How to use OmniMic and PCD to find the Relative Acoustic Offset

[Jeff B.]

(This was previously posted in TechTalk, but I added it here too - Jeff)

I have written and article on how to use OmniMic (or other measurement system) and my Passive Crossover Designer Excel tool to determine very accurately the relative acoustic offset between a woofer and tweeter's acoustic center for accurate crossover design simulation.

Finding this value is not difficult if these steps are followed, and once found will allow for very accurate crossover design simulation using my Passive Crossover Designer. Without knowing this value phase relationships will not be accurate, and as a consequence, the accuracy of the modeled response will be reduced as well.

As you will see, perfect measurements are not necessary. To make this point my example was done in my family room intentionally to show that you can do this the same way.......

To read the rest of the article please Click here to download pdf file (http://www.box.net/shared/ouxjjsx0m8bs00cil5iq) with graphs to explain the details of this method.

Jeff B.

[arlis_1957@yahoo.com]

thanks jeff. this time it was easy to comprehend.

[dankwijoti]

Awesome, much apreesh Jeff. I have been using the PCD, but lack the mic to use it to its full potential. Hopefully that will be remedied soon.:D

[DanP]

Nice first post in this part of the forum, Jeff. I've been using this method of finding acoustic center offsets since you posted a thread about it in the main forum a while ago. I'm still impressed with how well it works.

Dan

[dcibel]

Just noticed there is a forum for the Omnimic!

Thanks for this Jeff, tried it out tonight, totally works and it also a simple procedure.

I am a bit confused though, as I tried this out for an econowave speaker that is in the works, crossover parts being ordered later tonight. With this method, it determined the offset between the woofer and tweeter is 0.115 meters, which is a lot more than what I think the physical dimension between the drivers is. Is there something funky that goes on with horn loaded speakers that causes a greater acoustic offset than the actual physical dimension?

[ernperkins]

I'm using Jeff's method, but not having much luck getting the waveforms to match in PCD. Because of room reflections I have to limit my gate time (I use HolmImpulse) to 3.5 msec (~ 300 Hz). Jeff's article states he uses a gate time of 8 msec (~125 Hz). I'm wondering if my extremely short gate time is the cause of my problems. Thoughts?

I've tried various HolmImpulse options: detect first peak on each measurement (no locking), detect first peak locked (woofer and woofer/tweeter measurements locked to tweeter peak) and casual impulse locked. I've also tried running the files through Response Modeler/Hilbert Transform to generate minimum phase. All to no avail.

Regards,
Ed

PS - Sorry about cross post in Omnimic forum. Perhaps I should just fork over the dough.