I wasn't pulling the optimal tuning frequency out of the air. You said the tuning frequnency would be 45hz at 7.3" long port. My question is can I run a hpf that is 50hz thru my mini DSP to the MBM even though its tuned at 45hz?. I don't want it bottoming out.
I did not say Fb, which you call the tuning frequency, is 45 Hz, it is 38 Hz. So this number has nothing to do with the response per se, or where you set your crossovers. It is the box air cavity resonance. Normally it lies pretty much between the two peaks of impedance in a ported enclosure.
The point is that your speaker will be 6 db down at 45 Hz. So this makes it the optimal place to put an LR crossover.
It will not make a whit of difference whether you actually put the crossover at 45 or 50 Hz. You will NOT hear the difference. The real point is that this is the optimal box for what you are trying to do.
However as I said before this whole concept of mid bass modules is total nonsense in my view. If you do do it you want to make it a composite fourth order LR. So that as I told you would be electrical second order LR high pass and fourth order LR low pass at 45 to 50 Hz range.
The whole problem with all this is that because of the overlap of crossovers you get what is called band pass gain. Crossover points should be at least two octaves apart, and preferably more. So that means you need to cross those mid bass units over to your mains no lower than 180 Hz. At that frequency you will localize them, and so they will need to be right up against your main speakers.
The main point is that good speaker design keeps the number of crossovers to the minimum. A good full range pair of single driver speakers can sound incredibly good.
Every crossover added is a quality spoiler.
So any decent sub should be able to handle up to 80 Hz without a problem. Any decent set of mains should be highly capable of handling above 80 Hz at power. If either fail in that regard they are just bad speakers and need thowing out.
So the wise designer finds ways to eliminate crossovers and NOT add them.
