLs, each one is another source. By the time you’ve piled on enough to get adequate SPL, you’ll have a comb filter problem of epic proportions! The sound will be brittle, unintelligible and wishy-washy.

What you need are bigger speakers. At the size necessary for a gym, the speakers may be broken up into separate high-mid-low cabinets with different directivity specifications and power ratings. Don’t panic! This can actually work to your advantage, as the better speaker manufacturers typically provide software tools to help you pick the right cabinets and orientation to achieve the desired coverage and SPL. Handy!

A big trend these days is towards ‘Line Array’ speaker systems where units are stacked in tall columns so as to provide a more directed sound projection, and therefore reduce the amount of perceived room reverberation. Separate cabinets will often require external amplification and processing. These are readily available in professional models that may make you jealous of the price and features. Most speaker manufacturers will recommend amplifier power, delay and crossover settings for your cabinets, but you still need an analyser like SmaartLive to properly time the system. You should already have an analyser from your work in home cinema, so that’s no problem!

Once your PA systems are up and running, you’ll need the analyser again to tune the speakers to the room. A digital parametric EQ is your friend; you will probably need to supply one with the lecture hall system, but the processor for the gym may offer EQ as a DSP function. Techniques for tailoring the response of PA systems are too numerous to discuss here, but you can find the popular methods online.

Finally, know when to throw in the towel. Big PA systems for auditoriums, theatres and other performance spaces can get very complicated very fast when you start adding stuff like multiple line arrays; audience warmers; delayed side and back fills; subwoofers; stage monitors; wireless systems for mics, instruments and personal in-ear monitors; analogue, digital or Ethernet snakes carrying audio between the stage, front-of-house, monitor world, and equipment racks; and separate consoles for front-of-house and monitors capable of mixing 48 (or more) channels each. There are people who specialise in these systems. It may be to your advantage to seek them out for a consult or referral.

Remember, you want to keep your client happy. Making the PA at his kid’s school worse is not the way to do that!