Just a small question i cant find any Google answer to.

What excactly does it do to reverse mount one driver in a dual BR box enclosure?

I know it has something to do with elimination of 3th harmonic distortion.

But what else? Any other pros and cons on doing this?

We are talking a plain BR box with both drivers in the same chamber and on the same baffle. 

Since we are busting myths and false info here from now on:

So not. Not even remotely like that.

First and foremost: It depends on drivers used.

What push-pull does is reduce nonlinear distortion in the acoustical output of the said cabinet, excluding the port effects. Hence it depends on linearity of said drivers how much the effect ends up being. With lower quality (cheap) drivers the effect may be up to 50% reduction on both second and third harmonic distortion, it's not always like that.

Biggest reason for nonlinear distortion is suspension stiffness asymmetry and second is inductance variation. 3rd thing is the actual BL, force factor, variation depending on which direction the cone is moving. This is caused by the plateset assembly and non symmetrical structure of it.

As we can see on the pic below, there is a slight variation on Kms, the suspension stiffness and barely any change on BL. But Inductance varies quite a lot, which indicates that there is no shorting rings on this driver and thus doing a push-pull may have a slight benefit as it would make Inductance symmetrical. But on BL or Kms there is barely any benefit. So effect wouldn't be anywhere near 50% reduction.

When using a driver with aluminium or copper shorting rings, they already stabilize the inductance variations and thus push-pull would make even less sense. But on cheap driver where you don't have shorting rings, got BL symmetry issues and your suspension isn't symmetrical either, benefits can be huge.

And that's why you see / don't see it that much in the modern world as it's not always deemed necessary, unlike the olden days when drivers weren't as well engineered as they are nowadays.

And effect is very audible on drivers which actually benefit from the push-pull.

Drawback on push-pull is that spider flexing noise and cooling air noise can be severily audible from the driver with magnet pointing out. For example certain driver range from well known European manufacturer uses specific cooling method where air is compressed inside the plateset on a backward motion of the cone causing very loud bangs and noise even at low excursion rendering those drivers unuseable on push-pull design.

Edit: On the left and middle graph on both rows, right side is coil out = cone forwards and left is coil in = cone backwards.

edit: whoops, misread it as bp, not br, so forget that there front chamber

One would think, wouldn't he?

Well, as Qes calculation has BL squared as divider, your 5% just became 10%. 30x30=900, 31,5x31,5=992.5

It is pretty typical chinese driver. First one originates from Mediterranean area :)

Really Nice with such great answers. Thats going right into the wisdom vault 👌