Thin unsealed panel over bass absorber to reflect high end?

[dfeiszli]

Hello, first of all, thanks for such a great forum -- I have learned a ton reading on this site!

I'm a renter, and I recently moved into a house with a larger space for my studio (still rented, though.) I'm having some issues with the acoustics in my tracking room area (see diagram.)

I just installed a false ceiling with 4" 703 spaced 4" from the ceiling. That in itself has made the room a little too dead in the high frequencies, but I still have some low frequency modes causing dips and peaks. Putting 703 in the corners does a decent job of taking care of this issue, but there's also some flutter echo between the two long walls. Putting a few panels of 703 on one wall takes care of that too, but then all my high end is completely gone.

Since my space is rented, I can't do much with construction, although I'm good at creating low-impact false ceilings and free-standing treatments. I'm looking to make things that can be used in any future room I may have, rather than treatments specifically tailored towards this one, so I don't think making tuned bass traps would be a good idea. Helmholtz resonators seem interesting, but I definitely need to treat lower frequencies than those work on, although I might make some after I get the bass sorted out.

What it seems like I need are treatments that reflect or diffuse the high end, and absorb a large chunk of the bass range.

So, Question #1: If I put a layer of thin plywood (1/8"?) under the false ceiling, but in such a way that it's not sealed so it's not acting as an extra leaf or a complete ceiling, would that still keep the ceiling working as a low and midrange absorber but reflect some high end? I'm thinking of maybe creating a checkerboard pattern so half the ceiling is cloth-wrapped 703 and the other half has a plywood facing over the 703. (So, in those plywood covered spots it would become, from floor up: 1/8" plywood > 4" 703 > 4" air gap > existing ceiling.) Will the plywood let enough of the lows and midrange pass through so the 703 can work as it has been on those frequencies?

(Similar) Question #2: If the concept in the 1st question would work, how about this one? I marked an area on the diagram as a proposed new wall. What I'm thinking is thin plywood again, run from the floor up to where the false ceiling starts but not up to the actual ceiling. Therefore, it wouldn't be a sealed wall, so it wouldn't be acting as a new leaf (right?) or a panel trap, but would just be to reflect some highs and break up flutter echoes. I'd put 703 in the corner behind the wall as a bass trap and possibly along the length of the wall as well.

(Theoretical) Question #3: I guess from my other questions it's clear that I'm a little confused as to what the difference is between a wall that reflects all frequencies and a reflective surface that reflects a certain upper frequency range and lets lower frequencies pass. On one hand there 703 with a facing, which reflects highs and passes almost everything else. On the other hand I guess would be a massive solid concrete wall that reflects everything down to 20Hz and beyond.

What I'm interested in is what's in-between those two -- is there a formula that will tell me what frequencies a given density of material will reflect and which it will pass (like 1/8" plywood in the questions above?) And, am I right in thinking that if a material is thin enough to not reflect bass frequencies, does that mean that whatever trap is behind it will work on the bass frequencies as if the material wasn't in front of it?

Thanks very much for your help!

[knightfly]

Yeah, you got it basically right; simple and cheap test would be to buy some painter's plastic drop cloths - get the heavier ones, typically about 1 mil thick, and temporarily tape these up in front of your 703 ceiling, do a section at a time; this may be all you need to bring back the "sizzle". If it works for you, but you want your cloth covers to show instead of plastic, then add plastic to more and more absorbers til the room is a bit TOO bright - then re-do these with plastic UNDER the cloth. Should come out about right...

1/8" panels would do this to lower frequencies, so if you actually measure your room's response and find it needs more high mids as well, then try the light panels.

Each room is its own unique puzzle, due to differing construction techniques and materials - this is why it's necessary to "cut and try" to some degree... Steve

[dfeiszli]

Thanks, Knightfly -- great idea about the plastic, I never would have thought of that. I'll start experimenting; I don't really have the resources to do accurate measurements of the room response, so my plan all along has been to go by my ear, which is really what I'm trying to satisfy in the end anyway!

There's one more related question I have about Helmholtz absorbers, specifically ones like John's corner absorbers -- since one element of their construction is 703 spaced across and away from a corner, would these also absorb the same amount of bass that the 703 would by itself in the same spot? Or is there something in the construction of the rest of the unit that keeps the lower frequencies from passing and being absorbed by the insulation? According to John's Recording Manual, they're really only low-mid absorbers, but is it possible he means that's just for the tuned frequencies, but they'll also absorb some more broadband bass frequencies below that? If they'll work below their tuned frequencies, I might make some of those for my corners, filled with as much 703 spaced as far out as I can fit.

Thanks so much for all the help!
Dan

[Ethan Winer]

Dan,

> If I put a layer of thin plywood (1/8"?) under the false ceiling <

Even 1/8 inch plywood will reflect bass more than you'd like. Try heavy cardboard or card stock.

--Ethan

[dfeiszli]

Hi Ethan, thanks for the reply. Do you have any rough or specific measurements about what frequencies might be reflected with the 1/8" ply? Acoustics seems pretty counter-intuitive to me in a lot of ways, and I'm trying to get a feel for how things work. I'll definitely check out both your cardstock suggestion and Knightfly's plastic sheeting to liven up the response; since it seems like either or both of those would be easy to do, I'll probably leave the high end for last, after I have the bass and then the midrange sounding good to me.
Thanks again,
Dan

[bpape]

Just as another thought, you can take Kraft paper or heavy shipping paper and use spray adhesive to attach it to the 703 - probably the corner ones would be best. This is relatively easy to do.

[Ethan Winer]

Dan,

> Do you have any rough or specific measurements about what frequencies might be reflected with the 1/8" ply? <

It depends partly on how rigidly it's mounted. Maybe 200 Hz? This is just a WAG (wild-assed guess) so don't hold me to it. You could even measure this yourself by putting a large wood panel between a speaker and microphone and measuring what get's through. Whatever frequencies don't get through are being reflected (or possibly absorbed).

--Ethan

[dfeiszli]

It depends partly on how rigidly it's mounted. Maybe 200 Hz? This is just a WAG (wild-assed guess) so don't hold me to it. You could even measure this yourself by putting a large wood panel between a speaker and microphone and measuring what get's through. Whatever frequencies don't get through are being reflected (or possibly absorbed).

--Ethan

Thanks for the WAG! I'm thinking about doing some tests to measure it before I build anything, so if I do I'll post them here.

How would the rigidity of the mounting affect the result, do you think? Would a more rigid mounting reflect lower frequencies?
Thanks,
Dan

[Ethan Winer]

Dan,

> How would the rigidity of the mounting affect the result <

If it's rigid it might vibrate as a unit which means it might resonate. I honestly don't know exactly how that would affect the reflectivity versus frequency, but somehow it seems like it should.

--Ethan

[dfeiszli]

Dan,

If it's rigid it might vibrate as a unit which means it might resonate. I honestly don't know exactly how that would affect the reflectivity versus frequency, but somehow it seems like it should.

--Ethan

Makes sense, thanks. I'll post back when I have either some test results or some results on the room. Thanks everyone for the help!