Stuart, how do you attach the slats to the studs in the "inside-out" wall construction method, if they are not to touch the fabric, which is stretched over the studs themselves?
You can put thin spacers between the studs and the slats. Of course, you so need to take the depth of that "shim" into account when calculating the frequencies for the slots.
However, I probably should not have been so vehement with what I said: you
can have the cloth in direct contact with the slats, as long as you understand what it will do. If you want a very sharply tuned resonator that will just hit a specific frequency very well, then keep the cloth away from the slats. However, if you want a more broad-band effect, so the slot covers a wider range of frequencies with lower absorption, then leave no gap: put the cloth and the insulation up tight against the slats.
Here's a graph of both cases:
Slots-with-without-air-gap-2.png
The cavity depth is identical here, at 90mm. The slats are also identical: 19mm, thick, 150mm wide, 5mm slots between them. The only difference is that for the blue curve, the cloth and insulation are placed 10mm behind the slats, while for the green curve, they are tight up against the slats. (In both cases, the insulation is 50mm of OC-703)- As you can see, the green curve shows much less absorption, but it also covers a much broader range than the blue curve (about two octave wider, in fact).
So it all depends on what you actually want to do. If you have a specific problematic frequency that you need to hit hard and tight, then keep the cloth and insulation away. But if you want to hit a broader range more gently, then put the cloth and insulation up close.
If I determine that i need:
-slot width - 1mm
-slat width - 200mm
-depth from wall - 90mm
-slat thickness - 25mm
to get the absorption Frequency of 74 Hz,
Right, but be careful there! Helmholtz resonators are very difficult to tune so precisely, especially at such low frequencies. If you make even a tiny mistake in building that, and the slot ends up at 1.5mm instead of 1mm, then you are about 10 Hz off! Way too far to be useful. If the depth is off by 10mm (say 80 instead of 90) then you are off by 5 Hz. Still a large difference.
Then there's the issue of effectiveness: in order to be effective, the resonant volume of the resonator needs to be about 1% of the room volume, at least, if you are targeting specific frequencies. Can you do that?
And finally, why do you need to hit 74 Hz? That's pretty low for a Helmholtz resonator. They don't work too well down that low.
Your other two questions are more about percent coverage: It works like this... If the open percentage (are of slots as compared to area of slats) is greater than about 10%, then the entire wall acts more like a broadband absorber, and should be calculated as though it were a perforated panel resonator. If the open area is less than about 5%, then it acts more like a series of individual Helmholtz resonators, each tuned to its own frequency. So once again, you need to take that into account when figuring out your slot wall plan.
- Stuart -
