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What do sub alignment sound waves look like?

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Have you ever wondered what the sound waves of your sub-alignment look like?

Coupled, Equidistant, Aligned

If your main and sub are right next to each other and phase aligned when equidistant then you can send them the exact same signal and expect the audience to be in the coupling zone.

Here you can see a ripple tank example of two sources encountering an audience represented by a white box. No reflective boundaries have been enabled.

Uncoupled, Unequal, Aligned

If your main and sub get separated, as is often the case with flown mains and ground stacked subs, then they are still phase aligned somewhere, but maybe not where your audience lives.

In this example you can see a power alley (coupling) where arrival times are match along the isosceles triangle and power valleys (cancellation) where the triangle becomes obtuse.

Uncoupled, Equidistant, Aligned

You can equalize the displacement problem with a physical offset or digital delay. Notice how the isosceles triangle rotates down towards the audience. Edit this example here.

Uncoupled, Equidistant, Misaligned 120º

What if you’re not sure if your main and sub are aligned when they are equidistant? You might fix the displacement problem with delay, but unknowingly leave an alignment problem. Now part of the audience is in the cancellation zone.

In this example I have introduce a 120º phase offset into the bottom source.

Uncoupled, Equidistant, Misaligned 180º

In this worst case scenario, you have equalized the distance offset, but accidentally put your audience fully into the cancellation zone. This could happen with an accidental polarity inversion or simply with two speakers that are not designed to be phase compatible when they are equidistant.

The moral of the story is to know your gear. Don’t assume a successful alignment has been created by only equalizing the distance offset.

FOH Engineers: How to sneakily check your system tech’s work

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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.

Other examples:

  • 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.

Breakfast measurements

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.

1: Math

  • FOH to Main = 30ft
  • FOH to Sub = 20ft
  • 30 – 20 = 10ft distance offset
  • 10 * 0.88 ≈ 8.8ms time offset

2: SubAligner

  • 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.

Can you remove reflections from live measurements for more accurate alignments?

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In my last article I found some minor success in validating subwoofer alignment indoors through the use of carefully chosen microphone locations and their average. Next up: Can it be done live in real time?

Thierry De Coninck sent me an article he had written outlining a procedure that would allow you to not only create these averages, but also set the delay locator for your transfer function based on distance to avoid any errors from reflections or lack of HF content arriving at ground plane. I also found an AES paper called Measurement of Low Frequencies in Rooms, which outlines a similar method through the post processing of IR measurements.

A new method is outlined for measuring frequency response at low frequencies. This method uses microphones arranged in a low frequency end-fire array to create useful directivity to discriminate against sound waves from rear wall reflections and reverberation. It also operates in the time domain, processing the acoustic impulse response as it arrives at successive microphones – a shotgun microphone writ large.

D. Murphy, “Measurement of Low Frequencies in Rooms,” Paper 9482, (2015 October.).

What interested me most about Coninck’s method, though, was the ability to get results in real time. Here’s the outline:

  1. Deploy two microphones (or more, potentially).
  2. Measure the furthest position setting its delay locator automatically or by distance from the source.
  3. Set the delay locator of the next measurement position manually based on the difference in distance to the source from the previous measurement. (eg. If the second measurement position is 1m closer to the source then offset the delay locator by 1m / speed of sound.)
  4. Verify that the LF phase of both measurements has some agreement.
  5. Create a live average.

Initially I thought I would create a script in MATLAB to automatically calculate the delay locator setting for each transfer function pair using pythagorean’s theorem, but looking at some initial results convinced me that, if possible, setting the delay locator automatically from the IR would be more accurate. Since my goal is to measure and align at head-height then the microphones should receive enough high frequency content to make this possible.

The Setup

Here’s a section view of the sound system at the Sojourn Campus Church here in Minneapolis. Adam Rollin from AVE was kind enough to let me tag along and do some tests during his normal system calibration work.

