# Riser as bass absorber



## mpednault (Dec 20, 2012)

How do I calculate the size of the inlet openings for the front of the riser?

My riser will be constructed out of 2x8's at the perimeter with the middle "joists" being 2x6's spaced at 12" on center. I'll fill the cavities with 6" paperfaced fiberglass batts "hung" from the top of the framing.

Can I just cut holes in the front framing member or will I need to install PVC pipe to attenuate the right frequencies? If I need to, how do I calculate the correct diameter and length of pipe?


----------



## bpape (Sep 14, 2006)

Just cut large irregular shaped holes so it doesn't even thing about tuning. The carpet on the front covering the hole is going to be high absorbing anyway so no point in trying to make it not absorb highs.


----------



## mpednault (Dec 20, 2012)

When you say "irregular" do you mean varying diameters or varying shapes? What's the smallest I should go with? I've got a 1" and a 3" hole saw. Won't a 1" hole not be big enough to be worth it? I have a jig-saw too if I need to cut bigger holes... So if I put a 3" hole between each "joist" will that be effective at absorbing the LFE's? Or should I vary the sizes?

FYI: My riser is only 48" deep, 96" wide and 8" tall. If I subtract out all the wood framing, that leaves me with ~16 cu. ft. of air space, with 13.425 cu. ft. of that filled with fiberglass batts.


----------



## bpape (Sep 14, 2006)

Do the 3". I said irregular as I don't want it to act like a port. You can just cut a little out of the diameter in a couple places to make it 'not round'


----------



## mpednault (Dec 20, 2012)

Perfect. Thanks!


----------



## Tonto (Jun 30, 2007)

Remember to cut holes at the back/top of each section to allow sound out as well.


----------



## mpednault (Dec 20, 2012)

Ah yes, thanks for the reminder.


----------

