Just wondering if I need to cut some 4”x10” holes on the top of my riser (covered by registers) or not.

My riser facts:
-Riser is about 18 inches high, by 11 feet wide (width of room) by five feet long
-Riser is in the back 1/3 rd of the room from wall to wall but decoupled from any wall
-Riser is lifted off ground by about 1 ½ inches by rubber feet
-Have two butt kickers in riser
-Riser is filled with Roxul Safe n Sound insulation
-Riser is enclosed on all four sides and top by ¾” plywood
-Bottom of riser is somewhat open - I layed 2x4’s flat at the bottom part of the riser leaving about 4 inch spacing between pieces. Plastic and mesh was layed over the flat 2x4 to hold the insulation off the ground.

Is my opening of about 1 ½ inches at bottom of riser enough for effective broad band bass trapping or do I need to add a bunch of 4"x10" holes near corners and back wall (covered by registers) on the top of the riser?

Thanks
Frank

See two pictures attached:
1. First shows riser covered by 3/4 inch plywood. Although the pictures shows the top centre part of the riser with a very large hole I did cover that with 3/4 inch plywooed too. The centre is where the butt kickers are placed.

2. Second picture just shows the bottom of the riser before insulation to give you an idea of the spacing between the flat 2x4.

Short answer is that unless you know how to do the math so to ensure that the trap is trapping exactly what you want/need, is that you will likely need to pay someone to do the design work for you. I haven't seen anyone but the pro who hang out here know how to do and they don't just give this service away for free.

Short answer is that unless you know how to do the math so to ensure that the trap is trapping exactly what you want/need, is that you will likely need to pay someone to do the design work for you. I haven't seen anyone but the pro who hang out here know how to do and they don't just give this service away for free.

Not looking for specific frequency trapping. I just want to determine if I already have enough access with the existing gap at the bottom to have an effective broadband bass trap or if I need to place additional holes at the top of the riser.

It can be done; but, not by drilling a bunch of holes in the decking. Plan on removing the existing decking to modify your framing a tad.

Short answer is that unless you know how to do the math so to ensure that the trap is trapping exactly what you want/need, is that you will likely need to pay someone to do the design work for you. I haven't seen anyone but the pro who hang out here know how to do and they don't just give this service away for free.

I am thinking that a little searching might find your results you seek. I have seen the topic discussed a few times. To what extent I can not remember, but I bet the info you desire is here somewhere. ;)

OK... Go to the Forum Jump drop down arrow and scroll up a bit and select Home Entertainment and Theater Builder so you will bring up all four sub forums that may contain info on using the riser as a bass trap.

I just did a little search using Riser Bass Trap and came up with almost two full pages of info that could help you.
Hope this helps!
Enjoy ;)

It can be done; but, not by drilling a bunch of holes in the decking. Plan on removing the existing decking to modify your framing a tad.

Thanks Dennis.

I am not too interested in any major modifications. At most I wanted to just add some 4"x10" register on the top.

In my first post on top, from the first picture you can kind of see the 1 1/2 inch gap that goes all the way around the bottom of the riser (riser is lifted by 1 1/2 inch rubber feet) . Currently that 1 1/2 inch gap is my only access point to the insulation inside the riser (Insulation in riser is suspended off the floor by about 3 inches ie. gap on floor (1 1/2 inches) plus flat 2x4 (1 1/2 inches) attached to bottom of riser and then insulation).

OK... Go to the Forum Jump drop down arrow and scroll up a bit and select Home Entertainment and Theater Builder so you will bring up all four sub forums that may contain info on using the riser as a bass trap.

I just did a little search using Riser Bass Trap and came up with almost two full pages of info that could help you.
Hope this helps!
Enjoy ;)

I previously did the search and read the responses. I did not read about anything that answered my specific question.

hey frank im digging up this old thread because i was looking at doing the exact same thing you did and was wondering how yours worked...

hey frank im digging up this old thread because i was looking at doing the exact same thing you did and was wondering how yours worked...