I put these details into the Sub Align calculator and could see we had a small phase span of 28º. I decided we would do our alignment at the last row and distribute the microphones evenly from there.

Here’s a comparison of all six measurement mics at about mid-depth of the audience after level and polarity match (1/24oct smoothing). You can see that we’ll have our work cut out for us in the low-mids.

Alignment

We used the Relative/Absolute Method to complete the main+sub crossover alignment. Here are the native solo measurements at 1m. The sub trace is offset -6 dB due to half-space loading.

At first I thought the wonky phase around 1kHz was there because the mic was too close, but we found the same thing in the far-field. I’ll have to save the story of that polarity inversion for another time, though.

I wanted to reduce the overlap and improve the alignment without adding a lot of delay. An LR48 HPF was inserted on the main at 42Hz. This left us with the pre-alignment value of 2.4ms of delay on the main and a polarity inversion.

Why the polarity inversion? Compared to other possible variations it allowed for less delay and improved alignment.

Here’s how we deployed the microphones.

Here are all six traces with the average of the main.

At first I though, “Success! We removed the reflections.” Then I noticed the wrap at 200Hz. What the hell?! You’re supposed to disappear!

I can tell it’s not supposed to be there by comparing the average with the near-field measurement.

Where’s it coming from? Is it a floor bounce, a ceiling reflection, or something else?

If it’s a floor bounce, shouldn’t we be seeing a 2.5ms peak in the live IR graph in every measurement? Not necessarily since any peak in the low-mids would be down ~40dB and hardly visible.

There’s one way to rule out the floor-bounce for sure. Here are the ground-plane measurements.

Here is the average ground-plane vs near-field. Good-bye 200 Hz phase wrap.

Why does this matter?

The wrap at 200Hz caused by the floor-bounce causes phase shift through the crossover region and therefore misalignment.

What’s the solution?

Don’t align to a reflection. Do align to direct sound by using the Relative/Absolute Method or SubAligner.

A distance measurement from the alignment position were almost matched so we decided to leave the pre-alignment delay value unchanged. Here’s the alignment at head-height.

Everything is within the corridor of 60º, but if you wanted to be picky you might decide to use less delay in the main to attempt to close the 30º phase offset at 70 Hz. This may create a misalignment, though, because the floor bounce is creating the extra phase shift.

Attempts to remove the phase wrap at 200Hz by varying the microphone positions over height and width were unsuccessful. A majority of the traces always included a dip around 200 Hz.

Conclusion

I started down this path a year ago in order to prove the results of the Relative/Absolute Method, and therefore, the necessity for a tool like SubAligner. The Catch22 is that the more you need SubAligner, the harder its results are to validate. If the phase trace is inactionable, SubAligner can’t fix that. It just ignores it. And while a measurement at ground-plane will help with visualizing the data, it will hurt the alignment because we don’t have ears on our feet.

Using an audio analyzer to validate SubAligner’s results is like trying to validate the presence of an optical illusion by taking a photo of it. It will only enhance the illusion.

Here are some takeaways from Sojourn:

  1. No amount of measurement data and mic positions can guarantee a reflection-free average in the far-field (especially with poor direct-to-reverberant ratios). The null created by a floor bounce or other boundary may be sufficiently common to each individual trace that it ends up in the average.
  2. Since a measurement cannot be guarantee 100% reflection free, there is always risk for misalignment. Gathering more data in a strategic manner can help reduce this risk, but the only real solution is to abandon the audio analyzer for a more appropriate tool.

SubAligner vs. L-Acoustics Preset Guide (K2 + SB28, Kiva + SB15m)

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The L-Acoustics Preset Guide is not only a valuable resource for subwoofer alignment, but is also one of my inspirations for creating SubAligner. If L-Acoustics can create alignment presets for their speakers then I should be able to scale the same method to any other speaker pair.

It also provides a quality check for results from SubAligner: Do they match? Why or why not?

