Will covering a bass trap w/perforated hardboard reduce effectiveness? - AVS | Home Theater Discussions And Reviews
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post #1 of 16 Old 08-06-2014, 06:27 PM - Thread Starter
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Will covering a bass trap w/perforated hardboard reduce effectiveness?

Quick version:

Will the 3/16" perforated panel shown in the pic reduce the effectiveness of OC 702 behind it?

Details:

I'm covering bass traps (6" thick OC 702 x 24" x 60") with grasscloth fabric.

For looks and with no small effort I made the faces curved.

I carefully measured, cut, and stapled the grasscloth to the frames.

I made the frames a bit oversized in depth so if I had a problem with the grasscloth buckling or wrinkling, I could put batting in to fluff it out.

Didn't work, there are wrinkles I can't get rid of.

So I'm redoing them with perforated hardboard panels to match the frame curvature and give a surface for the grasscloth to follow.

My concern is that with only 5% open area the panels will impede effectiveness of the traps.

The panels are 3/16" thick and even with the curvature applied (see pic; if you're thinking it doesn't look like 6" thick it's because 4" is in the wall cavity behind it), it flexes noticeably with moderate finger pressure.

So likely it's transparent to bass but I'd like someone with more experience to verify.

Increasing the open area by drilling the holes bigger is a nonstarter (about 5000 holes).

I could groove them to increase flexibility, but I'd rather not as that would still be quite tedious.
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Noah

Last edited by noah katz; 08-06-2014 at 06:36 PM.
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post #2 of 16 Old 08-08-2014, 04:39 AM
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How about using a metal grill instead?
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post #3 of 16 Old 08-08-2014, 07:44 AM
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How about using a hole saw/drill bit to make every 5th hole 2"? I've done that before and it cuts through very quickly.
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post #4 of 16 Old 08-08-2014, 08:56 AM
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you can model the effect of the holy panel here:

http://www.acousticmodelling.com/multi.php
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post #5 of 16 Old 08-08-2014, 09:28 AM - Thread Starter
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Quote:
Originally Posted by Audio Geek View Post
How about using a metal grill instead?
I considered that real perforated etc metal was expensive.

I looked at hardware cloth but thought thin wood-based material would be easier to attach and give a more uniform surface.

Quote:
Originally Posted by JohnDean View Post
How about using a hole saw/drill bit to make every 5th hole 2"? I've done that before and it cuts through very quickly.
I actually tried that but too slow considering the number of holes.

Quote:
Originally Posted by JackNC View Post
you can model the effect of the holy panel here:

http://www.acousticmodelling.com/multi.php
Neat calculator, thanks.

Also I have no idea what to choose for "Porous model", but leaving everything on the left at defaults and with the data for the perforated panel I got "Absorber configuration is not valid."

Water under the bridge at this point; turns out the panels were so stiff that applying the curvature caused the sides of the frames to bow out.

To make them more flexible I cut 1/8" deep x 5/8" wide grooves between every column of holes; this took a few hours, so not too bad.

Now the frames don't bow out and the panels deflect easily with finger pressure so I'm confident they're transparent to bass.

Noah
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post #6 of 16 Old 08-08-2014, 10:01 AM
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It won't absorb highs with the hardboard, which is probably what you want (maybe).
The bass will go right through it; 60hz will go through a solid block of 5/8 drywall, 5/8 OSB and 6 inches of insulation + air gap, with only a few db's of reduction... which is a standard house construction, that's why you can hear bass cars zooming by (although most of that weakness is windows and doors, which bass also goes through.)

Even in my theater, distant thunder <105db sounds like someone jumping on a sheet of 1/8 plywood outside; and that's more than double the thickness of what a standard house construction is.

