As John’s Excel calculator only provides 1 absorption frequency, 1 slot and 1 slat dimension:
1. How can one predict the central frequency of the absorber when using multiple slat slots dimensions?
Apply the equation to each individual slot, to predict the probably resonant frequency of that specific slot.
2. Is it possible to target multiple frequencies that way???
Yes, but I'm not sure why you would want to do that in the low mids. For what purpose?
3. What should I input as “Depth from wall” if the absorber front face will be 12° angled?
If the depth changes, then so does the frequency, so you would not have one single tuned frequency for that slot: Rather, it would be tuned to a range of frequencies between the frequency at the point where the depth is greatest, and the frequency at the point where the depth is least.
That said, I'm wondering how you would go about building the structure for a slot wall where the face is angle 12° with respect to the actual wall. What is your plan for that? And how will you seal that, to ensure that it is fully air-tight?
4. How thick should be the insulation?
That depends on what you are trying to achieve. The insulation does actually change the resonant frequency slightly, and also lowers and broadens the Q, so if you are looking for low levels of absorption across a wide frequency range, then make the insulation thick. If you want to only target specific frequencies very accurately, then make it thinner and constant depth.
5. Is it possible to predict which higher frequencies will be reflected?
Yes: All of them!
OK, it's not really that cut and dried, but assuming you are using reasonably thick, solid, rigid wood, the slot wall will reflect pretty much all frequencies above the highest tuned frequency. Depending on how the wall is designed and built, the transition from absorption to reflection might be fairly abrupt, or it might be spread out over an octave or more. The width of the slats roughly governs the frequencies of reflection. If you use small slats, then you'll be reflecting highs mostly. If you use broad slats, you'll also be reflecting mids
There's also the issue of "percent coverage" or "percent open area". If the area of the slots themselves, as compared to the total area of the slats, is less than about 5-10%, then the wall acts more like an array of individual Helmholtz absorbers, each tuned to its own frequency. But if the percent open area is greater than approx. 20-30 %, the entire wall will act more like a general broadband absorber, with no specific resonant turnings.
Also, do take into account that a slot wall is also somewhat diffusive, especially at higher frequencies. So it's not just a matter of resonance and reflection, but also absorption and diffusion. That's why it is such a versatile tool, in the hands of an acoustic designer. With careful design, you can get it to do lots of things at once, in a controlled manner.
- Stuart -
