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:
- Deploy two microphones (or more, potentially).
- Measure the furthest position setting its delay locator automatically or by distance from the source.
- 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.)
- Verify that the LF phase of both measurements has some agreement.
- 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.
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.
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.
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:
- 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.
- 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.