tomlang Messages: 10 Registered: February 2011 Location: Asheville, NC
Chancellor
How about thinking "inside the box"?
Build the 4pi per plans, cut out (but save) the front baffle to the Monet's and literally put the smaller (if it fits) 4 pi box inside the Monet box just behind the grille.
I've looked at a lot of designs and had a long hard talk with these cabinets. They told me that they really want to be 4pi. The plan would be to remove the baffles and install an airtight horizontal divider. The woofer would live in it's own enclosure of whatever volume we decide. The picture shows it at 23" from the bottom which gives 3.5 ft3. I've also drawn 2 ports of 3" diameter. I would take frequency response measurements and post them here. I'd make any suggested changes and ask Wayne for his approval before affixing the pi logo.
Wayne Parham Messages: 18786 Registered: January 2001
Illuminati (33rd Degree)
Just watch for lower midrange ripple resulting from internal standing waves. That's always the risk with large two-way loudspeaker designs. The positions of the midwoofer, port(s) and damping material all are very important.
The Omega driver's recommended vented enclosure is 3.28 ft3 with an Fb of 48hz. I think the 4pi is 3.14 ft3 tuned to 42hz. I could keep the woofer centered in its enclosure or move it up closer to the horn. Perhaps I should make the woofer enclosure slightly larger an use closed cell foam to decrease it to desired volume. That way I could move the foam if I needed to change the geometry.
I will test for standing waves, but any tips on box size, tuning frq, and driver location would be appreciated.
Wayne Parham Messages: 18786 Registered: January 2001
Illuminati (33rd Degree)
The midwoofers used in the four Pi models work well in 2ft3 to 5ft3 cabinets with Helmholtz frequency between 35Hz and 40Hz. So that part is pretty easy.
It's dealing with the internal standing waves that's tricky.
I found free online software called Boxnotes that calculates internal sound reflections to minimize standing waves. The Monets should be about an inch less tall, and a few inches narrower with the woofer offset just slightly. That ensures none of the internal reflections stack up on one another. I'll change the volume with closed cell foam which is easier, lighter, and less reflective than mdf.
Wayne Parham Messages: 18786 Registered: January 2001
Illuminati (33rd Degree)
That sounds reasonable. Be sure to measure and pay close attention to the response between 200Hz and 400Hz. The standing wave nodes above that are damped very effectively by the fiberglass insulation. It's the lower frequency nodes - the ones in the midrange - that the insulation can't damp very well. Actually it's the anti-nodes - the highest-pressure points - that cause the problems but semantics aside, we're just worried about response anomalies under 500Hz caused by internal standing waves.
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