Bghead8che
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- Jul 1, 2019
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When summing two identical woofers, you just add 6dB to its SPL. If you're using the method of aligning the low-end response tails, it doesn't really matter. I will note that if you haven't, you should also measure the midrange driver too. I don't know what the crossovers are for the speaker
So I'm not sure of the top of my head why this happened so dramatically @Bghead8che , but its basically the phase of the two measurements are not time aligned correctly. This is good for anyone following this thread, so for reference, this is the way the port and woofer are summing in @Bghead8che's measurements:
View attachment 193424
Whenever you see various dips like that in succession ("comb filtering") when summing measurements it's a sure fire sign that there's a timing issue.
If you go to the 'Impulse tab' and look at the two measurements, you'll see that the references timing is very different. Close to 0ms for the woofer, but -27ms for the port:
Woofer:View attachment 193419
Port:View attachment 193422
Whenever I see comb filtering like that, you can try using REW's "alignment tool" to sum the graphs and have control over timing as well.
(All SPL> Controls> Alignment tool). Here you can control both the gain (dB) of the response and the timing, and see what would happen if you changed the timing.
View attachment 193425
That showed that modifying the timing of the port by adding ~26-27 ms gets us a respones more like what we'd expect. A good way to do this is to tap on the little slider and hold the left right up or down arrow on your keyboard and see the response change live.
View attachment 193427
So I'm not sure why that difference in timing happened. You can see from the alignment tool that delaying the port by 26.75ms is equivalent to moving the port 30 feet/ 9m away lol. But that gets us closer to what we want. You can then tap on "aligned sum" to save the response.
Alternatively, you could make a copy of your responses, go to the impulse tabs, click on controls, and set t=0 at impulse start. That can cause issues during far field measurements but should be fine for these nearfield ones.
EDIT: On closer look, the measurements have a note saying you used an acoustic timing reference. Was that on purpose?
View attachment 193430
For some reason that seems to have screwed things up.
Also these measurements are using the 90 degree calibration file. Not a big deal for these frequencies, but FYI it should be the zero degree one.
You nailed it! I had a timing measurement on for both measurements which I had used previously for in room measurements. I am going to turn that off and change my calibration file. I'll measure the midrange as well though it may be difficult because it has a large phase plug. In your guide you say to just measure to the side of the phase plug.
<<When summing two identical woofers, you just add 6dB to its SPL. If you're using the method of aligning the low-end response tails, it doesn't really matter. I will note that if you haven't, you should also measure the midrange driver too. I don't know what the crossovers are for the speaker>>
I'm not sure if I understand you here. Are you saying if you align the low-end tails you do not need to add 6db? How exactly do you "add 6 db?" Do you add a 6 db offset?
In the guide you wrote the following in the notes:
<<Ideally, the port and woofer should be measured at the same 1-meter SPL as your on-axis, but you might find this is too loud when the microphone is so close to the sound sources. For passive speakers, the bass response will rarely vary dramatically at different SPLs, so just use the loudest SPL you can get away with. >>
I'm a little confused by this. Are you saying instead of measuring the close to the woofer and flush with the port you should measure them 1 meter away? Sorry, this wasn't clear to me.
I am going to remeasure the midrange, both woofers, and the port. Hopefully you can then help me put them all together correctly. Then we can compare the final response to the official posted response and see how close we come!