Estimating Acoustic Offset

Ed_Perkins

I'm helping a friend get started in loudspeaker design and a question came up about estimating acoustic offset.  He doesn't have measurement gear so we're using manufacturer IB files and Jeff's Response Modeler to sim the baffle step, low end woofer response and the modified impedance curve.

 He's decided to use XSim and we'll use that to create the minimum phase (derived phase in XSim) .frd and .zma files. Which brings me back to the estimated acoustic offset (mod delay in XSim).  I seem to remember that an estimate for a "typical" 6 ½" woofer and 1" tweeter was about 1 ½".   And I don't remember if that estimate was valid for minimum phase files or not.  Do you guys have any opinions or guestimates?  I suppose we could always punt on the acoustic offset, but I feel an estimate based on the experience of this group is better than none.

 Finally, I don't use XSim that much and always get confused on the sign of the mod delay.  Do you put a positive mod delay value in the woofer?  I'm used to the negative value used by PCD.

Thanks guys…

 

   

jhollander

I.5 is close imo. XSim is positive for the woofer. When you hover over the cell it says positive moves away from the mic.  The Blender is better for the baffle imo as you get to set the tails and see what you minimum phase file looks like.

Ed_Perkins

Thanks John.  

dynamo

I have found 1” to be accurate for a standard dome and standard 6.5”.  That is my go to for that driver combo when modeling.

The estimated offsets I use for modeling with a dome:

5” - 19mm
6.5” - 25mm
8” - 50mm

Dont forget to then account for Y axis differences using Pythagorean theorem. Use tweeter as the 0 point, determine hypotenuse to the middle of the woofer  from listening distance, subtract listening distance from hypotenuse, and add that number to to the z offset.

Programs like PCD allow you to enter x and y distances and crunches it for you, but iirc you need to do it on your own in xsim.

So for a 8’ (96”) listening distance and a ctc spacing of 6”, you would have 96^2 x 6^2 = sqrt of 9252 = 96.187, -96 listening distance = .187” farther travel to listening point than the tweeter. If 1” is estimated offset difference, the offset is 1.187” in a TM configuration with a 6” ctc.

PWRRYD

Remember that the offset numbers are different when using WinPCD (or PCD) versus Xsim.  

Ed_Perkins

Are you referring to the difference because PCD uses both Y and Z values where XSim just uses a single value?   Yes, I was aware of that.  I found an thread from 2015 where it was brought up and Bill Walso said he was going to add the Y/Z option to XSim.  But I think he got caught up with XSim 3D and it never happened.

PWRRYD

Yes, that's what I was referring to.  You probably know your Y-offset.  So take your estimated Z-offset and use Pythagorean's theorem to calculate the woofer's offset in Xsim.

jhollander

Good point on the Y separation/ hypotenuse.  My 1.5 est was all in.

Kornbread

From @dcibel; Load your measurement of the two drivers playing together as an overlay. Draw a schematic out just like you wired them when you measured them together, and adjust delay value on one of the drivers until the response overlaps. For best results, it's important to turn off smoothing on all plots.  Worked for me.

Ed_Perkins

I think you missed the point that my friend doesn't have any measurement gear!  

Kornbread

A mic is only ~$60, whoops, looks more like $75 now.