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Helmholtz riser questions

post #1 of 10
5/7/10 at 9:42pm
Thread Starter 
I've build an 8' x 4' x 12" riser similar in design to the one here:

http://www.bobgolds.com/HelmholzRiser.GIF

The surrounding frame is made from 2 x 12's. Internally, I've created one large partition of 4' x 4' and two smaller ones of 4' x 2'. The base is 1/2" plywood and top is 3/4" hardwood plywood. All inside compartments are sealed with Liquid Nails.

The room is 13' x 23' x 8' ceilings, with the riser positioned with the long side parallel to a short wall about 2" away.

Some questions about this arrangement:

1) Does it make any sense to turn this riser into a Hemholtz resonator? If so, what size holes should I make to the front of each partition? Is tubing required, or will the holes alone be sufficient? I'm not as versed in acoustic theory as I'd like to be, I just want to verify I'm looking in the right direction before I dive in too far.

2) What sort of insulation is appropriate for filler? The 8-pound Rockwool I used for my bass traps would be very expensive for this volume. Will pink R-13 or similar suffice?

3) How else might I use this riser as a bass trap if the Helmholtz idea is not appropriate?

Thanks in advance for your thoughts.
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post #2 of 10
5/7/10 at 10:58pm
Hi,

Unless you know the specific low frequency response of your room, I would avoid making your riser into a Hemoltz resonator. Hemholtz resonators are tuned to specific frequencies based upon cavity opening sizes and depth, and can be useful IF you know which frequency specific to your room to tune for.

Just having a big cavity filled with insulation sitting in your room is enough to call it a base trap. However, be aware that not having openings of any kind can also cause your riser to be a VERY effective resonator. Also, depending on the cavity size, certain frequencies of 1/4 wavelength could be effectively absorbed too much and be the cause of a suck out in the room. In effect, very similar to a null, but it is not a modal issue. Not knowing the difference could cause you to chase your tail so to speak. Best wishes!
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post #3 of 10
5/8/10 at 9:26am
Thread Starter 
I was hoping to put off room measurement for a little while. I though perhaps the effective range of an HR might be wide enough to encompass a room mode, but I suppose not. What would you do with this riser in the interim?

If I seal it, screw down the top, and drill holes, I end up with an HR. If I seal it but don't screw the top, I have a membrane absorber (right?). If I open the entire front to expose the fiberglass, I have a broadband absorber, though with an area of only 8 sq ft.

What if I screw down the top and do nothing else? Would that sealed (and fiberglass-filled) box have a negative impact on the room?

As well, for a carpeted room, what percentage of the room's wall area should be covered with broadband absorption traps?

I have nine 2'x'4'x4" traps on my walls. Total wall and ceiling area, not including floors, is around 425 sq ft. My traps cover 56 sq ft, or about 13%. The room still sounds like it's resonating, but I need a number to shoot for before I buy material for more traps so I don't end up dampening the life out of it. The plan right now is to buy 12 more 2x4'x2" sheets and cover most of the ceiling with them.

Thanks again for you help.
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post #4 of 10
5/8/10 at 10:16am
Quote:
Originally Posted by KnightRT View Post

I was hoping to put off room measurement for a little while. I though perhaps the effective range of an HR might be wide enough to encompass a room mode, but I suppose not.

The at the very least calculate the modes

What would you do with this riser in the interim?

Cut some holes in each cavity

If I seal it, screw down the top, and drill holes, I end up with an HR.

Not necessarily. Remember that the HR tuned frequency is a function of opening size and depth of that opening

If I seal it but don't screw the top, I have a membrane absorber (right?). If I open the entire front to expose the fiberglass, I have a broadband absorber, though with an area of only 8 sq ft.

You have a volume of the entire riser as long as it filled with fiberglass. Not just the opening square area.



What if I screw down the top and do nothing else? Would that sealed (and fiberglass-filled) box have a negative impact on the room?

Maybe...don't know until it is measured but it is a risky proposition

As well, for a carpeted room, what percentage of the room's wall area should be covered with broadband absorption traps?

Define traps? There are a number of different methods to treating a room. Broadband absorption is but one element. Coverage and type are a function of the room design.

I have nine 2'x'4'x4" traps on my walls. Total wall and ceiling area, not including floors, is around 425 sq ft. My traps cover 56 sq ft, or about 13%. The room still sounds like it's resonating, but I need a number to shoot for before I buy material for more traps so I don't end up dampening the life out of it. The plan right now is to buy 12 more 2x4'x2" sheets and cover most of the ceiling with them.

