If you are a FOH sound engineer then you often work on sound systems that someone else has set up for you and you need some way to make sure it was done properly. Whether it’s your best friend in the most famous venue in the world or the local AV provider in small town USA, you’ve got to cover your ass.
Before I go any further, it should be clear that I’m exaggerating here to have a little fun. Open communication is the first best strategy for a safe and successful live event.
No matter the conditions or the people, you always need to verify that things are in place so that you can do your job to the best of your abilities and make your client happy. In this short article I’m going to focus on one of the more difficult things to verify: sub alignment.
We all want a solid low end and for the audience. It’s one of the main reason they even leave the house.
Certain bits of research have found that upwards of around three-quarters of what people determine is high quality sound comes from the low frequency content. It’s really important to get the low end correct at live events.Adam Hill
So if someone else set up the sound system, how can you check their work without redoing everything yourself or generating mistrust?
How much delay did you put on the mains?
A well-formed question can reveal a lot about someone’s work and can be thrown in as part of a getting to know you conversation.
Take a look at the speaker placement and make a quick estimate about how the sub alignment should play out. In a common situation with ground based subs and flown mains, you can assume that the subs will need some delay to equalize the physical distance offset.
If the mains are 10ft up in the air, you can estimate about 10ms of delay. But instead of asking, “Did you delay the subs 10ms?”, be sneaky and say, “Hey, how did the sub alignment turn out? How much delay did you have to put on the mains?”
You expect them to say, “Mains? You mean subs, right?”
Despite an obvious distance offset, it has happened to me in the field that the mains were delayed 30ms. When I asked why the tech said, “Because that’s what Smaart told me to do.”
I totally get it. I’ve been there. You’re trying to measure with a single mic in some shitty middle school auditorium, and the phase trace is FUBAR. So you do your best with the time and resources and miss the mark. Then later there’s a forehead slap moment when you realize that a visual check renders that solution invalid.
- Did you have to invert the phase on any subs?
- I usually high-pass my LR buss, my matrix outputs, and then again in the DSP. Is that OK with you?
- What’s your favorite crossover filter? I always use Linkwitz Normalized.
- I’d prefer to use 4th order all-pass filters if possible.
I got this idea from my friend Clint. Imagine that you arrive at the venue in the morning. Nothing has been loaded in yet, and you’re still drinking your coffee. You can take a couple of measurements with your laser disto and either run them through SubAligner or do some quick math.
You’re tapping on your phone and no one is the wiser. A little later you ask the system tech if she can walk you through the output setting. When she does you’ll have a comparison in case a red flag needs to be waved.
Let’s look at two examples.
- FOH to Main = 30ft
- FOH to Sub = 20ft
- 30 – 20 = 10ft distance offset
- 10 * 0.88 ≈ 8.8ms time offset
- FOH to UPQ = 30ft
- FOH to PRX618S = 20ft
- Recommendation: Delay the Sub by 2.95ms and invert the polarity
You can even look at the plot and then have some expectation of what the measurements should look like in case that comes up.
Listen to color pulses
Color pulses are great for focusing in on different frequency ranges. Check out Merlijn’s article and download his examples.
You might load them onto a track in your virtual sound check or on the playlist with your reference tracks. Then, when you’re doing your listening tests throw one of those on and try inverting the sub polarity. In a well-behaved system in a well-behaved room, you should hear cancellation.
There are various factors that can break this test (see Don’t Align Your Subwoofer to a Room Reflection), but at least it will provide you with some potential reason for further investigation.
Have you tried any sneaky ways to check a sound system setup? Let me know in the comments.
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