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Old 14th April 2010, 05:43 PM   #1
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Question Push/Pull-coupled basses.

I have 4 basses lying around (Vifa M26WR-09-08) and I'm thinking about what to do with them. So I thought about a push/pull coupling, but not all internal as you would have in subwoofers. One of the basses mounted as if in a normal vented case and the other immediately behind it, mirrored - magnet to magnet. Externally it would look like a normal speaker la Vifa Vivace, only with much deeper bass. What are the drawbacks except higher cost and lower impedance?

Does anyone here have experience with a system like this, or comments about it?
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Old 14th April 2010, 05:56 PM   #2
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Similar yes, what you'd end up with is an isobaric configuration. The result will be a kind of superdriver that will only require half the enclosure internal volume to give the same output & response as a single driver. Obviously it'll be a bit bigger due to the volume between the drivers

I did something similar years ago, i had a pair of old Wharfedale TSR110 speakers which were 2.5 way, in other words one of the bass drivers was fed bass & midrange & the other was fed below 300Hz. What i did was rebuild the box but with one driver behind the other. The rear driver was fed the sub 300Hz signal & the front driver was fed the full range up to about 3.5Khz.

It worked very well indeed. I used a Kef ABR from the 104AB & a focal tweeter.

I ended up selling them for about 600 Not bad considering they cost me about 300 to build & were surplus to requirements...
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Old 14th April 2010, 06:03 PM   #3
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Similar yes, what you'd end up with is an isobaric configuration. The result will be a kind of superdriver that will only require half the enclosure internal volume to give the same output & response as a single driver.
Thanks for the reply. What would the resulting sensitivity for the (bass)system be compared to a single driver?

Last edited by Whalebiter; 14th April 2010 at 06:07 PM.
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Old 14th April 2010, 06:07 PM   #4
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If one driver is completely concealed inside the cabinet behind the front driver then you have created an Isobarik arrangement. Doing this you halve the size of the enclosure required by one driver but gain no efficiency nor max SPL. This arrangement is not generally reccomended for use above subwoofer frequencies.

Also, in this arrangement you get no vibration cancelling, which I think you are after? Since both drivers will need to be wired to move in the same direction. To gain vibration cancelling advantages, use 2 drivers, both exposed, typically one on either side of the cabinet. This requires a cabinet twice the size required by one driver but with increased sensitivity and max SPL also. Due to the side mounting, this can't be used much above ~250Hz.

If you mounted one driver on the front and one on the back and wired them in-phase, you'd have a Bipole. This can be used up to higher frequencies, but be aware that you will get a lot more reflected sound among other considerations.

EDIT: event horizon posted while I was typing
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Old 14th April 2010, 06:14 PM   #5
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Well the voltage sensitivity will be exactly the same, however, as the two voicecoils will be wired in parallel they'll consume double the current. Assuming they are 8ohm drivers your amplifier will see a 4ohm load at any frequency that the drivers receive

The force factor will double, QTS & FS will remain the same & VAS will half

I can see why (i think) that you'd like to mount the internal driver back to back so-to-speak from the front one. When two drivers are used for a sub or woofer & are on the exterior of the cabinet & one is monted conventionally & the other with the magnet exterior to the enclosure, they tend to cancel out a certain ammount of distortion. However, as you plan on mounting it internally i'd be more inclined to face the cone of the rear driver at the magnet of the exterior driver. You'll get a closer coupling between the two (which also means a smaller enclosure) & tbh i'm not sure that they'd cancel distortion so well as you won't actually get any sound radiation into the room from the internal driver.
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Old 14th April 2010, 06:26 PM   #6
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This arrangement is not generally reccomended for use above subwoofer frequencies.
I can see that it would become a phase-problem as it gets to higher frequencies due to the distance between the cones. Are there any recommended rules (of thumb) here?

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Also, in this arrangement you get no vibration cancelling, which I think you are after?
No, I really wasnt. I just don't like big speakers and this seems to be a good way around that.
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Old 14th April 2010, 06:57 PM   #7
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Yes, the halving in impedence will mean double the power is supplied to them, up to a point at least, not all amplifiers will supply twice as much output at 4ohm as with 8ohm loads. It looks like the max SPL with an Isobarik is 3dB less than with a single driver in the larger box, as modelled on WinISD.

I am not sure how high you can go with an Isobarik but I know that you want the drivers closely coupled, typically they are mounted cone-to-cone, but this isn't so good if you need high frequency radiation!

The Isobarik configuration is indeed a good way around a larger box, so it is worth considering I think
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Old 14th April 2010, 07:08 PM   #8
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As you are moving the same amount of air with an isobaric as a single driver, the physical output will be the same as a single driver. If winISD says it's 3db less then it's wrong

A pint can't become half a pint unless you drink some

Like you say Dr.EM, the amp will need to produce twice the power but in current. The output in DB will be the same as a single driver but it would need twice the enclosure volume.
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Old 14th April 2010, 07:23 PM   #9
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Must be the way it calculates it. Checking by putting the in power figures myself I get the same SPL from each configuration as one would expect. In this case, and probably most real bass applications, I put in 4 times the power to the Isobarik enclosure than the regular. Twice as much as it's 2 drivers and twice as much again as the smaller box lowers excursion (I assume this is the reason). Doing this creates identical SPL and Cone excursion curves.
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Old 14th April 2010, 07:33 PM   #10
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Originally Posted by Dr.EM View Post
Twice as much as it's 2 drivers and twice as much again as the smaller box lowers excursion (I assume this is the reason). Doing this creates identical SPL and Cone excursion curves.
Interesting. There might be a problem with winISD. Because the enclosure is half the internal volume (compared to a single driver) but the force factor is doubled it should cancel out. You wouldn't need to give any more voltage drive to give the same output as a single driver in a standard enclosure.

I know, i have built enough of the things since i started playing about with isobarics nigh on 15 years ago. I suggest you try modelling a single driver but with half the VAS & see if you get the same output in half the enclosure size, if you don't i'll be very surprised
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