[NoDOHC]

Good call on the math error! I should have just given the original equation. Apparently the algebra is a bit rusty (I didn't think it looked right, but I couldn't find the formula solved for L in the book).
The natural frequency formula is correct though, I just checked it.

Anyway, I like where you are going with this. Something to remember on the Plenum is that the two rotors are 180 degrees out of phase and have a 270 degree intake duration. This means that both rotors take in air at the same time.

This is looking good!

There are more advanced formulas for varying cross-sectional-area runners, but I don't have the ambition to enter them into paintbrush and you can probably find them online anyway without any errors.

If you want to do the math yourself, it is very simple to draw it up as a dynamic system and then find characteristic equations for it. I say this because you are most likely taking a course in dynamic systems right now or in the near future.

The Plenum has compliance as it acts as an accumulator. The runner has a resistance (dissipative) and Reluctance (inertial) element to it. The port closing is the disturbance function. The system is a lot harder with changing area.

Anyway, you will find that the wave intensity is a logarithmic decaying function and that the time constant is related to the runner smoothness (resistance) and the air velocity (inertia). I can't find the equation right now, maybe it is in a book at work.

I will try to find it and let you do the Algebra.