The main difference would be when the driver has a vented cooling system, where there is a small port you may see through the middle of the magnet. Other than that, if it is a sealed magnet, the difference is too minimal to notice audibly, even more with the right acoustical pressure, you could technically have the wall touching the back of the driver without any acoustical losses, but dont do that, because the baffle, with enough voltage to the driver, will flex and likely cause noise from the magnet hitting the port/wall structure, so, any amount of depth is ok with a sealed magnet, but a ported magnet, allow about 3" average to be safe for any driver. There can be a calculated figure for the pressure losses/gains at that single point and the noise it may cause, but not needed. Just use 3" to be safe with any amount of power/driver.
