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Old 10th May 2005, 10:51 AM   #21
Jennice is offline Jennice  Denmark
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Planet 10,

do you know how essential it is, for the drivers to be mounted aligned, on-axis?
It could be tempting to mount the drivers with an off-set, therby using the space between the front woofers' magnets to accomodate the magnets of the "hidden" woofers. Considering the depth of the structures, this could save 10cm in the depth.

I don't want the "hidden drivers" to be placed too near the rear wall of the "overall" cabinet.

As long as it's a sealed volume of air being moved my the drivers (in the isobaric chamber), alignment shouldn't be important in my imagination (?)...

Any comments?

Jennice
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Old 11th May 2005, 01:37 AM   #22
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Quote:
Originally posted by Jennice
do you know how essential it is, for the drivers to be mounted aligned, on-axis?
It could be tempting to mount the drivers with an off-set, therby using the space between the front woofers' magnets to accomodate the magnets of the "hidden" woofers. Considering the depth of the structures, this could save 10cm in the depth.
Basically, the small the coupling chamber the better... with 4 woofers i'd mount them similar to my drawing above, which means 2 chambers.

dave
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Old 11th May 2005, 05:55 AM   #23
Jennice is offline Jennice  Denmark
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Hi Dave,

I see your point, and I don't like to "waste" more enclosure volume than needed, but the speakers will be in a room with two small children (technical hazard) and a spouse (aestetical hazard if she doesn't like 'them). Thus, I don't think I can get away with two "free" speakers like you have on the outsides of your drawing.

While we're at it... If I recall correctly, you have shown several projects where you compensate for baffle step loss with an extra driver on the rear. This baffle step is 6dB, right?

Jennice
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Old 11th May 2005, 06:02 AM   #24
azrix is offline azrix  United States
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Quote:
Originally posted by azrix
As far as I know, you have that correct. You should gain 3db for doubling cone area once (2 drivers), another 3db for doubling again (4 drivers), and none from impedance since that didn't change. Which gives you a 6db net gain.
I have to recant this. You do only gain 3db at 1watt when you double your drivers, but it's not because of cone area, it's because you double your Vas, and doubling Vas gets you a 3db boost in efficiency. Doubling displacement (cone area) gives you a 6db boost in total output. So, given two of the same driver wired in parallel, you should have a 3db boost in efficiency due to increase in Vas, and doubling of power handling, which also gives you a 3db boost if you have enough power, giving you a 6db total boost, but at half the impedance.

With regard to driver arrangement, there are a million different ways you could do it and I don't know how small you want your speakers. I personally would consider having all four drivers, one on top of the other, on the front baffle, but that would be getting close to one meter tall just for the bass drivers. May be too big for you. Baffle step is a bit tricky. If you are building a floor standing speaker and have your bass drivers very close to the floor, then you should not need any baffle step since your bass speakers will be radiating into a half-space just like your mid and tweeter. You can also cross over to the bass drivers at baffle step and use the increase in efficiency of having multiple drivers counteract the decrease you get from baffle step.
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Old 11th May 2005, 06:17 AM   #25
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I wasn't really clear. I intended something more like te attached. Still push-push, but as tight a space as possible (blocking the empty corners (or using PVC or an octagonal shaped coupling chamber wouldn't hurt)

dave
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Old 11th May 2005, 06:33 PM   #26
simon5 is offline simon5  Canada
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That's nice, but I heard there is some thermal problems associated with the outside woofers because the chamber is so small. Did you hear about this planet10 ?
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Old 11th May 2005, 06:57 PM   #27
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Quote:
Originally posted by simon5
I heard there is some thermal problems associated with the outside woofers because the chamber is so small.
There could well be... i a home hifi situation, it probably is not too serious.

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Old 12th May 2005, 08:11 PM   #28
AndrewT is offline AndrewT  Scotland
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Hi,
you quoted some box volumes for 1 & 4 drive units.
41L for 1 & 164L for 4; giving 82L for 2.
In isobaric format double the number of drive units and half the volume. Now you have double the power handling but the same Xmax.
Taking the 82 litre box with 2 drivers then go isobaric and have 4 drivers with 4 times the power handling and same Xmax in a 41L box (too small to be worth the effort). and gaining 3db in efficiency. (90db?? can someone confirm).
I have simply restated what has gone before in case you missed the points being made.
The problem may be that you run out of cone travel at low frequency (same Xmax) but I think the volume(SPL) by then will be deafening.
Does all that make sense?
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Old 12th May 2005, 08:33 PM   #29
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Officially, the 4 x isobarik box will give me the same output (ignoring impedance effects on the amp(s)) as a pair in a box 2x as big.

Power handling is something that i have no concern about. Besides the smaller box, i get the mass trapped between the 2 drivers to lower the Fs subtely, the extra layer of cone to isolate what happens in the box from outside the box, and the push-pull-push-push motor which will not only reduce vibration passed to the box, but will help linearize the drivers over a single example. I lose the air the extra 4 drivers would move, but i think 4 12" in my room will be more than sufficient.

As was recently pointed out in one of the threads, increasing the number of drivers doesn't actually increase efficiency (flying in the face of common knowledge i had to go back and look at it from 1st principals to see that this is probably true (ignoring potential gains from improved coupling with the air))

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Old 13th May 2005, 08:14 PM   #30
Jennice is offline Jennice  Denmark
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I have studied various pictures of isobaric designs... is it correctly understood, that minimizing the volume of air between the isobaric pair is important (which could be why most make them front-to-front mounted)?

Jennice
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