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Questions / Planning for Big PA Party Speakers

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  • #76
    Hey all - I think I was just trying to beat the lower woofer into submission too hard. In the attached PCD screenshots - way down at the bottom - you can see the Secondary Woofer part where I've added a 4mH iron core inductor. I was trying to make this woofer roll off at the 95 dB target level following either a BW1 or LR2 curve. That was forcing me to use something like an 8mH inductor and a series resistor to dampen out the huge inductor spike that made. Clearly that series resistor is a BAD idea in the woofer section, especially on a PA cabinet where the woofers might actually try to pull 400 - 600 watts from the amplifier.

    Once I realized that I didn't have to be so aggressive (my first 2.5way sim ever..) things started to fall into place a bit more. The attached model shows 2x10 SB Audience Bianco 10MW150 + B&C DE250 1" CD. I used manufacturer FRD/ZMA traces from spec sheets - ran them through Jeff Bagby's diffraction / boundary sim to apply baffle step for a 15" x 35" baffle, and vented box response for a 2 cuft box tuned to 65 Hz by two 4" ports of appropriate length. Box models were spliced into the woofer FRD (I used the same 8 ohm woofer FRD twice in PCD), and minimum phase FRD / ZMA files were extracted. I made some assumptions on the driver offsets to establish a Z-offset, and started playing with XOs. The PCD model is using active XO for the HF and top woofer, with passive XO coil in-line to the lower woofer.

    1) I think something isn't right with my impedance, as the system impedance is down below 4 ohms until ~700Hz, and the impedance peaks for the vented arrangement are barely 6 ohm. Removing the 4mH coil did not change the system impedance, so I'm not sure what I did to mess that up. I don't think I should actually get Impedances that low outside of the woofer dips around Fb, should I?

    2) Cancellation around the XO point. It's difficult to tell, but PCD is showing me a tiny bit of cancellation out of both the top woofer and the tweeter around the XO. It's ~ 1/2 a dB between 700 and 1000 Hz on the woofer, and another 1/2 dB on the tweeter between 2250 and 2780 Hz. I'm curious if that is normal / expected in a 2.5 way crossover design, or is there something else I'm introducing by modeling active XO with passive additions?

    The Biggest question of all though - How can I tell if the little bit of added complexity and effort is worth it on a PA cabinet? Sure, I want it to sound fantastic and have as little interference between the drivers as I can, but how likely am I to notice the difference? I don't have a baseline to compare against short of spending $40 extra on two inductors to test it out.

    Looking forward to your thoughts!

    (Edit: Full size photos on Google Photos Album:

    Click image for larger version  Name:	Simmed 2.5way TMM DE250+2x10MW150 Normal.png Views:	0 Size:	360.4 KB ID:	1456629 Click image for larger version  Name:	Simmed 2.5way TMM DE250+2x10MW150 Reverse Null.png Views:	0 Size:	360.6 KB ID:	1456630
    Last edited by KEtheredge87; 11-13-2020, 04:22 PM. Reason: I tried to be cute and upload full screen scrolled snapshots of PCD. PE's forum software shrunk them down to the point you can't read anything if you pick them and zoom in! and I wasn't going to make
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