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Old 20th February 2009, 03:38 AM   #1
the joz is offline the joz  Australia
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Default Help with a concept

Anyhow I think I have alluded to what I'm planning else where around here but I thought I'd just put it out there.

I'm planning on using 2 x12" Exodus Shiva drivers in one smaller sealed box about 100litres+ approx.
Both these drivers will have their magnets facing each other,yes one cone facing forward the other back,sharing the same cabinet volume.

I've been told by some that this way I can get away with using smaller enclosures because somehow the drivers are tricked into acting like the box is bigger than it is, while others are telling me that I must simply double the box volume because I'm using two drivers.
I know this is what B+W with their small round subby and some others do.
But where does the truth lie?

Also will having a rearward facing driver have any detrimental effects??


Anyhow ontop of these boxes will sit my MTM's..

Sigh!
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Old 20th February 2009, 04:04 AM   #2
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Isobarik or Isobaric loading it's called. There are different ways of doing it and yes you can use two drivers in a box meant for one or even half that. ie: a box that's 1/2 the size of the normal box for one driver.

Here's another way of aligning the drivers, same result.

http://en.wikipedia.org/wiki/Isobaric_speakers

and the way you suggested, I think

http://www.members.shaw.ca/pferlow/isobarik_boxes.htm
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Old 20th February 2009, 04:41 AM   #3
TerryO is online now TerryO  United States
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Isobaric loading is, or was, a method that reduced distortion when playing, although at the cost of two drivers. The Exodus Shiva-X drivers utilize XBL2 motors and already have somewhat reduced distortion. I'll offer some advice: Don't use isobaric loading, but instead build two subwoofers. Two woofers will produce the same SPL output as an isobaric with half the excursion. Lower excursion usually translates into reduced distortion levels. As an added bonus you can position two subs to curtail the nulls or eigenmodes that you will always have in any room.

In the old days, when Cal was young, they didn't have the really good subwoofer drivers like the original Shiva and later the Shiva-X and so isobaric loading was conceived as a method to have elevated levels of bass with somewhat reduced distortion. With the quality of the modern drivers (and Shiva-X is very good quality indeed!) isobaric loading is no longer needed. However, if reduced size is the main factor, then by all means go for it.

Best Regards,
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Old 20th February 2009, 04:52 AM   #4
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Hi Joz, I think I know what you are talking about.

I have been told that when drivers are mounted front to back, sharing the same volume the box can be somewhat smaller than the normal.The one test box I built the shared volume was 90litres for a reasonable F3; using a calculator for a Qts of 0.8 with one woofer gave 30 litres, drivers do need to be in parallel tho.
Sound is omni-directional at these low frequencies

There is a thread, search for "Tardis" subwoofer
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Old 20th February 2009, 04:54 AM   #5
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Quote:
Originally posted by TerryO
eigenmodes
Jiminy cricket man. I had to go look that one up. How's a guy your age able to remember words like that?

Oh and Joz? - what Terry said. He is a man of much wisdom. Many many years of wisdom...
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Old 20th February 2009, 04:55 AM   #6
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Quote:
Originally posted by Moondog55
"Tardis"
Oh my, that brings back painful schoolyard memories.

Oops, OT
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Old 20th February 2009, 05:06 AM   #7
TerryO is online now TerryO  United States
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Quote:
Originally posted by Cal Weldon


Jiminy cricket man. I had to go look that one up. How's a guy your age able to remember words like that?

Oh and Joz - what Terry said. He is a man of much wisdom. Many many years of wisdom...
I think I do pretty good for someone 87 years old.

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Old 20th February 2009, 05:25 AM   #8
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Joz,

I just read your other thread and now I get it.

The people who are telling you that you have to double the box are right. I was answering how to do it if you want the box to be smaller.

Can you take Dave's push push box and fire the drivers sideways rather than front and back?
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Old 20th February 2009, 05:58 AM   #9
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Hi Cal , an "OLD"?? Doctor Who fan ??

The test box I built would suggest double less at least 15%, providing that walls only are lined and the boxes are not fully stuffed.
I did not understand the explanation I got, my maths stops at 8th grade ( use it or lose it )
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Old 20th February 2009, 06:05 AM   #10
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If you put 2 drivers in 1 box, you need to double the volume. (ie double the Vas, double the volume).

0ounting the drivers push-push and tightly coupling them will give you active vibration cancelation and you can get away with less actual box (you still need to make sure it is stiff enuff not to ballon (avoid MDF))

2 subs are usually better then 1, but i fell the advantages of push-push are significant So to me, 2 subs means 4 drivers (just got 4 Trio8 and 2 SDX10, that makes 3 subs -- this could get interesting)
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