W-Frame or H-Frame dipoles? performace issues?

[johninCR]

That's some great work. It brings up some very interest questions, especially since the actual and predicted SPL tracks so closely below 150hz.

First, he predicts a rise of 12db from Fequal up to Fmax. My understanding from Linkwitz's work is the peak should be at +6db.

Second, while panel resonance and/or 1/4 wave resonance could explain the peak around 180hz, the steep null in the next octave looks like it has to be the dipole null at 2 x Fmax. What is causing the dipole peak behavior half an Octave earlier than expected? I hope it means that something about the manifold alignment makes what should be a 100hz Fequal baffle according to Linkwitz in free space, a 60hz Fequal in the real world. Maybe the rear pressure wave needs to clear the rear cavity by some distance before it starts its true free space expansion in all directions including back toward the front, because the peak at 180hz indicates almost a 1 meter separation distance vs the 60cm distance in his cab drawings. This is quite a significant difference, and may be part of why I get deeper bass than expected in folded alignments, which I always attributed to room benefit. More likely though, it is because he measured at only 2 meters, but that still doesn't explain the early Fmax.

[Rudolf]

John,

I believe it was GM who told me that the effective length of any resonator (be it quarter wave pipe or BR port) has to include some correction factor, which should be roughly half the diameter of the tube/pipe/port. So your explanation of the lowered resonance seems logic.

I´ve seen some math on the net for calculating that correction factor for oddly shaped openings, but different approaches seem to give different results.

Rudolf

[johninCR]

Rudolph,

GM likes to use the visualization of a pressure or sound wave as a bubble travelling through a pathway, and if we visualize what is happening as a wave being a bubble, then it makes sense that the bubble can't immediately expand in the forward on axis direction with its full potential because the cab is in the way.

If this is true, then it's good and bad news for those of us who advocate manifolds and U-baffle shapes. While we can get some significant free ride in terms of "D", Differential of front/rear wave travel distance, the cost will probably be at the expense of lowering the frequencies where resonances begin.

Time to consult the physics experts. I like free rides and I address resonances with damping anyway.

[Davey]

Free rides in physics?

Regarding Stefan's excellent effort... Simulations are great....to a point. But when the real-world measurements show a significant discrepancy then there's only one conclusion. The simulation is incorrect.

I believe/understood Ingemar was continuing to work on the program to try and make more accurate the "box," "tube," and "mouth" parameters. Also, it's not clear the numbers for those parameters used by Stefan are correct. You'll note that he only included half the volume and distance of the chambers in his CAD model.
It just goes to show that (in conjunction with the drivers) this is a more complicated acoustic structure than it would first appear.

I think the 180Hz peak in his measurements is indeed the cavity resonance. This is consistent with "W"-frame enclosures I've built with similar dimensions.

Cheers,

Davey.

[johninCR]

Originally posted by Davey
Free rides in physics?

Regarding Stefan's excellent effort... Simulations are great....to a point. But when the real-world measurements show a significant discrepancy then there's only one conclusion. The simulation is incorrect.

I believe/understood Ingemar was continuing to work on the program to try and make more accurate the "box," "tube," and "mouth" parameters. Also, it's not clear the numbers for those parameters used by Stefan are correct. You'll note that he only included half the volume and distance of the chambers in his CAD model.
It just goes to show that (in conjunction with the drivers) this is a more complicated acoustic structure than it would first appear.

I think the 180Hz peak in his measurements is indeed the cavity resonance. This is consistent with "W"-frame enclosures I've built with similar dimensions.

Cheers,

Davey.

Davey,

OK, how about a free ride compared to common theory due to the actual physics?

How sure are you that is wasn't panel resonance? I've had tremendously audible panel resonances with unbraced folded forms. Damping clears up the cavity resonances, with no sonic detriment to my ears, but they need to be avoided on the front side. I hope to avoid them using non-parallel surfaces and small drivers.