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Old 23rd July 2012, 07:04 PM   #21
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Quote:
Originally Posted by adason View Post
I experimented with subwoofer with two 15" woofers on opposite sides, and heard big improvement in push-push (dipole) configuration in comparison to push-pull (bipole). Even decided to take some measurements.
keele line array, bad start...
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If the drivers were acoustically in phase then they are both bipoles irregardless of the orientation of the 2nd driver. A diple would have all the same bass issues as a single driver on an open baffle.

dave
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Old 23rd July 2012, 08:01 PM   #22
adason is offline adason  United States
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sorry, no idea what you are saying Dave, can you elaborate?
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Old 23rd July 2012, 08:20 PM   #23
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I'm saying that i don't think your use of dipole & bipole are correct.

In each case are the drivers both acoustically in phase?

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Old 23rd July 2012, 08:52 PM   #24
adason is offline adason  United States
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My subwoofer looks exactly as the plans of your "Twin SDX10 Push-Push Prism" subwoofer, that is one large box with two drivers, one on each opposite side.
There are only two ways to have it.
Bipole <- [box] ->
Dipole ->[box]->
Arrows indicate the woofers phase.

Last edited by adason; 23rd July 2012 at 08:55 PM.
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Old 23rd July 2012, 08:54 PM   #25
adason is offline adason  United States
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I do not change the orientation of the driver...
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Old 23rd July 2012, 09:01 PM   #26
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My experience matches this B&W whitepaper:
If your volume displacement requirement can be met by a single woofer up to 15" - 18", this is the best sounding solution. I have not built a PPSL 2-woofer counter-force design, but I guess the single slot offers the same coherent wavefront advantage plus higher efficiency and displacement.


B&W Whitepaper on Bass
Advantages of Producing a Coherent Wavefront

One remarkable fact was consistently noticed during the development of the new 380mm bass unit for the Nautilus™801, and that was that a single large and stiff bass cone always sounded better than a number of smaller cones, even though they may well have had the same aggregated properties. One possible explanation for this is the concept of the production of a “coherent wave front”. This will be produced by a single large very stiff cone, which can couple with the air in a uniform manner over the whole of its surface area unaffected by differences in loading over that area. This behaviour is to be compared with that of several cones which, even though they may be closely spaced, will still leave gaps of “uncoupled air” between them. The very stiff cone material of the large single driver, which is a thick sandwich of Kevlar® reinforced paper fibres with a very stiff skin, makes it less responsive to local changes of acoustic impedance or unbalanced modal pressures either behind or in front of the cone.

A “coherent wavefront” simply means there is either a constant or a smoothly changing phase relationship between neighbouring parts of the wavefront. So even if two drivers are relatively close together compared to a wavelength. Also, even if their contributions are equivalent to a single large driver, their different acoustic environments will mean that their outputs are slightly different, in terms of both amplitude and phase.

Furthermore, the air between the drive units is not being driven at all and this will translate into a change in phase across the resulting wavefront as the air tries to “fill in” the lost contribution. One can postulate that, at low frequencies, air can “spill off the edges” of the individual cones more easily in an array of small cones, which obviously have more edges for it to spill off, than from a single large one. For instance, two 12 inch drivers have a combined circumference of 1630mm whereas one 15 inch cone has a circumference of only 1037 mm. It is also interesting to note in this context that the radiation resistance and reactance at low frequencies of one 15 inch cone, is actually larger than that of two 12 inch cones, even though the area of the 15 inch cone is 0.02m2 smaller than two 12 inch cones. This is because the change from a steadily rising radiation resistance characteristic at low frequencies, to a horizontal one at high frequencies, occurs at a lower frequency with one large driver than with two smaller ones. Mutual radiation impedance effects will redress this imbalance to some extent, provided that the two individual drivers are close enough together for one driver to acoustically load the other, though it will only be totally redressed if the array of small drivers produces a totally contiguous surface in all directions.
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Old 23rd July 2012, 10:13 PM   #27
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Quote:
Originally Posted by adason View Post
My subwoofer looks exactly as the plans of your "Twin SDX10 Push-Push Prism" subwoofer, that is one large box with two drivers, one on each opposite side.
There are only two ways to have it.
Bipole <- [box] ->
Dipole ->[box]->
Arrows indicate the woofers phase.
Push-Pull is used to indicate one driver facing out, 1 facing in, acoustically wired in phase (electrically out of phase). A bipole as you have said.

My SDX10 PPP is also a bipole, drivers are wired in phase(electrically & acoustically -- ie both push out at the same time). Wired acoustically out of phase (dipole) you would have almost no bass.

Are your arrows indicating how you wired them acoustically or the orientation of the woofer cone?

dave
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Old 23rd July 2012, 10:15 PM   #28
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Quote:
Originally Posted by adason View Post
I do not change the orientation of the driver...
Then it isn't push-pull, it is a dipole, and you would expect huge difference because the bass would be cancelling.

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Old 23rd July 2012, 10:31 PM   #29
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Quote:
Originally Posted by LineSource View Post
My experience matches this B&W whitepaper:
Almost all of that is only relevant at higher frequencies than a sub works at. The 2 woofers would need to be > 1/4 wl c-c to start causing phase issues. At 100 hz, that is 33 inches.

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Old 23rd July 2012, 11:08 PM   #30
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Very interesting posts here and lots for me to digest!

Quote:
Originally Posted by planet10 View Post
I would not compliantly mount the woofers. It would reduce the push-push coupling (in most cases) leaving energy in the basket reducing the system's DDR.

dave
Even if I fixed the pair of drivers together, back-to-back, very securely probably at both the magnet structures and at the bolt holes in the drivers' frames (using part-threaded rods or similar)?


The B&W whitepaper on bass reproduction gives food for thought. The comparison though seems to be mainly between a single driver vs. multiple drivers with approximately the same (total) radiating areas.

I wonder if some of B&W's arguments wouldn't apply if the multiple driver sub had massively more combined diaphragm/cone area than the single driver sub? I can understand that 'air spill' from cones will increase with increasing cone circumference (or combined circumferences) but I'd guess that 'air spill' will also become more significant with increased cone excursion - something that's likely to happen with a single driver sub.

I take B&W's point about a coherent wave front and spaces between multiple drivers. I had thought of mounting the multiple drivers from inside the cabinet then chamfering the outside of each hole in the cabinet - sort of like the flare often seen at the room end of reflex ports (or to look at it another way, a very, very short exponential horn) - so that it matched up with the adjoining one. I could increase the cabinet wall thickness at these points to give me more wood to work with. Again though, I suspect the gaps between multiple drivers becomes less significant as cone excursions reduce (which happens when radiating area is very large).

Also, B&W's paper is about bass and sub-bass. I wonder if their comments would apply to sub 80Hz only?
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