This is a small hack to stack multiple ventilator together. I tried this some time ago but the effect was disappointing. My guess is this was the case because the air after a ventilator is pretty turbulent. So in this design I added some guides to make the air more colaminar. This setup works way better and is very silent.
BTW: I also tried a 100mm ventilator normaly used in the shower but this was even worse and just produced a lot of noise.
The thin ring can be removed, it is just necessary to prevent the small fins from warping