I haven't seen too many before and after bass trap measurements threads, but I would imagine that the bass isn't "trapped" by very much.
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post #7 of 16 Old 08-08-2014, 10:15 AM
 
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A real easy test for this type of thing is can you put your mouth on any part of it and comfortably breathe through it with no additional effort or discomfort? If you cannot do that, then you don't need to be firing soundwaves through it.
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post #8 of 16 Old 08-08-2014, 12:23 PM
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Quote:
Originally Posted by noah katz View Post
I considered that real perforated etc metal was expensive.
What about chicken wire?

http://www.homedepot.com/s/chicken+wire+fence?NCNI-5

Not sure how clean it'll look covered, though.
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post #9 of 16 Old 08-08-2014, 01:00 PM - Thread Starter
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Quote:
Originally Posted by BassThatHz View Post
It won't absorb highs with the hardboard, which is probably what you want (maybe).
Doesn't really matter; they're out of the way in the corners and all of my speakers are CD.

Quote:
Originally Posted by BassThatHz View Post
I haven't seen too many before and after bass trap measurements threads, but I would imagine that the bass isn't "trapped" by very much.
Not totally absorbing is different than not attenuating at all. and house insulation has much lower low freq absorption than OC 702.

In any case I heard an appreciable reduction in boominess when I added them at all four corners and in a big opening in the back wall.

Quote:
Originally Posted by corndogggy View Post
A real easy test for this type of thing is can you put your mouth on any part of it and comfortably breathe through it with no additional effort or discomfort? If you cannot do that, then you don't need to be firing soundwaves through it.
Not sure I'm getting your point, but bass easily goes through things that air can't.

Quote:
Originally Posted by Audio Geek View Post
What about chicken wire?
Too flexible to hold shape.

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post #10 of 16 Old 08-08-2014, 01:40 PM
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Lots of relevant info, video and links at Realtraps, Ethan Winer"s site, the real lowdown.
you'll want a desktop icon, it is almost too much for just one visit

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post #11 of 16 Old 08-09-2014, 05:35 AM
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So are these curved panels 6" thick? I am not familiar with OC702, but I have used OC703 on my theater walls and ceiling. How much difference in price is there between OC703 versus OC702? I would imagine that your bass traps are very expensive! Around here a 2ft by.4ft panel of 2" thick OC703 cost almost $80! I can just imagine what a 6" deep and really wide bass trap would cost!

In my old theater room I just used stacked triangles of pink fluffy in each corner to work as my bass traps, but now that I have a real theater room that I want to look really nice, I must find a prettier solution to my corner bass traps.

Any suggestions? Perhaps it could check on some OC702 and do a build similar to yours, assuming that it's no where near $80 per 2" thick 2ft by 4ft panel!
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post #12 of 16 Old 08-09-2014, 12:54 PM - Thread Starter
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I may have misremembered the number, maybe I have OC 703.

They're built up from flat 2" pieces with the top layer making the curve cut from different thicknesses/angles; now that I added the curves frames/hardboard that was wasted effort.

It's been years, but IIRC it was about $125 for all of it from a heating/insulation supply.

Or maybe I did it in two rounds and that was just one of them; can't remember.

Noah
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post #13 of 16 Old 08-09-2014, 02:45 PM
 
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Quote:
Originally Posted by noah katz View Post
Not sure I'm getting your point, but bass easily goes through things that air can't.
Do you think it is optimal to use a solid-core commercial-grade steel-plated door panel for a subwoofer grill? Why not? Bass will go right through it after all. I mean, it really will. I used to work at a place that had steel external walls, several inches of insulation, then drywall interior, I'd roll up in my car and about jolt people's teeth out. So, would steel wrapped fiberglass backed by drywall make a good subwoofer grill?
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post #14 of 16 Old 08-09-2014, 07:38 PM - Thread Starter
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Still not getting what that has to do with anything I said.

Oh, you mean my original concern is totally unfounded; got it.

Noah

Last edited by noah katz; 08-09-2014 at 07:42 PM.
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post #15 of 16 Old 08-10-2014, 01:33 AM
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get some perforated metal plate--the stuff the pro-audio guys use for their grills. it is pretty much acoustically transparent, not too expensive, and should hold the shape you are shooting for.

Listen. It's All Good.
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post #16 of 16 Old 08-10-2014, 05:05 PM - Thread Starter
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As I said
Quote:
Originally Posted by noah katz View Post
Water under the bridge at this point...

Noah
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