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Sound Advice

Curing An Out-of-phase PA – by Chris Zackoor

Here are a few things to keep in mind when coming up against an out-of-phase sound system in a club situation. Remember the house engineer and you are on the same team so you don’t want to offend him/her by jumping down their throat insisting that the system is out-of-phase. Have a little tact in the situation – it goes a long way!

Most engineers – including myself – will pull out their favourite CD to tune the system. It’s not always at that instance that you will notice the problem but you will know that something is missing. What I mean by missing is that there is hardly any bass or bottom end. When this happens you’ll notice that the system has an empty sound to it, a cancellation of frequencies. You know that there’s bottom end, just not enough of it. Assuming that all the bottom end cabinets are matched they will be 180 degrees out-of-phase with each other and this is your problem. At the same time you’re listening to the system walk around the room so you can assess the situation. There is a good chance that you will be clipping the bottom end amps. When you have come to the conclusion that the bottom end is out of phase pull out your trusty Brooks Sirens Systems (BSS) phase checker and phase test the bottom end. When you have the proper phase of all the bottom end components you will then have no phase shift between components, collectively producing coupling cabinets. When finished with the bottom end you might as well go ahead and phase test the rest of the system.

Now what if it’s not as easy to test the system because you don’t have or can’t afford a phase checker or some other time aligning crossover/processor? You will have to trust your ears and your know how to phase test the system. It may take a little more time to do but here are a few pointers.

Starting with the bottom end again, because in my opinion, it is always the easiest to perceive it being out of phase. It doesn’t matter if the system is in stereo or mono, put on some program music and walk to the centre of both the left and right PA stacks or to where the PA will have the most coupling (on-axis). Assess the situation from thereby listening to the system and observing the amplifiers status lights. Then stand in front of either the left or right stacks (off-axis) and assess the situation. You should notice the difference between standing on and off-axis you will have to test the phase of each bass cabinet or pair of cabinets. Keeping in mind that in a club situation you may have 2, 4, 8 or 16 low-end drivers depending on the size of venue so you better get busy!

Let’s say you have two low-end cabinets per side loaded with 15″ or 18″ drivers and that each cabinet is on either side of the amplifier. Turn one side of the amp down and listen to the cabinet by itself. A quick test is to turn the other side of the amp up to hear if the low end is coupling with the other cabinet or not. You may have to do this a couple of times to make sure. When doing this you will definitely notice a difference between it being in or out-of-phase.

Chris Zackoor is FOH/tour manager for the Gandharvas – currently on tour in the US.

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