[Paul Spencer]
What is push pull driver mounting?
It's a way of mounting pairs of bass drivers which provides lower distortion. In a monopole, the second driver has the magnet facing out and the polarity is revered so + becomes - on the input wires and vice versa.
Over what range is it useful?
It is useful for bass where the magnet and basket assembly of a driver has no significant impact on the sound. In the midrange this method can't be used, since the magnet and basket create an acoustic shadow. Around 120 Hz is probably the upper limit of usefulness for this mounting method.
Free shorting rings!
If you have two identical drivers, then you effectively get "free shorting rings." A similar result is achieved, however this method may be considered more effective for bass since it is done mechanically. Asymmetries caused by flux modulation are cancelled. The result is lower 2nd order hamnonic distortion.
Easier to drive
According to the
[ACI subwoofer manual], push pull drivers are an easier load to drive since resistance and inductance are more stable.
The concept is like those manually driven railway karts. Two sit on each side facing the other two with a level in the middle. While two pull the level towards each other, the other two push, and this moves the kart. Due to the mechanics of the motion, those pulling would be stronger, but since they are operating in push-pull mode, the total driving force is made more even.
Adire Tempest example
Consider some of the earlier versions of the Adire Tempest 15" driver (models up until around 2003). In Dumax measurements it has an Xmag and Xsus of 18mm. Xmag is the point where BL drops to 71% and Xsus is where 25% of resting Cms is reached.
If you look at the chart you will see in both the suspension and BL curve, the outward and inward stroke are not the same. Xmag in the outer direction is over 18mm and in the inward direction (towards magnet) is less, but 18mm is the average. This is more marked in the suspension. Xsus in the outward direction is about 22mm but only 14mm in the inward direction. If you look at Kms at xmax, Kms is around 13 on the inward stroke, but less than 4 on the outward stroke!
Using a second push-pull mounted driver will cancel these asymmetries both in BL and suspension restoring force and thus lower distortion.
Applications
Consider push pull wherever you have two or more drivers which will integrate well when placed together. If you have only two drivers, you may find a better integration when located in different places in your room. This mounting is especially useful for infinite baffle, where four or more drivers are common.
I have found push pull results in a subjectively tigher sound. Mounting my 2nd run AE speakers AV12 drivers in this way, I was surprised how much of an improvement it was. Current versions of the AV12 now have two shorting rings, and may benefit less from push pull.
Examples
[TL subwoofer]Chris comments about his use of push pull mounting:
"A variation I almost didn't try was mounting one of the woofers backwards (with the magnet facing out) and then wire the drivers out of phase. The theory is that woofer cone movement is not perfectly linear. In response to a symetrical sine wave the outward motion of the cone does not match the inward movement. By mounting the drivers front to back and wiring out of phase the non-linearities cancel out. I figured that with a high quality driver and limited excursion that the cancelation effects would not be audible. Boy was I ever wrong! The bass was notably cleaner and controlled with lots of slam. Subjectively there seemed to be a little less bass which is a clear indication of less distortion. After hearing the difference I would never consider any other topology than out-of-phase push-pull. It is more than a little odd looking but it fits with my non-conventional theme."
Note: Push pull is not out of phase. If one driver were mounted magnet out, then this would be true, and they would behave as a dipole. Reversing polarity as well means that both cones move in to compress the air in the box, then out together. Clearly the writer understands this, but I make this point to avoid confusion.
[Pro Sound Web forum discussion of push pull mounting for Wayne Parham's 12pi bass horn] Note Wayne has used push pull mounting since Eminence indicated the cost of adding a shorting ring effective at bass frequencies to the Lab12 driverwas prohibitive.
Wayne comments that push pull cancels asymmetries caused by flux modulation.
[Linkwitz Lab] Both Phoenix and Orion woofers are push-pull mounted
