Originally Posted by Samps
I've read that is reduces nonlinearities between the two drivers and reduces distortion. If a particular motor design has some form of irregularity on its inward stroke, the inverted driver will help cancel that out, and vis versa.
Not very common but M&K, TAD and even Cerwin-Vega made them. The main issue is one of the drivers is inverted which demands that the drivers are very quiet with little vent noise. You also are limited in what drivers you can do depending on impedance you desire.
I have a variant called Push-Pull Slot Loaded or PPSL subs. They are used in my garage because the slot is just additional box size and coupled with two 15's--they are some big boxes!
The main reason I built them was not distortion reduction but they don't vibrate which is important for my line arrays placed on top of them. The other point was the drivers are protected inside the slot from flying parts, people, drinks or freight moving around in my garage. I use two 15's in an 8.7 cubic foot enclosure tuned to 24Hz with the slot compression at 2.2:1. The battle continues over at DIY Audio about what compression ratio gains how many dB's in efficiency and so on.
I've been using mine for almost 6 years and like the protection of the drivers in the slot and they don't vibrate which is eerie when they shake the entire garage and house.
The efficiency gain, distortion reduction and increased cooling is just frosting on the cake. Never mind the comments of "you mounted one of your subs wrong" or "Arrays and goofy backwards subs--don't you build anything normal?"
My inhouse subs are not push-pull or PPSL--just 15's in traditional ported end table boxes. Since you have to use two drivers and either build something to protect the inverted driver or go PPSL, they are just too large for inhouse use. There are over 800 posts at DIY Audio forum about PPSL, very interesting and spirited discussion concerning that alignment.
A major reason is they look weird--you can run regular ported or sealed speakers with inverted drivers also--not common to see that! As far as push-pull goes, I've seen many horn designs that use that alignment but you can't see the drivers in the first place. All part of the compromises one makes when building subs--pick your poison.
If you have subwoofers behind a screen and have the space--going push-pull is a nice option and easy to change back if it scares the children. Push-pull and PPSL are tradtionally commonly used in infinite baffle subwoofers so they don't shake the walls and inside the plenumn for protection reasons. Not a good idea to have an inverted sub around kids, they have the habit of playing with wires so not kid friendly.