I have a set of DIY two-ways consisting of AE TD15M & BMS 4550 on QSC waveguides. I've been using a miniDSP 2x4 for a long time, but want to design a passive crossover for gear simplification.
I only have a USB mic, single-channel measurement setup. The obvious drawback is timing measurement. I could likely work with a few assumptions and get close enough with a VituixCAD design, but would like to do all I can to get this right. I also cannot really physically change the driver mounting at this point.
With the 6 following measurements available, all 200Hz-2kHz, taken at the y-axis mid-point:
woofer, normal & inverted;
tweeter, normal & inverted;
parallel woofer & tweeter, in phase & tweeter inverted;
Could I use trace arithmetic in REW to extract the correct z-offset? The IR of both driver together is mostly that of the tweeter and I cannot really make out the woofer's impulse.
I thought I'd set tweeter IR peak at t=0 for the together-in-phase trace, same for the tweeter-normal trace, then subtract the later from the former, hopefully leaving the woofer's IR peak 'legible', at its now correct offset. I suppose the same could be achieved by adding the inverted trace instead.
Does this sound sane? Is there a better way with these of different measurements?
Oh, I posted this in Multi-Way thinking it might get more views than in Software, but if more appropriate, I don't mind if the post is moved there.
I only have a USB mic, single-channel measurement setup. The obvious drawback is timing measurement. I could likely work with a few assumptions and get close enough with a VituixCAD design, but would like to do all I can to get this right. I also cannot really physically change the driver mounting at this point.
With the 6 following measurements available, all 200Hz-2kHz, taken at the y-axis mid-point:
woofer, normal & inverted;
tweeter, normal & inverted;
parallel woofer & tweeter, in phase & tweeter inverted;
Could I use trace arithmetic in REW to extract the correct z-offset? The IR of both driver together is mostly that of the tweeter and I cannot really make out the woofer's impulse.
I thought I'd set tweeter IR peak at t=0 for the together-in-phase trace, same for the tweeter-normal trace, then subtract the later from the former, hopefully leaving the woofer's IR peak 'legible', at its now correct offset. I suppose the same could be achieved by adding the inverted trace instead.
Does this sound sane? Is there a better way with these of different measurements?
Oh, I posted this in Multi-Way thinking it might get more views than in Software, but if more appropriate, I don't mind if the post is moved there.
You could do something along these lines, there are many ways. However one simple way could be to use the acoustic timing reference in REW, which was put in place to offer USB mic users an option.
AllenB,
Thank you. I haven't looked much further into this specific REW feature, though I remember using similar in HolmImpulse many years ago. I'll read up some more on it.
I'm not certain I can make sense of the results of my subtracted impulse; the polarity seems inversed compared to the woofer alone.
Thank you. I haven't looked much further into this specific REW feature, though I remember using similar in HolmImpulse many years ago. I'll read up some more on it.
I'm not certain I can make sense of the results of my subtracted impulse; the polarity seems inversed compared to the woofer alone.
Use HolmImpulse. Measure the tweeter, then under “analysis,” set the t=0 to the just measured tweeter and all other measurements will be with reference to that one. Thus the phase response for all measurements comes out correct and you don’t have to separately input the offset.
Maybe REW can do it too, as Allen says, but I haven’t tried it. Holm, I’ve used for years and it works perfectly.
Maybe REW can do it too, as Allen says, but I haven’t tried it. Holm, I’ve used for years and it works perfectly.
A very quick ballpark offset can be got by using your current dsp xo settings, but with any delays removed. Run a sweep in REW and look at it in the 'spectogram' window. Zoom in to the frequency where your xo is and any jump up or down on the graph will show a timing offset. (sometimes it jumps about a bit, probably due to phase issues)
As Alan said, the acoustic timing reference in REW works very well, and does not take long to set up.
Rob.
As Alan said, the acoustic timing reference in REW works very well, and does not take long to set up.
Rob.
Here's a write-up by the late Jeff Bagby. His method was very popular when single channel measurement systems were the norm. As Jeff mentions in his paper you may or may not need to create minimum phase versions of the three measurements. Some single channel programs (e.g. Omnimic and HolmImpule) automatically set t = 0 to the impulse peak. Jeff called these "quasi-minimum phase" measurements. You could still make minimum phase versions of these measurements (i.e. add high/low tails and perform the Hilbert-Bode Transform), but it really isn't necessary.
Delay is right at amplitude maxima, not sure but you might see it with RTA so tune live. Sine tone at crossover, sweep the delay and note where is the maximum amplitude. If its tough to spot you can probably see both sides when the amplitude drops, and its in between 🙂
That said, its fun experiment to listen to music and scroll the delay with mouse and honestly I found it pretty hard to hear any difference until phase is so much out that there is audible difference in amplitude, a frequency response dip forms.
That said, its fun experiment to listen to music and scroll the delay with mouse and honestly I found it pretty hard to hear any difference until phase is so much out that there is audible difference in amplitude, a frequency response dip forms.
I used this method when a USB mic was my only tool. Mastering this method helped further my understanding of acoustics. Allen's suggestion to use the REW timing reference feature is the quickest way to get the information
Thanks for all the replies and hints.
My measurements may have already been done using the REW timing reference after all, but I really should do them again to make sure. It's a bit of a pain to set up the 5cu.ft 125lb boxes up for measurement, so not something I can do alone quickly, but I'll try to find some time. I have a bit of reading-up to do in the meantime anyway. 🙂
My measurements may have already been done using the REW timing reference after all, but I really should do them again to make sure. It's a bit of a pain to set up the 5cu.ft 125lb boxes up for measurement, so not something I can do alone quickly, but I'll try to find some time. I have a bit of reading-up to do in the meantime anyway. 🙂