timing reference help

[Richard Northcutt]

Hi to All,
This is my first post on this forum so go easy on me. I'm trying to do loop back connection on the reference channel (i.e. dual channel measurements) for a 2 way passive crossover design. My equipment is Dayton emm-6 mic with Scarlett 2i2 interface. I believe the connections are correct but my difficulty comes from processing the information in REW before exporting to crossover design software. Measurements were taken at one meter on the tweeter axis on a flat baffle with typical woofer below tweeter arrangement. My question to start with is how do I account for the delays between woofer and tweeter? I see the ref time present in the ir window. In this case, woofer =3.13ms and tweeter= 3.00 ms. I will attached the .mdat file for the experts to have a look to see if everything appears correct.

Best Regards,
Rich

 

[jtalden]

The measurements look good. I haven't used VituixCAD2 or XSIM so be sure to follow their measurement processing and entry instructions.
I can only confirm that with this mic location the MR needs 0.13 ms less delay (or the TW needs 0.13 ms more delay) for the 2 drivers to be aligned throughout the range of 2 - 9 kHz. The phase tracks very closely through that range. I assume 0.13 ms is the number they would require to identify the relative offset of the 2 drivers to put them is phase.

I also noticed that both drivers measured as being negative polarity. The 2i2 does not invert polarity so either they are both wired wrong or possibly you have inadvertently set the REW soundcard preferences to invert the soundcard input.

 

[Richard Northcutt]

Hi jtalden,
Thanks for taking a look at the measurements I posted. I do have a few newby questions if you don't mind replying to.
1. How did you determine polarity has been flipped? What graph shows this?
2. Do I need to use t=0 offset and shift both measurements by 3.00 ms? I.e closer to t=0 point.
3. If so, is this done before or after gating measurements.

I have been looking over past posts on the subject matter but much I have stumbled upon is dealing with room corrections or active crossover solutions. My apologies for such simple questions. Just trying to learn the software.

 

[jtalden]

Welcome to the forum.

1. How did you determine polarity has been flipped? What graph shows this?

The impulse overlay chart I posted shows the initial rise of both impulses are in the negative direction. You can invert them in REW impulse controls instead of remeasuring them if you like. Inverted or not all the modeling will work out correctly in terms of XO modeling.

2. Do I need to use t=0 offset and shift both measurements by 3.00 ms? I.e closer to t=0 point.

Yes, I would think any passive XO designer would probably handle the offset okay, but I would shift them both 3 ms as that will need to be done someplace in order to interpret the phase correctly and REW is a good and easy place to do it.

3. If so, is this done before or after gating measurements.

The order doesn't matter in REW, but the phase will look strange until the shift is made.

 

[Richard Northcutt]

jtalden,
Once again thanks so much for your expertise! That is great that I do not need to re-measure. I believe your assistance has me pointed in the right direction. I can't think of any other questions at the moment.

Best,
Rich