Never ended up adding the registers. I originally had the 1 3/4" gap all along the bottom so I just left it like that. I did end up adding about 12 bass traps (6" thick x 4' x 2') in various corners of the room. I bought base trap cases from Ready Accoustics and some rigid insulation locally to make the traps.

Look here in Ethan Winers forum, bottom line is you needs lots of surface area, i.e., lots-lots of vents:
Home Theatre:Riser/other Broadband bass traps (http://forums.musicplayer.com/ubbthreads.php/topics/2032618/Home_Theatre_Riser_other_Broad#Post2032618)

So does only the top count for surface area? If you have the riser elevated, the whole bottom surface is exposed to air current. 8x4= 32sq ft..

From what you are trying to accomplish, make a broadband bass trap, but giving it limited surface area (thin 1 3/4" gap around the perimeter of your riser, not 32 sq ft) won't let it really affect bass as a broadband bass absorber should - that's why Ethan suggested I put in as many 4 x 14 vents as I did for my just re-did riser.
IMHO, from what I've read, yours will be tuned to some freq and narrowly absorb around that?

I'm curious if Frank D took some REW measurements and could "cover the gap" and take another one to see his end result, would be nice to see for common knowledge sharing.

Look here:
Ethan Winer acoustic forum READ THIS FIRST (PLEASE!) (http://forums.musicplayer.com/ubbthreads.php/topics/1651343/READ_THIS_FIRST_PLEASE#Post1651343)

Many people ask about the difference between the wood panel traps shown in my article Build a Better Bass Trap (http://www.ethanwiner.com/basstrap.html) and rigid fiberglass panels mounted straddling the corners. In my opinion wood panel traps are great in larger rooms, but are less appropriate for rooms the size of a bedroom.

The great thing about panel traps is they absorb well at low frequencies without impinging very much into the room. The less than great thing is they are tuned, and absorb well over a range of about an octave only. So for a complete treatment solution you need at least two types: low-bass and high-bass. But that means you need twice as many traps altogether, since each trap absorbs only half the range. In contrast, bass traps based on rigid fiberglass absorb a much wider range, so treating all the corners puts full-range absorption in every corner.

As I see it, the best way to treat a small room - say, less than 15 by 20 feet - is to use broadband absorption such as rigid fiberglass panels in all the corners, including the ceiling corners. This is usually enough to make a very big improvement. But if more trapping is still desired you can then put wood panel traps on the walls alternating the low-bass and high-bass types. For very small rooms, smaller than 10 by 16 feet, you'll do fine using rigid fiberglass only.

From what you are trying to accomplish, make a broadband bass trap, but giving it limited surface area (thin 1 3/4" gap around the perimeter of your riser, not 32 sq ft) won't let it really affect bass as a broadband bass absorber should - that's why Ethan suggested I put in as many 4 x 14 vents as I did for my just re-did riser.
IMHO, from what I've read, yours will be tuned to some freq and narrowly absorb around that?

I'm curious if Frank D took some REW measurements and could "cover the gap" and take another one to see his end result, would be nice to see for common knowledge sharing.

Look here:
Ethan Winer acoustic forum READ THIS FIRST (PLEASE!) (http://forums.musicplayer.com/ubbthreads.php/topics/1651343/READ_THIS_FIRST_PLEASE#Post1651343)

Probably too hard for me to cover the vents all around at this point as I have several other bass traps nudged in the back and have carpeted my riser etc.

Saw your post over at Ethan's forum and it definitely looks like you achieved some benefit out of adding those registers. Generally from Ethan's response to you in your post on Ethan’s forum he says the more registers the better. That makes sense since you are exposing more surface area as you are trying to make a broad band bass absorber. However since I had already carpeted etc. and I was planning on adding 12 corner type bass traps then the marginal returns to me would have probably been minor had I done the work to add the registers.