A majority of the Preset Guide deals with loading the appropriate preset into the amplified controller for a particular speaker.

The Preset Guide describes the recommended loudspeaker configurations for each system, with the corresponding factory presets and the main resulting acoustic properties.

But there’s a big section at the end called Pre-alignment delay values.

For some loudspeaker enclosure combinations, it is necessary to adjust the delay values for time-alignment. Refer to section Pre-alignment delay values (p.66).

This is where a method for crossover alignment is described to be used “If no acoustic measurement tool is available,” or if I might add, if actionable data is otherwise unobtainable. When might actionable data not be obtainable? Please see Don’t Align Your Subwoofer to a Room Reflection.

The procedure goes like this:

  1. Measure the path difference
  2. Geometric delay = path difference / speed of sound
  3. Alignment delay = geometric delay + pre-alignment delay

Let’s see how the results from SubAligner stack up against the official guide.

K2 + SB28

If you were to attempt to align these two speakers with a distance offset only, you would have a pretty bad day.

k2 with SB28

The preset guide recommends a polarity inversion and 0.5ms of delay in the K2 output.

SubAligner recommends a polarity inversion and 0.92ms of delay in the K2.

A difference of 0.42ms is only about 4% at 100Hz (or 14º). I haven’t interviewed the person who created the preset guide, but I’m happy to see that the human and robot result are in good agreement.

Is there an alternative solution?

If you would like to use less delay or create a crossover region with less overlap you can open the Filters pop-up in SubAligner. The first solution you’ll see is a 4th order Butterworth filter at 73Hz.

Kiva + SB15m

These two speakers are only about 80º apart out of the box so I wouldn’t worry about the alignment too much.

The preset guide recommends 1.4ms of delay on the SB15m.

Meanwhile SubAligner recommends 1.8ms of delay.

It’s interesting to see a difference of 4% again. My assessment, they agree.

SubAligner does give some recommendations for filters, but they provide no significant reduction in delay or overlap.

dV-DOSC + dV-SUB

When equidistant, these guys are ready to party.

Notice that 4% difference again? If I had to guess while splitting some hairs, I would say that the SubAligner results are giving more weight to the lower frequencies of the crossover region while the L-Acoustics recommendations give more weight to higher frequencies. The L-Acoustics result will produce a tiny bit more summation at the top of the crossover region and will allow you to turn your sub up louder in relationship to the main with less misalignment. On the other hand the SubAligner result gives a tiny bit more summation at the low end of the crossover region and allows you to turn the sub down lower in relationship to the main.

It’s also interesting to note that all of these alignments produce a crossover bandwidth of 1.7-2.1 octaves and if you average all 220 L-Acoustics alignments in SubAligner you’ll get 1.44 octaves.

Have you tried any of these presets in the field? What were your results?

Don’t Align Your Subwoofer to a Room Reflection

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Here’s a plan view of a sound system with two mains, a sub, and a measurement mic at a fixed location. If I asked you to align this system, would you prefer to start by muting the left side to align the right side to the sub or would you like to play both left and right together with the sub?

plan start

My guess is that you’d probably start by muting the left side to align the right main to the center sub, especially if you’re in a hurry and indoors where your measurement is prone to even more contamination than from just another loudspeaker. If so, you’d be in good company based on a few polls I ran.

What if I show you a prediction at 80 Hz with both left and right mains playing that reveals that the mic is in a power valley?

plan prediction 80Hz

You’d definitely want to mute that other side to do your alignment now because you know it’s going to contaminate the measurement and make your phase trace go crazy.

What if I rotate the room 90º and ask you to do the same alignment?

Would you change your strategy?

Nothing has changed so hopefully you would do exactly the same as before and mute one side to do your alignment.

What if I cut off half of the room and enable the bottom boundary?

Let’s make it look more like a section view.

section view 2

Everything is in the same place. Nothing has really changed for your alignment.

You might say, “Sure, I’d like to mute the other side, but that’s impossible.”

