Tweeter Polarity problem
Okay, so I've been having a problem with a pair of two-way towers I've been working on with some extra parts laying around. I measured all the drivers in the cab on top of a 5 or so foot stand I made. Then I time aligned them in xsim by measuring the woofers and tweeter together in all three combinations woofer 1+tweeter, woofer 2+tweeter, and woofers 1&2+tweeter, then matching the graphs in xsim.
I then constructed the crossover with the components I had on hand and got this.
Notice that the tweeter's polarity is reversed to put the drivers in phase. Everything looked at least halfway decent until I started measuring. Unfortunately, I was so frustrated yesterday that I deleted the measurements, but describing what was happening should work. Basically, when I measured the speaker with everything in phase it matched the xsim prediction perfectly. Don't mind the 105dB sensitivity, this is incorrect because for some reason my umik was reading high. All the drivers are level matched and accurately measured.
The dip was exactly how it should look as well as tweeter and woofer levels. Everything was looking perfect and I flipped the tweeter polarity to ensure everything was correct, but it wasn't. When I reversed the tweeter polarity a larger null presented itself. Basically, the drivers were even more out of phase.
I messed around and made sure everything was wired correctly. It was. I did a bunch of off-axis measurements vertical and horizontal and the null presented itself every time. Xsim has always been fairly accurate for me but this is the first time I've done a crossover with inverted polarities for time alignment and I'm not sure what the problem is. Do you guys have any ideas? Like I said these are extra parts laying around so I don't have many components to switch out. That said I also don't have much invested aside from yesterday's failed attempt either.
@123toid it is a tmm design completely two way. I'm aware of the issues that this alignment has, and I'd rather make it a 2.5 way, but I didn't have the extra components laying around. the woofers entered into xsim are the individual responses and they are both time aligned separately with the tweeter. I'm certain everything was wired correctly.
I thought maybe my alignment might have been incorrect, but no matter how I align the drivers, there is no configuration where I get large dips both in-phase and out of phase. I'm about to remeasure everything since it's cooling down outside and I'll try again.
Here's the best thing to do. Whenever the woofers are going to be playing the exact same frequency response, measure them as one driver (woofer). So measure the frequency response of both woofers playing together and their impedance. Just make sure they're wired how you're going to wire them at the end (aka series or parallel). Then measure your Tweeter response. Finally measure them all eyes together to find your z offset. So in Xsim you'll actually only have one woofer and one tweeter that you're messing with. Once you do this, I think all your issues will go away.
For anyone that's reading this, that might not know. When taking a measurement like this, just keep your mic at Tweeter level when you take your frequency response.
Okay, so I redid my way of doing z offsets and got slightly different values. Then I read your method and tried it as well. I got a curve that looked identical so I had some reassurance that my numbers were now correct. I then hooked up my crossover and the graph matched well enough.
I think the inconsistency could be due to the alligator clips I was using because I measured them later and they were about 1ohm each.
The problem with the measurement is that the tweeter was in phase with the woofers when I did the measurement, not out of phase like in xsim. I measured it wired our of phase too and got a large dip again, so I'm pretty confused. It's not impossible that something was wired wrong but I checked like 20 times and I've never had issues with any of my other builds. Maybe the tweeter itself is marked incorrectly but I've used it before without issue so I don't know.
Overall, I got the response I was looking for but I'm still unsure.
Every other build I've done I have used an extra inductor and a notch filter to get the desired phase I want and maybe I'll just stick with that. I'll definitely have to try it on something else before I plan something based on an inverted tweeter design.
Why don't you zip the measurement files and I'll take a look at them. If you have the measurements the way I told you to, those would be the ones I would want. Because that's my preferred method of doing a TMM.
Something doesn't seem to be adding up. Just a double check. How far away are you measuring your response? And where is the microphone in relation to everything?
It wouldn't matter if your Tweeter polarity was marked wrong on your tweeter. Because your measurements are going to reflect it either way. Meaning if you measured it as a positive and a negative, it's always going to be measured as a positive and negative the way you hooked it up.
@123toid It's really not worth it. The measurements were taken at 1 meter at 2.8ish volts that was calibrated with my multimeter playing a 60hz tone. The mic was level with listening axis which is halfway in between the tweeter and top woofer. I'm positive they were done correctly. I'm gonna make some mtm bookshelves sometime next week with the drivers instead. I made a test box last week and did the measurements and ordered some parts for it. They also use an inverted tweeter, which I wanted to test with these towers.
If you want to see those measurements you're welcome to but I've given up on the towers for now. I'm just going to assume that I wired the tweeter backward when taking measurements with the crossover, even though I'm almost certain I didn't.