Tour Blogs

Fun with Phase

Since I’m carrying extra subwoofers on this tour, my daily task is to make them play nicely with the house subwoofers. Some days this is easier than others. Luckily the Clair BT218’s that we are carrying are just simple front loaded dual 18’s, and it’s very easy to make them cooperate with house subs. My “ideal” deployment with the BT218’s is a mono block in front of the stage. I say “ideal” because I can’t do any fancy subwoofer arrays on this tour. However, I can eliminate some of those annoying power alleys, and I can get the most impact out of my subs in the center. Additionally, when I have a separate send for the house subs, I can delay them further and steer the LF as needed.

Here’s one of the various house systems and my subwoofer deployments this tour:

[IMG]http://i39.photobucket.com/albums/e194/HarfordSound/system.jpg[/IMG]

How does one go about aligning subwoofers? The process itself really isn’t that complicated, it’s just being able to figure out if the measurement you’ve taken actually means something. The first step of the process simply involves capturing the house subs, and seeing what they look like. Here’s a screenshot from today, where the house system was an older EV X-array rig:

[IMG]http://i39.photobucket.com/albums/e194/HarfordSound/housesub.png[/IMG]

Some days, depending on the type of subwoofer and phase response, I’ll end up high passing it or EQ’ing as needed. This sub however was already pretty good looking out of the gate.

The next step is to shoot my subs and see what kind of delay time and phase response I end up with.

[IMG]http://i39.photobucket.com/albums/e194/HarfordSound/bt218.png[/IMG]

Once I get the phase to smooth out, I’ll subtract the delay time of my subs from the house subs and that should give me the “golden number.” I forgot to write down the delay times today, but I know I needed 10ms of delay for my subs to line up with the house subs. Remember, we’re not really looking at the magnitude response here, but mainly the phase response. Obviously we want the magnitude response to improve as I add my subs in, but I’m more worried about a smooth phase response. Here’s the combined subwoofer’s response:

[IMG]http://i39.photobucket.com/albums/e194/HarfordSound/combined.png[/IMG]

Everything looks good together, the phase stays smooth, the magnitude response looks good and most importantly, it sounds louder. One thing to really take into consideration is that these are not perfect measurements by any means. They are down and dirty in sometimes less than ideal circumstances. It’s my job to figure out what looks “good enough” in a short frame and make it work for my show.

After I phase align the subwoofers, the listening process begins. I start with pink noise and punch my subs in and out to make sure the LF is actually getting louder. Once that is confirmed, I’ll play some tracks. Walking the room, I’ll look for nodes and power alleys with and without my subs, and I’ll listen up close to make sure the transition from their subs to mine is smooth. A lot of times, I’ll end up playing with the delay a little more to spread the LF out, tighten it up, or to steer the LF.

When it’s all said and done, the addition of my extra subs adds another dimension to the show, which adds more energy to the crowd and gives me the impact I’m looking for.

Evan

You give up on the B2's? You carry Clair subs now?
S
  • S
    Silas Pradetto
  • April 25, 2011
Evan, wouldn't subtracting simple delay times still not take into consideration phase at all? You could be opposite polarity and still have matching delay times.

When aligning, wouldn't you overlay the phase traces, making necessary delay, crossover filter type and slope, and all-pass changes until the phase matched as perfectly as possible?
Silas Pradetto;bt76 said
Evan, wouldn't subtracting simple delay times still not take into consideration phase at all? You could be opposite polarity and still have matching delay times.

When aligning, wouldn't you overlay the phase traces, making necessary delay, crossover filter type and slope, and all-pass changes until the phase matched as perfectly as possible?
Silas,

1. Its trivial to invert the polarity on one bandpass or the other and look for a null to insure that your subs and mains have the same polarity

2. Evan is rarely in positions where he has the freedom to change the subwoofer specific processing. The Clair subs have their fixed processing, and the house subs will also have their processing. Even if he could change the processing on the house subs, it is rarely prudent to mess with their highpass, lest you damage them.

3. The way Evan is doing these alignments, the apparent phase is at your discretion. He is manually changing the reference delay to center the zero group delay section of the arrival over the center of the bandpass, and then comparing the reference delay offsets. I know this because I taught him how to do it ;)

4. The phase match that can be achieved between vented boxes adjusting only the delay time is pretty good. If you mentally overlay the two phase traces above on top of each other, you'll see they are in close alignment through the center of the subwoofer bandpass. Do this enough times (as Evan has) and you don't even really need to overlay the traces anymore, and can quickly eyeball it.

5. The lowpass/highpass electrical filter combination that is in front of the subwoofer and amp tends to dominate over the acoustic phase behavior of the box/driver combination. Since the electrical LP/HP filter corners of any two quality vented pro sound subwoofers are reasonably similar, the measured phase responses of both boxes generally have similar(ish) phase response.
S
  • S
    Silas Pradetto
  • April 27, 2011
Hey Phil, thanks for the clarification. I know Evan knows what he's doing, but the way he explained things above seemed a bit oversimplified. You clarified his process a lot; I just didn't want anyone reading the article to think that subtracting delays times from the Smaart delay finder was the proper way to phase align a rig. Thanks again for the info.
K
  • K
    Evan Kirkendall
  • April 27, 2011
Phill nailed it. I did skip a lot of the details, as the focus of the blog isn't teaching someone how to do it, but rather just showing the steps to get it done.
E