Maybe this dissonance is reflected in these polls. Or maybe I just didn’t phrase the question clearly.

Why is that?

Most of us have made our peace with the first scenario in plan view. You don’t necessarily need to align with both sides on because the left side will arrive with a slightly lower level and the right side will be slightly more dominant. So you mute the other loudspeaker and not think much of it.

If you rotate the room 90º the story doesn’t change except that house left in plan view has become the mirror image—living on the other side of the floor—in section view and you don’t have the luxury of turning it off. That’s the conflict. Your phase response becomes compromised and you never had the opportunity to prove to yourself what is was like in the absence of the mirror image.

Measured indoors, the phase trace can be hard to interpret or even so jagged that you think “Why bother?” or resort to a gratuitous amount of smoothing which is not the same as muting the mirror image.

This has made field tests of SubAligner in small rooms a real problem. I have done four tests in Canada, Spain, Greece, and Minnesota and they have all gone the same way. All of the measurement and listening tests are good until the mic goes to head-height. In the direct field it’s clear, but as soon as the room gets involved it falls apart.

This is a pretty big problem since I’m trying to sell a product that is supposed to create an audible result. If you can’t hear an improvement, why would you buy it?

SubAligner is a simple web app that uses a method of comparing relative distance offsets in main+sub alignments to pre-alignment values in its database. It short: it saves time on sub alignment.

I didn’t invent the method. I learned it from Merlijn van Veen. He got it from the L’Acoustics Preset Guide (Time alignment with geometric distance) and d&b Line Array Design along with his own field experience. The process is also described in the 2011 AES e-brief Time Alignment of Subwoofers in Large PA Systems.

From what I can tell the method is solid, but (and this a big BUT) you won’t always be able to hear or measure the results. It requires a leap of faith.

Fight the room

A floor bounce will cause a negative shift in the average phase measured at the microphone. Delay may be introduced to correct this local misalignment with unknown effects on the rest of the audience.

Here are two matching sources. With no boundaries enabled they create a perfect line of summation along the perpendicular bisector or median plane between the sources.

matched

If the floor is enabled the central power alley seems to be pushed up.

It is tempting to add delay to steer it down. Here the summation pattern has been outlined.

The entire shape is pivoted around the main speaker back into alignment so that the ground sub is properly delayed.

That should do it, right?

Damn it. I’m not sure if that’s better. There may be more of the audience in the blue color (lower level) now. It got better in one spot, but the rest of the audience has suffered.

Measuring LF indoors is scary

One challenge to alignment decisions is the excessive ripple encountered when measuring indoors, especially in smaller spaces.

Here’s a prediction of a subwoofer measurement using the previous example with 4 boundaries enabled. The more reflections, the more ripple, the harder the story is to read.

In this case the envelope of the measurement is still visible. You can see the trend and and indication of the original uncontaminated trace.

Let’s look at field data from a real room featuring two more walls. Here are measurements for a main+sub alignment that I attempted at a small church north of Minneapolis.

sub alignment messy
20% coherence blanking

Unless the audience is close enough where they can practically touch the subwoofer, most audience members will suffer from poor direct-to-reverberant ratios. Most audio professionals, myself included, would throw up their hands and say, “Forget about it.”

Average out the floor bounce

If measuring at a single location indoors is highly prone to error, how can better data be obtained?

Let’s measure at eight positions across the audience and average them together. Here’s the example from the beginning of the article with the addition of a standing height audience.

Here’s all 8 measurement of the main combined with its mirror image, plus the average.

Here’s what that average looks like compared to a solo on-axis measurement without the mirror image (ie. anechoic).

The mirror image’s delayed arrival time varies throughout the audience such that it averages out. The jaggedness is inversely proportional to the relative level offset between the main speaker and its mirror image.

If you are wondering why there seems to be more phase difference around 100Hz, it’s because those measurements made closer to the speakers have greater magnitude and a greater influence on the average. This is generally helpful since we’d like to average out the nulls of the comb filters. Here is an unweighted average for comparison. 

