Recently I had the opportunity to help my friend Nick work on the calibration of some new components at a great looking venue in Cincinnati called The Redmoor. We met on TeamViewer and recorded the entire thing so that it could serve as a combination of consultation and training. If you’d like my help on your project, you can schedule an appointment here.
In this post I’ll walk you through some of the EQ and crossover alignment steps we took.
First, gather materials. I checked Tracebook for the HDL10-A and STX828S. No luck. I found the GLL file for the HDL10-A on the RCF website, though. I opened that in the GLL viewer, built an array with the settings I expected to see, calculated the balloon, opened the transfer function, and exported it to a file.
The get the subwoofer data into my audio analyzer I used VituixCAD2 to convert the image on the spec sheet into response data. Then I imported everything into Crosslite.
The sub’s native response will allow me to experiment with different low-pass filters.
Next I needed to choose a target. Since we are looking at anechoic responses, it makes sense to use a flat target, but recently I have started using a +6dB slope in the low end because I have found that that will push the crossover region up, which is a better representation of what will happen in the field when someone inevitably turns up the subs.
I’ll start by inserting those filters recommended by the manufacturer.
Next I’ll apply some initial EQ and gain to make a better match of the target.
Should we design an overlap or unity crossover? Let’s do both!
Before I we even look at the phase graph, let’s measure some slopes. We know that the sub’s slope is 24dB/oct because it was a flat line before the LPF was inserted. Switching to view Data Pre in the bottom window, we can look at different HPF slopes on top of the HDL10-A response and find that it is 48dB/oct. This is a clue that the phase response of the main will be steeper than the sub.
Switching to the phase graph we find that that is in fact the case.
Let’s try adjusting the sub’s LPF to match the response of the main.
Now the sub is too steep. Let’s split the difference with 36dB/oct. Now we have a nice match with only about a 30º maximum phase offset between them.
The filter change required a small gain adjustment. Here’s the sum.
What should we do about that bump? We could leave it alone and say we like it, but let’s insert symmetrical filters to restore the response to the target.
Now let’s try the unity class crossover.
The HDL10-A already has a steep HPF so I am reticent to make it even steeper, but I need main and sub to meet at -6dB. I know that the DSP at The Redmoor is a Venu360 so we’ll only have access to basic EQ filters. Let’s try the least steep HPF available, 6dB/oct. I’ll adjusted the delay by 0.5ms for a slightly better phase alignment.
Let’s try a 12dB/oct option.
Let’s zoom in and compare them. Which one is better? I don’t know, but now we have some options.
Now let’s see how this actually played out in the field.
We started by verifying all settings and taking eight measurements of the HDL10-A through their coverage area.
We applied EQ towards the target and took more measurements to verify the EQ and prepare for alignment. We exported averages from Smaart and imported them into Crosslite. Here’s the phase graph.
At this point I realized that we could have made our work a lot easier by starting out with a ground plane measurement very close to the speakers and without any processing to get cleaner data for alignment. But, we were running out of time so I decided to simply apply 3ms of delay to the sub and move forward with this solution.
Here’s what the final measurement looked like.
Let’s see if we can improve on the alignment I came up with in the field.
Interestingly, measured in the room, the HDL10-A appear to have a 24dB/oct slope, not 48dB/oct as expected. Maybe this is a result of one of the user definable settings on the back.
The STX828S appear to have an 18dB/oct slope, even though we used the recommended 24dB/oct slope on the LPF.
How can we equilize this relationship? 24 – 18 = 6, so we can add a 6dB/oct LPF to the sub, right?
But that will add another 3dB of attenuation at the cutoff frequency, which we don’t want because we are trying to simulate what would have happened if we would have used a different LPF from the beginning.
One option is to simply switch the LPF to zero magnitude. That will give us a steeper slope without affecting the magnitude. Of course the magnitude won’t be accurate, but we can still research the phase alignment.
The result is better alignment without any additional delay.
I should make it clear here that a zero magnitude filter is not something you would normally find in a DSP. It is a special kind of simulation that Crosslite offers for research purposes. The closest thing you would find in a DSP is an all-pass filter or within a variable architecture FIR filter.
How do we know if we are making an improvement? We can see the phase come into better alignment and we can see the sum go up, but I find it helpful to have a goal of perfection to compare it to. In SATlive you would load the Perfect Sum trace. The workaround I used in Crosslite was to simply import the data a second time, but this time without the phase.
In this graph you can the the perfect sum target with two delay options. Both of the options include the new zero magnitude LPF.
How do you prepare for crossover alignments?