John Sayers' Recording Studio Design Forum Archive

when building helmholtz resonators, does the width of the cavity matter, or just the depth? for instance, using the helmholtz calculator, it says i will have a resonant frequency of 195Hz, and i'm supposed to build a 3 foot deep cavity with a 1 inch slot width and 1/2 inch deep x 5 inch wide slats. do the cavities have to be any specific width, or if i build one cavity in both rear corners of my control room that are each 3 feet wide, would that be ok? thanks in advance!
-mark

Mark, the surface area of the resonator doesn't change the frequency center; however, it DOES change the AMOUNT of absorption at that resonant frequency. So the larger you build the absorbers, the more they will absorb at design frequency.

For typical slat absorbers, you can figure about 80-90 % absorption at resonance with a fairly steep fall-off within an octave or two to either side; so if you have 10 square feet of surface in the absorber's face, you would have 8 or 9 sabins of absorption at resonance (.8 to .9 coefficient, times 10 square feet) -

I started a re-make of our slat calculator some time ago that would calculate the rough TOTAL absorption around resonance, but have yet to find time to finish it; for now, hopefully the above will help... Steve

ok, a couple of other quick questions... my control room is going to be 9'h x 14.5'w x 21'd. what would be the best sized slat absorber(s) for my room, if i were to build them into the back wall of my control room, with 4' of available lateral space on EITHER side of the back wall? i used the helmholz calculator, and for a frequency of 198Hz came up with an 18" deep cavity with 1/2" slat spacing using1/2"x5' slats. would this be close to what i should use? higher frequency? lower? i really appreciate your help, and if you want some kind of consultation fee let me know and i'd be happy to send something your way. thanks again!
-mark

bizump

First, I've yet to see a drawing for your room; is it a rectangle, splayed, double-splayed, ceiling flat, vaulted, a-frame, etc??!?

I'm asking because if it's a pure rectangular room with parallel ceiling/floor, then you have a few coincident modes to consider (although overall the modal DISTRIBUTION looks good)...

Also, I would think twice about making slats only 1/2" thick - you'd be better off using 1" and changing spacing to meet the desired frequency center; too thin and they may "ring" longer, and cause more problems than they solve.

If you have a floor (and elevation) plan you could post, it would help a lot.

If you want to donate, the best way to do that (since I've not spent a ton of time just on you personally) is to use the Donation button at the top of the page; that will help EVERYONE by helping defray costs of keeping the site running; that way, ALL the "lurkers", bearded or otherwise can benefit - and thank you... Steve

ok, here is my rough floorplan. i don't have any cad files yet, so i just quickly drew it by hand, .25"=1'. if you need any other dimensions, please let me know. the helmholtz resonators are about 18" deep, and the available width is up to 4'6" on each side. about how much surface area do i need for the helmholtz traps? the height of the room is 10', with parallel floor and ceiling. i don't have the elevation at this time. however i will also post a picture of what i was planning on doing with the ceiling around the mix position. thanks in advance for the help, suggestions, and/or constructive criticism.
-mark

and i was thinking of doing something like this for clouds over the mix position.

bump again

With that much space between your head and the rear of the room, I'd use widely varied slat and slot widths to make those a more broadband absorber but with some diffusion effect from the slat edges - I would also be careful about putting the RPG's that close to the couch (unless they're 'way overhead?) - you generally need at least 3x the longest well wavelength in distance from a diffusor so it doesn't sound specular and "phasey" -

ON your side walls, there isn't enough angle to keep from needing absorption at first reflection points - don't try to change it, just be aware that absorption patches will be necessary in a couple of places on the side walls.

I like the cloud arrangement; should work well... Steve