If your room is resonating, or muddy in the low frequencies, then you have other problems that fuzzy stuff on the walls will not deal with. There is more to calibrating a room for acoustics. Design, treatment and equalization is the game plan for all rooms.


Thanks again for you help.

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post #5 of 10
5/8/10 at 11:25am
Thread Starter 
So we're not speaking in abstracts, here's the room as it appears now:





The subwoofer positioning isn't finalized yet. The mains and center are close to where they'll be to stay at the right angle to the absorption panels on the walls.

These are my calculated room modes, though they don't account for the asymmetrical soffit or the three-foot extension behind the screen:

24
43
49
70
73
86
97
122
129
140
146
170
172
195
210
215
219
243
257
280
300
343
350
386
420
429
490
560
630
700

The traps I'm talking about are just 8-pound mineral wool in wooden frame, 4" deep, with acoustically transparent cloth and no backing.

I'm limited to what I can do with the structure of the room. I hope it's not so willowy that this acoustic treatment is a fruitless exercise. FWIW, the floor under the carpet is concrete.
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post #6 of 10
5/8/10 at 11:46am
Hi,

So what you are showing me is that the screen is dictating how this room is laid out? Do you have a plan, or are you just throwing in a riser? How many rows will you have? In the current configuration, it is unlikely the second row will have a very good sound presentation. What do you plan to do with those surrounds? They seem awfully far back. Are they dipoles? Direct radiators? Where will the seats go...etc., etc. Best wishes!
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post #7 of 10
5/8/10 at 12:01pm
Thread Starter 
There will be four theater seats bolted to the riser for a total of one additional, eight-foot-wide row. The room layout was preplanned. It's a compromise of a variety of factors. The screen, projector offset, physical space, sightlines, door clearance, construction costs, and feasibility were all issues. Since the riser weighs the better part of 200 lbs, I assure you, no one is throwing it anywhere. I can barely move it along the floor.

The surround speakers are conventional except in box shape. Sound presentation is actually a bit better from the rear row, though the volume of the surrounds is a compromise between the two seating areas. A better design may be to angle them forward a bit. Hm.
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post #8 of 10
5/8/10 at 12:21pm
Quote:


....drill holes...

Maybe. Maybe not. You could end up with a low pass filter instead.
So here's the way it works. If you don't want to measure, don't be attempting to build an HR, Low pass filter, or buy stuff for the walls.

If you're not sitting under the soffit, the modes there are "so what?"

Only one subwoofer? You're already behind the curve.

The screen isn't doing squat for the LF response in the room; but, it is certainly a big adverse element for your HF response.

Am I being harsh? Yes. You're talking of building a complex system (HR) without a clue as to what your room really needs. My advice is to either do it right (which might include paying a professional), or not at all. The more likely result of the path you're taking is the room will sound worse and you will have substracted money from your bank account. I'd guess you don't even know at this point what the stuff you've hung from your walls is even doing ... good, bad, or otherwise.

Measure the room! Now! ... and at all seating locations. That's the next step. If you don't want to take that step now, then stop and put your money into savings.
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post #9 of 10
5/9/10 at 9:27am
Thread Starter 
Dennis, this is a learning experience. I don't have an unlimited budget. I'm sure it's of great amusement to you that I'm using a single subwoofer or that I haven't torn down a wall, but I have what I have and I'd like to make the most of it.

Room treatment is a means to end. I don't enjoy it. I'd rather be listening to music. If I thought I could avoid devoting even more time to this beyond the ungodly amount of reading I've already done, I would. But it's apparent that this room needs more, and I'd already reconciled before your post that measurements would be necessary before going further. The tone isn't necessary.

I'm concerned about the riser because I have to finish it by Friday to mount seats on it, and I may not have time to properly measure the room in the interim. The existing treatments are primarily to damp reflections above 100 hz. If the data sheets on Rockwool are even vaguely accurate, they should be doing just that.
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post #10 of 10
5/9/10 at 1:12pm
None of my "harsh" commentary has anything to do with budget, tearing down walls (or otherwise making a mess). It had only to do with the fact you want to build an HR without knowing what needs to be fixed (there could be better, less expensive ways).

You've got to know what problem you're trying to solve, before you apply a fix ... I guess that feeds back to the budget question. Measure the room, make a change and measure again. It's tedious; but, it works very well.
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