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Old 18th November 2011, 05:24 PM   #1
eccdbb is offline eccdbb  United States
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Default opposed woofers

If you wire two woofers with the same polarity, but face them in opposite directions like back to back, what is the summed output for low frequencies on the woofers' axis? Would it be close to 0, because the woofers are moving in opposite directions? Would the 3D polar response be like ---8---, something like a donut?
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Old 18th November 2011, 05:33 PM   #2
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Below the baffle step the speaker is essentially omni-polar, they act as if they were one big driver. At a frequency determined by the depth and width of the cabinet there is a dip (less deep if the cab is wider than deep), and above that you have a bipolar response.

dave
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Old 18th November 2011, 07:35 PM   #3
Francec is offline Francec  Australia
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My question is why would you want to do it?
If you face woofers back-to-back and wired them with opposite polarity, that would halve the Vas* so you could use a cabinet just over half the size. Now that is a tangible benefit.
The setup proposed gives no benefit, afaik.

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Old 18th November 2011, 08:52 PM   #4
Scott L is offline Scott L  United States
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Default Too much confusion

ISOBARIC loading is the type of loading that creates 1/2 VAS.

Loading 2 woofers in a common enclosure, magnet facing magnet, wired in phase acoustically, doubles the Vas. This can be a good thing if you can couple the drivers structurally, as it cancels vibration. An associated problem is, unless they are also very close to each other, there will be time delay smearing depending also on the frequency range covered.

As far as the OP's question- the answer is that the outputs cancel.

So again, why would you want to ?
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Old 18th November 2011, 08:58 PM   #5
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I think he just wants to know the radiation pattern that results from pushing or pulling the air in opposite directions at once. Beaming in both directions at higher freq, omni pressurizing the room at lower freq. Not "0" not "8" not donut.
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Old 18th November 2011, 09:00 PM   #6
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Yes, Scott, Francec is describing isobaric.

But re: the original question, it's unclear whether he's asking about two dirvers in free air or connected with a box or tube. back-to-back is not clamshelled... I was assuming some box or tube between. But with both in free air, yes as soon as it got low enough not to beam it would cancel around each driver.

Last edited by cyclecamper; 18th November 2011 at 09:04 PM.
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Old 18th November 2011, 10:05 PM   #7
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Quote:
Originally Posted by Francec View Post
My question is why would you want to do it?
If you face woofers back-to-back and wired them with opposite polarity, that would halve the Vas* so you could use a cabinet just over half the size. Now that is a tangible benefit.
Wired like that you end up with a complex to build dipole (ie OB) without the no box benefit of an OB and twice the driver cost. Bass would be determined largely by the box width convolved with the IB FR.

You may well be thinking an isobarik pair. Not the question.

There are very large benefits to a push-push woofer, one of which is active force cancellation dramatically reducing the energy input into tha box. Everytime i can, i use woofers in this configuration.

dave
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Old 18th November 2011, 11:02 PM   #8
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And depending on how far away you can position the loudspeakers from the rear wall, placing one driver on the front and one on the rear will compensate for baffle-step losses.

IE below the step the drivers act together as an omni source, reinforcing one another. Then, as frequency increases, the width of the cabinet 'prevents' the higher frequencies from the rear driver from meeting the front so there is no reinforcement. Of course in a real room the walls will reflect sound all over the place, so quite how effective this 'using the loudspeakers cabinet as the acoustic filter' would be in practise I do not know.
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Old 18th November 2011, 11:08 PM   #9
Scott L is offline Scott L  United States
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Default correction ( i have sumtimerz )

Quote:
As far as the OP's question- the answer is that the outputs cancel.

So again, why would you want to ?
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I have thought about this further: The outputs would not cancel completely.
As mentioned above, it would be a complicated di-pole.

If two drivers were in separate boxes, faced each other, and wired out of phase, the outputs would cancel.

Fun thread, yes ?
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Old 18th November 2011, 11:36 PM   #10
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Here is a simplified illustration of what (i understand) we are talking about.

Click the image to open in full size.

Wired in phase there is no substantial difference in base performance (<< baffle-step) no matter what faces you put the woofers on. ie same as 2 woofers in phase on the front. Omnidirectional.

When as (illustrated) we approach baffle-step frequency, the on-axis FR of a face with a driver has a dip from cancellation, (>> bafflestep) you have the output of the front driver + room reflections of the back driver.

If you measure on axis of a face between the 2 woofers (ie the drivers are side mounted), there is no dip and FR starts falling with the 90+ degree dispersion of the driver (+ room)

dave
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