Increase TL With Holes Inside a Wall

AVare · Post by **AVare** » Tue Aug 10, 2004 5:44 am

I couldn't believe at first, but looking over the figures, holes inside a wall can increase TL by several dB in the low frequency range! In

http://irc.nrc-cnrc.gc.ca/irc/fulltext/ ... brn217.pdf

test results for 190 mm acoustic block (slots on side to let sound in) that do NOT SHOW a decrease in low frequency TL in the tested range. Figure 15 for quick checking.

Of course I tried to find the same construction without the slots, but best I could find is

http://irc.nrc-cnrc.gc.ca/fulltext/ir586.pdf

figure 3, upper right graph.

Thinking about it as described in BRN-217 it makes perfect sense. Would I have have ever thought of putting holes INSIDE a wall to increase TL at certain frequency ranges? No.

z60611 · Post by **z60611** » Wed Aug 11, 2004 4:43 am

Can you find any acoustic concrete block that doesn't have a center helmholtz frequency of 125hz ?

AVare · Post by **AVare** » Wed Aug 11, 2004 10:51 am

Can you find any acoustic concrete block that doesn't have a center helmholtz frequency of 125hz ?

Thats too easy.

The blocks used in the BRN-217 test center around 200 to 250 Hz. Day and Campbell make one that centers around 180 Hz. Besse Masonry makes one that centeres around 250 Hz. Here is a link to the pdf:

http://svc174.bne149v.server-web.com/be ... &pageID=12

Might be a bit expensive shipping blocks from Australia though, especially air.

I am not that familiar with acoustic blocks. Those are the results of my rough search. What frequency are you wanting to center on?

Andre

z60611 · Post by **z60611** » Wed Aug 11, 2004 11:32 am

The resonance frequency of a double stud wall. Anything between 30hz and 60hz, and less than 8" thick.

AVare · Post by **AVare** » Wed Aug 11, 2004 12:20 pm

The resonance frequency of a double stud wall. Anything between 30hz and 60hz, and less than 8" thick.

Reading a little bit between the lines (properly, I hope) are you trying to achieve high isolation with a concrete block wall behind the double stud wall?

If you haven't read the text above the graph in BRN-217, it indicates that the high TLs at low frequencies is due tot he block cavity effectively increasing the air gap. Not due to the absorption of the block. The graph shows somewhat in that the TL of the painted only wall increases at the cavity peak, but this is not increased significantly by the gypsum.

After studying the two referenced documents and doing a bit more research on the web I get the feeling that the best bang for the buck for effective LF isolation is acoustical block with the gypsum on the slotted side. Whether mounting the gypsum on 75 mm studs would cause a decrease in performance, I haven't been able to answer to my satisfaction. I hope it would not decrease performance, because then we would have a high TL wall, relatively cheap, close to standard construction with the capability to run cables inside and maintain a good aesthetic.

Good luck!
Andre

z60611 · Post by **z60611** » Wed Aug 11, 2004 1:34 pm

Avare:

Reading a little bit between the lines (properly, I hope) are you trying to achieve high isolation with a concrete block wall behind the double stud wall?

Nope.
The double stud wall has a resonance at which there is a lower TL.
What I was talking about is to damp that resonance, by building a helmholtz absorber out of concrete in the 8" gap inside the double stud wall.
It seemed to me that's what the BRN-217 was doing - except that they used the typical acoustic concrete block, which has a resonance between 125hz and 250hz. (the first one I found was 125hz) Well, 125hz doesn't do me any good since a double stud wall is pretty good there.

Admitidly, building a double stud wall and filling it with concrete blocks is insane. (It's much simpler to build a concrete wall and attach gypsum to both sides.) But what if one could build, out of concrete or something else, little massive helmholtz absorbers tuned to the resonance frequency of the double stud wall, to increase the wall's TL at the wall's resonance. Perhaps the bottom 2', inside the wall, could be a bunch of standard 4" concrete block, with a cover over the top with a helmholtz hole in it. Mount the whole heavy thing on some sort of isolation plate, so that the walls are still reasonably decoupled.

AVare · Post by **AVare** » Wed Aug 11, 2004 1:59 pm

Okay, I understand what you are thinking about now.

Why not design some tuned absorbers that can be easily built into the wall?

If the data is any hint of the LF performance I find it hard to imagine a construction that would improve on it with out doubling the cost.

In the construction that you are describing, which side would you place the slots on? The other side would still be m-a-m spring, from what I understand of what you wrote.

Interesting concept.

Andre

z60611 · Post by **z60611** » Wed Aug 11, 2004 3:12 pm

I'd place the slots on the top. I don't think any other direction would give the depth/volume you'd need. i.e. the wall is 8' tall and 8" cavity in a 10" thick wall, but the concrete block would be 3' tall and 4" thick.
But I'm concerned that the concept would create a quadruple leaf wall.

Alternatively, if it's a basement slab, then a ditch could be cut in the slab where the wall is to go -- and this helmholtz absorber dropped into the ditch.