Passive radiator questions

Tom_S

I have been revisiting some of my old projects - trying to improve a few where I just didn't do them justice the first time around. I've just started gluing up properly sized boxes for the TCP115-4. A 1 3/8" port should fit in the box with an elbow. But the velocity is going to be too high according to WinISD. Of course larger diameter ports would be longer than the height of the boxes. But I do have a pair of ND140-PRs that I tried with the previous TCP115 boxes I built. Those boxes were the wrong tuning for that woofer, but I noted the passives bottomed out really early. The displacement of the ND140-PR is over 3 times that of the TCP115, but their FS (44Hz) is lower than my desired box tuning of around 55Hz. Is this just a combination that won't work together or does the box volume somehow change the PR FS?

jr@mac

The problem with most of the PR available us out of the box, they already tune too low. 

Tom_S

I wonder if I can dremel off the weight mounting stud and raise the FS of these things enough to work.

DanP

The PR resonant frequency is not necessarily the frequency it will tune a box to.  You'd have to model it to see the exact effects, but in addition to lowering the tuning frequency by adding mass, you can raise the tuning frequency by reducing cabinet volume (which of course has other effects) or increasing the number of PRs (equivalent to enlarging port diameter).

jr@mac

You can also raise tuning by adding additional radiators. As it stands though, most PR simply tune too low for their matching drivers - think the Dayton SD/DS series. Frustrating getting a good low end model out of them in many cases. 

DanP

Yeah, I've noticed that if I'm trying to use a single PR per woofer, but I almost always use a 2:1 ratio, so it hasn't been an issue for me.  It is kind of a bummer that they can't just start higher though.