Field test

Ian Robertson from GerrAudio in Canada graciously agreed to test this with me.

The setup included eight Lina and a single 900-LFC.

After consulting the Sub Align calculator we could see that the phase span from front to back was very small (28º) and position eight was chosen for the alignment with SubAligner. (at this point I should probably apologize for titling my app SubAligner so similar to Merlijn’s calculator Sub Aligner or even Waves Sub Align plugin, but the URL was available so here we are)

Here’s are the measurements at position eight with the crossover region highlighted.

Here are the solo measurements of main and sub at each microphone position along with their respective averages.

Here are the averages alone.

I have to be honest that I wasn’t sure this would work, so I was really happy to see the alignment clearly visible, but also the similarilty with SubAligner’s predictions.

The first time we tried it, it didn’t work. My mistake was in calculating the coupling zone and alignment position in Sub Align based on a ground-plane height instead of head height, which has a much smaller phase span.

Step-by-step

Here’s is the method Ian and I used to test SubAligner.

I should first note that if you’re measuring outside away from boundaries you might not need to do any of this. These steps will be necessary indoors and anywhere where signal-to-noise ratio is low.

In order to find a global average free of local anomalies measurements must be balanced in level and phase. In level, in order to not tip the average towards one measurement with greater amplitude (eg. positions closer to the source). In phase, in order to not tip the phase trace too far in one direction or another.

XOVR position

Use the Sub Align calculator to find an alignment position in the center of the coupling zone. To do this, simply divide the entire phase span by 2. If the result is more than 120º, use 120º.

  • Example 1 – Alignment phase span 226º: Enter a max phase offset of 210/2 or 105º.
  • Example 2 – Alignment phase span 436º: Enter a max phase offset of 120º.

Measurement positions

Use at least eight positions distributed equally around the XOVR position. This may seem counter intuitive at first if the XOVR position is not near the center of the audience, but if there are an unequal number of measurements on one side then the average will favor that side and will appear to be mis-aligned. Of course, this is all much more critical with very large phase spans. If the phase span is 10º then the distribution is less critical.

Results indicate that a subset of eight measurements locations spread over the target audience area represents a rational solution to characterize the loudspeaker system response.

Optimum Measurement Locations for Large-Scale Loudspeaker System Tuning Based on First-Order Reflections Analysis

Most designs will have an asymmetrical relationship between the main, sub, and audience, and therefore require an asymmetrical placement of microphones.

main raised up to an acoustic center of 20ft

Level

Start at the closest position in order to avoid overloading the mic preamp. Use this position as the magnitude reference. At each new position reset the mic preamp to match this reference and reset the delay locator to the main. This will equalize magnitude and phase offset introduced by distance.

Back to the original question

When faced with an unknown number of mirror image reflections in a room, what do you do?

Do you rely on what you know to be true, or speculate on what might be true?

What you know to be true:

  1. Phase offset caused by distance offset is predictable and therefore alignable. Geometry tells us exactly where to align and what parts of the audience will be in the coupling zone and what areas will be in the cancellation zone.
  2. The pre-alignment values you created for this array were validated at the warehouse. You know they work.

What we can only speculate about:

  1. Attributes of the speaker’s mirror image living on the other side of the floor and every other wall.
  2. The validity of LF measurement data under poor D/R conditions.
  3. Where the null caused by floor bounce will end up at once the room is full of humans.

Back to Milaca. If we took that same system outside and measured on a calm day under near anechoic conditions I’m 95% positive we would get the data and see alignment. Inside where reflections dominate, suddenly the analyzer is showing me a different story. Unfortunately, it’s not a video game where you can turn off the walls. (the irony of Phase Invaders is not lost on me here)

Too much noise will render any measurement system ineffective. If it’s not working, try a different tool.

What has been your experience with trying to get actionable data indoors. Let me know!