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Old 21st June 2005, 09:08 PM   #11
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Simon,

Thanks for that port length on the cube.

If I really can reach my goal of 25hz with OB, then I'll try an easy 4th order BP first. Clamshell the Tempests on the cube and put it face down directly over 18" driver cutout on the big box. With the 2 drivers the cube will weigh close to 200lbs, so just some foam rubber will make a good enough seal for testing. Do you think iso loading a 90L sealed rear chamber will get me a wide enough pass band from 25hz down with the 360L front chamber? If I can avoid cutting my cube, I'd like to because it has a beautiful layer of 1" hardwood laminated over a 3/4" well braced plywood box.
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Old 21st June 2005, 09:16 PM   #12
GM is online now GM  United States
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Quote:
Originally posted by simon5
Hi, I want to know why you guys prefer bandpass since the group delay is worse and the output is lower?

Why not go even more crazy? Two dual chamber bass reflex boxes. 10 cu.ft main chamber with 5 cu.ft small chamber, using 6 four inches diameter ports 28 inches in length. One chamber will be tuned around 10.5 Hz and the other around 14.5 Hz. I guess this would beat bandpass IMHO.
Greets!

First off, in the intended BW our hearing acuity isn't good enough to notice the high GD and will be decayed enough higher up so that it's not an issue. Second, my 'BP' is a conical compression loaded horn that morphs into a catenoidal one once the expansion hits the ceiling, so GD isn't an issue.

WRT to peak SPL capability, mine has a great enough compression ratio and much lower vent losses to achieve considerably more efficiency than any of the other designs posted so far, so in theory will 'blow their doors off', including yours.

Like I said previously though, the room will probably 'bleed off' a considerable amount of all this craziness, so the differences between the various designs probably won't be all that much. Indeed, had I known about the other boxes JCR has lying around, I'd of just designed around them.

GM
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Old 21st June 2005, 09:50 PM   #13
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Quote:
Originally posted by GM


Indeed, had I known about the other boxes JCR has lying around, I'd of just designed around them.

GM
I wasn't thinking about using those boxes until I saw the dimensions Dan recommended. That made me realize very little construction work would allow me to utilize the only significant boxes I have laying around gathering dust, which makes it pretty much a no brainer on the way to go.
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Old 22nd June 2005, 05:10 AM   #14
simon5 is offline simon5  Canada
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Quote:
Originally posted by GM
WRT to peak SPL capability, mine has a great enough compression ratio and much lower vent losses to achieve considerably more efficiency than any of the other designs posted so far, so in theory will 'blow their doors off', including yours.
Oh, I agree with you on this, a horn is way better than anything else in the SPL department. My question was only about bandpass. Dan said bandpass and you said that's the only thing making sense to you too.

Then I check and see exactly zero advantages for a 6th order bandpass box over a ported box, that's why I asked my question. Why prefer something who looks inferior to me? There's probably a reason that I don't know? Especially since you are both good subwoofer designers with alot more experience than me.

Quote:
If I understand group delay correctly, it won't be an issue because I have a variable distance setting for the sub on the receiver, so I can adjust timing that way, plus it is only going to be used for HT.
Group delay is not constant delay over the whole subwoofer bandpass. You can't compensate for it, unless you can set a different delay at each frequency, something like a 10 band delay EQ, which I think isn't in use alot yet! The group delay is usually a big peak of delay just a few Hz under the tuning frequency.

Yes, you don't need to cut your boxes, WinISD Pro simulations are really close to Dan Wiggins recommendation. Since the boxes are bigger, you'll have a bit more efficiency.
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Old 22nd June 2005, 06:45 AM   #15
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I've come up with a new idea. I've had good results with the Decware housewrecker subwoofer, a 6th order bandpass with a sizeable sealed chamber between the two iso loaded drivers. There is no pressure in the chamber and if you opened it up there should be no sound.

My idea is this, to mount the small box on top of the big one with the driver facing directly at each other, but raise the small box 8-10" above the larger box on 3 legs and leave that space open. That way the drivers are visible, I can easily get to them if necessary and even up close I believe there would be zero sound coming from that open area even at high output which would be a cool effect.

The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas, but I have a feeling that has no effect on response with a 6th order bandpass OR am I all washed up and the different chamber sizes and tuning will make the 2 drivers behave differently resulting in output from the center if I leave it open.
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Old 22nd June 2005, 08:41 AM   #16
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"" There is no pressure in the chamber and if you opened it up there should be no sound.""

Sealed or not a woofer produces soundwaves on both sides of the cone.

"" I believe there would be zero sound coming from that open area even at high output which would be a cool effect.""

Wrong. See above.

""The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas""

There's no such thing as "loose iso coupling". The drivers are either coupled together or they're not. Unless there was a flaw during construction causing an air leak.

""....resulting in output from the center if I leave it open.""

Now your on the right track.
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Old 22nd June 2005, 01:09 PM   #17
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Quote:
Originally posted by Mikey p
"" There is no pressure in the chamber and if you opened it up there should be no sound.""

Sealed or not a woofer produces soundwaves on both sides of the cone.

"" I believe there would be zero sound coming from that open area even at high output which would be a cool effect.""

Wrong. See above.

""The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas""

There's no such thing as "loose iso coupling". The drivers are either coupled together or they're not. Unless there was a flaw during construction causing an air leak.

""....resulting in output from the center if I leave it open.""

Now your on the right track.

Mikey,

Yes the cone moves, but the other cone is moving too, but in the same direction because it's fed an identical but out of phase signal. The distance between these 2 equal but opposite sound sources is insignificant in relation to the wavelengths produced so they will net to zero except to the extent that the chamber differences affect cone movement, which I believe is the hole in the idea.

Just take 2 speakers and face them together say 1 foot apart. Switch the wiring polarity on 1 and play a mono low frequency signal. Magic, no sound. Haven't you ever used this approach to break in drivers quickly using high volumes without disturbing anyone? In fact, with the kind of frequencies we're talking about, the speakers wouldn't even have to face each other for silence.

That would make an interesting test. Identical drivers + 2 differently tuned ported cabs side by side with one out of phase + a mono signal. See how closely they imitate a 6th order bandpass enclosure.
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Old 23rd June 2005, 03:50 AM   #18
Mikey p is offline Mikey p  Canada
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johninCR , I'm really looking forward to your results. Let us know how you make out.
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Old 1st October 2005, 09:36 AM   #19
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Yep - I'm interested in what you ended up building too.


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Old 1st October 2005, 03:11 PM   #20
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I did a test run since I already had the boxes laying around to build Dan's suggested isobarik 6th order BP. It worked fine and as predicted.

I decided that I really didn't need that much potential output and have opted for a dipole arrangement consisting of 2 sealed shiva's in a coffee table immediately in front of my seating position and 2 sealed tempests under my screen firing out of phase.

The initial tests of the dipole arrangement worked perfectly and the room is taken almost totally out of the equation, leaving much cleaner bass. Due to the 3.5M separation distance I don't lose output going dipole. In fact, the out of phase wave reinforces the primary wave down to 17hz, so it's just a matter of flattening the sloped response which I have to do anyway with the sealed alignment.

To me, I give up room gain plus about 10db of max potential vs the BP alignment to achieve flat down to 13hz and I'll need to use the 2 additional drivers and more power. However, I realized I'll never use 120db at 15hz anyway, plus the dipole really keeps the sound in room drastically more so I'll be able to listen louder without disturbing others and/or causing structural damage to the house, all with better sounding bass.
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