Driving an open baffle opposing-driver sub-woofer

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#22
aagas said:
Background

See attached 2 (rather dusty) photos of an old open baffle opposing-driver sub-woofer project, I want to resurrect.

It uses:
  • a pair of 15" PrueAudioProject woofers, which PureAudioProject had Eminence optimize for open baffle.
  • spacers to connect the drivers
  • wrought iron table legs from which to hang the drivers
  • rubber tie downs as the suspension
In the past I drove this speaker with a stereo amp, one channel (same signal) to each driver.

Of note, in this kind of speaker, one needs to connect the drivers out-of-phase.

It worked great and matched very well with my large aluminum honeycomb DML main stereo speakers.

Next step

I have all the parts to construct a second one of these.
I don't have an extra stereo amplifier.

Key specifications of the drivers:

View attachment 1367434

Questions

I have a stereo amp: Hypex NCore 2x125W 4 Ohm
I used this amp to drive this sub in the past (one channel to each driver)

Can I use this amp to drive two of these open baffle opposing-driver sub-woofers (each has 2 drivers)?
If yes, how do I wire them to the drivers (especially given I need each subs drivers driven out of phase).

Alternatively, I can buy another Hypex NCore 2x125W 4 Ohm, but I'd rather not if I don't have to.

Thoughts appreciated.
Click to expand...

The specs for your amp modules can be found here:
https://www.hypex.nl/products/amplifier-families/mains-powered-ncore-family/nc252mp-oem

You have two amp channels, and each driver is 8 Ohms nominal. If you want to be able to have stereo operation, you have to use one channel "per side" and there are two drivers per side. The solution is to just wire the drivers in parallel, taking care to reverse the phase of one of them so both cones move in the same direction. This sort of driver arrangement is called a nude compound dipole. There is a whole thread on this somewhere in these forums.

The more baffle you use, the less LF losses. Usually the nude configuration is used when you want to operate the drivers as high in frequency as possible. In any case it is a relatively small and compact dipole system and with the proper equalization they can sound very nice, albeit at only modest maximum SPL levels.
 
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#23
This opposing nude woofers arrangement has bothered me for many years.... I really don't see any bebefit in it. Perhaps during the end of this week I can make a proto with two leftover 8" that lie around and measure it. Opposed woofers in a box is simple to understand.

Another mystery to me is woofer "slot loading". My guts say that it boosts only in extreme nearfield...
 
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#26
John knows better (with math skills!), but isobaric basically doubles the motor force to produce same spl. As well some nonlinearities in motor T/S won't become so prominent. Still same excursion is needed for similar spl. connecting parallel or serial affects impedance which has secondary effects to motor/amplifier co-operation.

Isobaric box was used more often before woofer "motors" bot better and amplifier wattage cheaper.

Nude opposing drivers with open cavity between cones is something that I don't understand "why". In-phase is waste of resources with modern drivers IMO. Opposite phase creates a quadrupole which is extremely different animal - backside of cones radiate haphazardly and cavity is pumping air in-between. Frequency/wavelenth change creates many interferences but different summation/cancellations...

https://www.acs.psu.edu/drussell/demos/rad2/mdq.html


https://www.acs.psu.edu/drussell/demos/rad2/latquad.gif
 
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#27
Juhazi said:
Nude opposing drivers with open cavity between cones is something that I don't understand "why". In-phase is waste of resources with modern drivers IMO.
Click to expand...

I know what you mean, and I've never actually tried it. But I might, one day, when I have time.

One argument in favour of nude OB drivers is that it maximises radiating area, for a given baffle size, which optimises the radiation pattern both up to and beyond the dipole peak. And if you're going to use nude drivers, for low frequencies, it makes sense to use big ones, in multiples. Arranging them one behind the other keeps everything compact. (And at low frequencies, the distance between the drivers isn't an issue, because wavlengths are so long.)
 
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#31
It's not a matter of believing something without question. There's reasoning and evidence in the links people have provided. If you think everyone else is wrong, maybe try explaining where they've gone wrong, why the measurements people have done are wrong etc.

I posted on the thread to answer the OP's questions, and then to respond to your questions. I'm trying to be helpful, with what (limited) understanding I have, so I'm not sure why you're having a go at me.
 
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#32
Offtopic - for Nico

Please search studies of Mellow&Kärkkäinen - driver vs baffle ratio
https://acris.aalto.fi/ws/portalfil...low_On_the_scattering_of_disk_source_JASA.pdf

12inch driver nude vs baffle dipole.pngbaffle loss 200 2000 20000mm edge.png
 
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#35
Actually the link I gave was not the one I meant (I'm Juha, not Tim ar Leo), sorry. Anyone can ask help from Tim https://www.frontro.co.uk/mellow-acoustics

The right one with pics is here https://www.semanticscholar.org/paper/On-the-sound-field-of-an-oscillating-disk-in-a-open-Mellow-Kärkkäinen/460d1b61e116d50ad3ed136152d8176e9984c28e

1728972189317.png


A cone or dome make deviations from ideal flat pistonic model. They can ce calculated too... Here is a peek to that issue by W. Klippel

https://www.klippel.de/fileadmin/_migrated/content_uploads/KLIPPEL_Sound_Radiation_Poster_01.pdf

Edge sim of a nude woofer
12inch nude ob onax edge.png15 dipole nude offaxis edge.jpg
 
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#37
So Ian is this the conclusion:
The whole arrangement does not strike me as a very effective use of a second driver and cabinet space compared to an H or W frame. I have no data how high in frequency the "compound woofer"can be used, but its radiation pattern will become more lobing than that of the two point source woofer.

Or rather Siegfried does not know. I can not recall him as having answers to everything while working with him. He was just another DIYaudio enthusiast.
So in effect this new arrangement is just another nebulous idea from a kitchen table scientist thinking he hit the jackpot
 
#40
I have met Siegfried Linkwitz once in 2013, a very nice gentleman was he! His basic science was allpied mathematics, in microchip world. His personal interest was in loudspeakers evolved to many wonderful things like Linkwitz-Riley crossovers and dipole loudspeaker development. His was open to other's ideas and adapted them after digesting all his life. Eg. His early dipole speakers had constant frame width, but after realizing baffle width effect to off-axis radiation he designed his LX521. Best of all he was systematic and wanted to share his learned skills to everyone via his homepage www.linkwitzlab and by numerous presentations at events and open house at his home.

John Kreskovsky (https://www.diyaudio.com/community/members/john-k.15987/#about) was even ahead of him in dipole design and still contributes at diyaudio sometimes. His homepage is still available at https://musicanddesign.speakerdesign.net/products.html

My third dipole guru is Rudolf Finke http://www.dipolplus.de/ https://www.diyaudio.com/community/members/rudolf.4207/#about
 
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