Helmholtz absorbers outside room boundaries

[nosfoe]

Hello everyone, we are currently working on a new post-production-studio with a slanted ceiling, and since we've got lots of space above the ceiling (at the lower side of the room) I had the idea to move some Helmholtz-absorbers outside the room and let the ceiling be the frontend, where the holes of the absorber go (see pictures). Theoretically this would place the holes of the resonator in the most ideal place, exactly on the room boundary where the sound pressure level is highest... but I've never done that. Any opinions on this?

[bert stoltenborg]

What the hell would you use Helmholzabsorbers for?


In stead of slanting the ceiling you should use the space that's now wasted to get a bigger room.
We-need-a-bigger-room.

[nosfoe]

In stead of slanting the ceiling you should use the space that's now wasted to get a bigger room.
We-need-a-bigger-room.

I know, but that's not for me to decide in this case. Fortunatley it doesn't make the room too small, the ceiling is more than 5m high at the highest point.

[bert stoltenborg]

It would make more sense to perforate the ceiling and put a shipload of wool on it than fiddling with HH-resonators.

[Zaphod]

I think a sheepload of wool is more appropriate

[bert stoltenborg]

There is so much wasted room there you could put in a living herd.

[nosfoe]

About wasted room: The pictures are just an example to show the idea of a resonator outside the boundaries of the room. Actually all of that space could potentially be used for resonating chambers.

[bert stoltenborg]

don't even bother.
fill it with wool, HH and so don't work.

[Scott R. Foster]

The solution to all non-trivial small room acoustic problems is a bigger room.... therefore, generally, secret chambers filled with hangerzorberfuzerhelmholtzianrezonatoraten should be converted to room.

[nosfoe]

After some reading I found the answer to my original question: yes, placing the entrance to the neck of a helmholtz resonator exactly on the boundary should work as expected. Everest describes some cases of this type of installation in churches, Master Handbook of Acoustic, page 217.

But to go with the bigger room/ using lots of wool-theme: I have a follow-up topic on this. There's a room that will probably be built soon looking something like this:

Lotsofwool.jpg

(118.34 KiB) Not downloaded yet

I am really curious how the huge chunk of wool in the top will affect the modes in the room. Everest writes about the original 'bass traps', holes in the ground or walls with a depth of a quarter wavelenght of the frequency to be absorbed, with porous absorbption in it. Since the pressure of at the entrance to the trap is at zero (at the design frequency) it should suck in more sound energy from surrounding areas. I imagine the huge ceiling trap might act like that and have an effect on the whole room. Of course some other treatment will be applied to the walls for the higher frequencies. Any thoughts if this will absorb enough modal ringing in the whole room?

[Scott R. Foster]

2 much wouldn't ya think?

[nosfoe]

the room is meant to be almost anechoic, i forgot to mention. so for that, nothing is too much i guess.

[jcgriggs23]

Making a small room "almost anechoic" is going to be tough to begin with - doing it with narrow band absorption (like Helmholtz absorbers) will be even more difficult. If you look at anechoic chambers, you'll see they mostly use large, broadband treatments. Same thing with the so-called "Zero Environment" room designs. And I doubt very much that the churches that Everest talks about are anywhere close to anechoic...

My $0.02 (Canadian Tire),
John

[nosfoe]

this is not about helmholtz absorbers anymore, it is about lots of wool, see above. maybe it should have been a new thread, sorry.

[jcgriggs23]

Sorry - I was confused by your most recent post, which seems to indicate that your plan is to fill a large area above your ceiling with wool and have openings between this and the room - which sounds like a large Helmholtz contraption to me. Better to have a bigger room with the fuzz inside, IMHO.

Regards,
John

[nosfoe]

ok, i see. what i mean is an acoustically transparent ceiling like just fabric, for example. so the modes "see" a much higher room than the spectator, with a massive porous absorber. i wonder if that will suck up so much bass-energy over time that there will be almost no need for further treatment for the modal ringing.

[jcgriggs23]

OK - you might want to look into the "zero environment" approach to recording studio control room design championed by Tom Hidley and Phillip Newell - it tries to approach near anechoic conditions using heavily trapped walls (apart from the front wall containing the flush-mounted monitor speakers) and ceiling similar to what you are proposing (although their design specifies "hanger" devices behind the false walls and ceiling rather than just masses of fuzz - there is some debate over whether the hangers would provide any real benefit over masses of fuzz)

One issue with this type of approach is the danger of over absorbing high and mid frequencies relative to bass (which is much harder to suppress), so you probably want to leave open the possibility of adding reflective and diffusive elements to the final room to add back some of the missing mids and highs to balance the bass you can't treat. Spaced wooden slats or diffusers mounted over the fabric facing of the walls and or ceiling - typically applied in spots rather than across the entire surface - can achieve this. Measurements and careful listening are key to getting the balance right.

Regards,
John

[Tubamark]

Lotsofwool.jpg

I am really curious how the huge chunk of wool in the top will affect the modes in the room. Everest writes about the original 'bass traps', holes in the ground or walls with a depth of a quarter wavelenght of the frequency to be absorbed, with porous absorbption in it. Since the pressure of at the entrance to the trap is at zero (at the design frequency) it should suck in more sound energy from surrounding areas. I imagine the huge ceiling trap might act like that and have an effect on the whole room. Of course some other treatment will be applied to the walls for the higher frequencies. Any thoughts if this will absorb enough modal ringing in the whole room?

Unless the density of that wool is far lower than normally used (maybe a low as 1-pound), this is not the best use of all that space. When the thickness of a given fiber absorber is increased, the total resistance to sound does too, even though the density may be constant. The wool would also have to be on mesh "shelves" or something to prevent settling to a higher density. Even though casual practicioners claim that OC 705 is always 'better for bass', the data says otherwise: common batt insulation performs better than OC 705 at depths greater than 3 or 4 inches.

The mucho-wool ceiling would certainly absorb some modal ringing, but again, is not making the best use of resources to do so -- Only one room axis has been addressed! if the wall treatment is of the usual variety (ineffective at low modal frequencies), lateral modes could still propogate.

This is (one of the) reasons why Anechoic chambers always use wedges or some other configuration that lets sound 'in', never a flat face.

--- Mark

[BengAT]

if you are using wool or any porous absorber then you dont want it at the wall.. its doesn't rely on sound pressure but the particle velocity so you want it where the maximum velocity is seen, its minimum is at the wall so you need to place it at least a 1/4 wavelength away from the wall in order to ensure maximum velocity at the surface. The reason you use velocity is because they absorb through viscous losses in the pores so you need maximum sound movement.

[bert stoltenborg]

if you are using wool or any porous absorber then you dont want it at the wall.. its doesn't rely on sound pressure but the particle velocity so you want it where the maximum velocity is seen, its minimum is at the wall so you need to place it at least a 1/4 wavelength away from the wall in order to ensure maximum velocity at the surface. The reason you use velocity is because they absorb through viscous losses in the pores so you need maximum sound movement.

This joke I already made a long time ago