Phase Alignment

Meyer’s VLFC is designed to cover the octave below a regular subwoofer (13 – 30 Hz), so building them into arrays requires some thought, because the wavelengths are so much longer than standard sub wavelengths that all spacings and timings have to shift quite a bit. (I have not yet had the opportunity to be in a room with one of these things but I hear that it is quite an experience.)

Yesterday I posted the following scenario quiz to my profile page, with only a handful of intrepid audio humans braving the complex phase alignment waters for the chance at the promised silver award for the correct answer. Some good guesses and solid reasoning, but no silver this time.

I think the situation will help illustrate a mechanism that will allow you quickly estimate alignment delay times in your head. It’s sixth grade math, but (nerdy) people think it’s really cool.

Here’s the situation:

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Here are two VLFC boxes that we want to use as the basis for an endfire array. Pretend we’ll be adding a third box to the line later; two illustrates the mechanism well enough. The goal is to delay the front box so it fires as the rear box’s arrival passes it. Predict this in MAPP XT and here’s what you get:

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The question was then posed: which box gets the delay, and how much? I included the above image of the physical layout for context, but pretend we only have the analyzer readout to go on. The first question is “which trace is which box?”

Looking at the magnitude trace (top) we see the blue is higher than the red. This means louder, which means closer to the mic. The mic is in front, so blue is the front box. Intuitively, now, we know that blue gets the delay. But we can also get this information from the phase trace alone.

My first Between the Lines post was all about how to read the phase trace, but this one image sums it all up:

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That’s all you need to know…no strings attached.

  • Flat phase trace – energy arriving in time with analyzer reference signal.
  • Trace sloping down left to right – energy arriving late.
  • Trace slopes up left to right, energy arriving early.
  • Steeper slope means more time offset.

So armed with this knowledge let’s look again at the MAPP phase trace.

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In the center of the image, around 25 Hz (damn, this thing goes low), the blue trace is trending flat (on time) while the red trace is tilting down left to right (late). So red box is the rear box, arriving later.

I picked this frequency range because it’s easiest to see, but it works anywhere.

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Up here, they’re both sloped up from left to right (arriving early), but the blue trace is a steeper slope (arriving earlier). So that gives us the same information: blue box is getting to the mic first, then the red box.

For the masochists, you can even do this down below 16 Hz if you like:

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It’s much harder to see, but down here, both boxes are sloping down left to right (late) but the red trace is a bit steeper (later). So again, it’s farther away.

This gets easier with practice. So now we’ve answered the first question: which box gets the delay. Blue.

The next bit is “how much.”

If we were looking at a main/sub alignment, we’d only care about the phase offset around crossover frequency, but this is a “full range” summation between two identical boxes with a linear time offset between them, so we can read it anywhere we want and get the same answer. Here’s the easiest one:

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The blue trace is crossing zero as the red trace is wrapping up from 180°. That’s a half a cycle offset at just below 50 Hz.

Period of 50 Hz is 20 ms, a half cycle is 10 ms, and we’re just below that in frequency so let’s so with 11ms.

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Perfect! But we can’t always count on it being so simple. Let’s pull the same number from another spot on the trace:

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Delaying a box will tilt the phase trace down (we’re making it later) and here we need to tilt it down by 180+60° = 240° which is two thirds of a cycle. At 63 Hz, a cycle is 16 ms, so two thirds of that is 10.7 ms. Sixteen is tough to divide by three mentally so I’d say two thirds of fifteen = 10 ms and you’re still right in the ballpark.

Don’t fall into this trap though: it’s very easy to look at the difference between the two curves around 31 Hz and use that for your math.

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Don’t do this!

Delay will push the blue trace down, not up, so we’re measuring the wrong way. Gotta go this way:

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Push down off the bottom and wrap around the top. Measured that way, it’s about 120°, or a third of a cycle. At 31 Hz (32 ms) a third of a cycle is 11 ms.

(MAPP doesn’t allow a lot of flexibility in how you view the data – it’s not really meant for that – but most modern analyzers will allow you to “scroll” the phase readout so you can view it in the center of the screen without the wrap.)

This might seem a little draconian – and typing it out feels like that to me, as well – but if you practice this, you can usually generate an approximate delay value in a few seconds.