I’m wondering if there’s a significant difference in the measured frequency response (40Hz to 120Hz) of an isobaric bandpass woofer (3-way) when the drivers are front-mounted face-to-face versus face-to-magnet. My intuition suggests that at these low frequencies, the difference will be negligible. I suspect that factors like enclosure volume and the quality of the seal will have a far greater impact on the sound than which side of the cone is facing forward. The latter might even favor a cone-to-cone configuration.
Thoughts
Since air acts as a spring this prevents a perfect coupling between the two cones. However, the smaller the air volume, the “stiffer” that spring is and closer it gets to “ideal” mechanical coupling. How much difference one volume makes to the other.. IDK. There is also the difference in mass of air contributing to moving mass. For bandpass, cone to cone is likely the easier option to incorporate. Then you get potential above benefits along with potential reduced distortion from averaging out surround shape flexing and non liniarities in inward vs outward cone movements.
It also depends on the driver, in that some are mechanically noisier than others as well as how much turbulence is created by the frame as the air moves past. In general there won’t be any substantial difference if mounted front to front or back to back.