Hi Hoiman
Here's the formula for figuring out how thick an absorber has to be (even at 250Hz) before it can affect (absorb) a given frequency. As you can see at 35Hz you are talking major dimensions.
Reflection Time Delay Formula:
First Null frequency = 1/ (2x TimeDelay)
Example:
2 ms reflection = 0.002 second reflection
First Null frequency = 1/ (2 x 0.002) = 250 Hz,
Then each 500 Hz above (250x2=500), 750,1250, 1750 etc...
AbsorberThickness Formula:
1130/ (4 x First Null frequency)
Example:
For 250 Hz absorption:
1130 / (4 x 250) = 1.13 ft thick.