I think you're conflating a bunch of different things here when you use the term "push pull".
To most people, the term "push-pull" would suggest that the drivers are out of phase (when one moves in the other moves out).
If you were to build a subwoofer that was wired or driven that way you would get almost no bass output at all.
(The wavelength of low bass frequencies is several feet - most of the air would simply blow around the box from one side to the other and back again.
Subwoofers with a pair of identical drivers on opposite sides, which are generally referred to as "force cancelling", have both drivers run
IN PHASE.
They both move out at the same time, and in at the same time; as far as air movement is concerned they are bipoles.
Vibration is largely caused by the fact that, as the mass of the cone is pushed forward, the motor assembly pushes backwards on the cabinet, causing it to move slightly backwards (and vice versa).
By arranging the two drivers to be symmetrically opposite each other, this force cancels out for the two drivers, resulting in less net vibration.
The M&K designs have a different basis (I had one long ago).
All drivers produce some amount of distortion... some of which may be due to the fact that the motor assembly and suspension may have some degree of asymmetry.
In the M&K subs, one woofer is mounted the normal way, while the other is mounted "backwards"; the idea is that any nonlinearities in either driver cancel those in the other one.
HOWEVER, the backwards driver is also electrically inverted; as a result both are actually operating in phase (both move outwards at the same time and inwards at the same time - relative to the inside and outside of the cabinet).
(I see M&K uses the term "push-pull", which is analogous to how the term is used in tube amps, and actually somewhat explanatory... but which can be confusing.
Both drivers are in fact pushing air away from the cabinet at the same time, and pulling air towards the cabinet at the same time.... so they are in fact operating in-phase.
If it makes the whole thing easier, think of it that the two
CONES are operating in phase, but the basket and magnet on one are flipped around.)
A passive radiator is directly analogous to a port..... the cone is the mathematical equivalent of a very large diameter very long port.
Therefore, at the tuned frequency, the movement of the passive radiator is in phase with the active driver, and so their outputs add.
However, at other frequencies they are
NOT in phase, and the passive radiator is subject to other forces which complicate matters (like the spring force from the surround and spider).
Note that, just like with a port, the passive radiator serves
TWO purposes:
- At certain frequencies it contributes output
- At certain frequencies it serves to control the movement of the active cone (to limit excessive cone excursion)
The main reason subs with passive radiators become excessively boomy is much the same as why some ported subs become boomy....
(because they're tuned very sharply, then not very well damped, to enable them to deliver a lot of bass at the tuned frequency - at the expense of a smooth or well-damped response)
Push-pull designs are excellent for vibration cancellation. Take a look at M&K pro subwoofer designs. As far as having opposing passive radiators on any system, the down side is the group delay. If all you want is a loud boom, like in car audio, then maybe you will be satisfied with that sort of design, but in the home the combined effect of two passives 180 degrees out from each other will mean your room will ring and you'll likely have so much overhang that the bass will turn from notes or sounds into one big drone. I don't even want to think about the room mode activation that would likely occur. The only PR based subwoofer system I have ever thought sounded musical and not boomy was made by Salk but required very careful setup to make it sound good in room.
If you really want to use four drivers and two PRs, my suggestion is to either build four small cube single driver systems to place around your room, or two push-pull systems to set up in a pseudo-stereo configuration, and sell the PRs to someone else.
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I have been thinking about a cube. But i was thinking 4 active and two passive. Don't kbow if this is actually a good idea though.
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