some questions about a helmholtz resonator

[bobmurphy]

hi - this is a thread i started over at ethan winer's forum and he recommeded that i ask i here......

i am only learning this stuff and would like someone to have a look over my plans.
i have built a piece of studing in my studio for the purpose of keeping the mixing area symetrical and was hoping to also use it as a helmholst resonator

it is in the shape of a triangle measuring aprox 8 feet by 3 feet . My plan is to drywall about half of it, leaving the rest of the face of it as an area for slating and sloting which is aprox 8 ft tall by 4 feet wide. ( i can adjust this more carefully when i calculate the tuning )

my main concerns are:
is the overall construction principle correct? is it ok to have half the front of the cavity drywalled with no slots in it and yet have slots in the other half ( also if anyone knows whether this makes a difference to the tuning calculations )
being new to this i would like if someone could confirm if i am basically on the right track - i am going to totally seal up the interior of this void so that it will be completely airtight EXCEPT for the slots in the front. Then i am going to partially fill the void with rockwool. ( BTW i have not been able to find information on how this affects the tuning ). Then i am going to cover the area to be slatted with material to keep the rockwool inside and last i am going to slat the area with timber.

i also have a concern that , as this resonator will be placed on the wall to the right of the mixing position, then will i need to put something similar on the opposite side or is this unnecessary.

thanks in advance.

[knightfly]

Last first - you definitely want a symmetrical mix position, at least to a few feet behind your head; so if you do it to one side, you should do it to the other.

The helmholtz/slat calculator on this forum is based on the same formula used in Alton Everest's books, and is presumably based on using actual Owens Corning 703 fiberglass insulation - this was about the only option at that time. The calculator is based on a constant cavity depth, and on some type of speaker cloth being placed up against the inside of the slats with the insulation (not sure of the original depth) placed right behind that, so that all air movement has to go thru the slot and thru the cloth and insulation in order to reach the rest of the cavity. Under those conditions, the results should be fairly close to calculations.

Changing the insulation to rockwool will change the flow resistivity of the insulation, and I'm not sure which part of the formula deals with that - I think it's the K factor; the 1.2 value is for increased EFFECTIVE depth due to slot inertia, etc.

Making the trap non-parallel will broaden the response, but this isn't predictable. The only way this would cause what one would EXPECT it to cause, is if you were to build individual separators between each slat, and run the slats VERTICAL so you were actually making a whole bunch of small, vertical traps (each with it's own depth) - otherwise, the volume behind the insulation will tend to AVERAGE the response, which (in theory, anyway) should make the trap less "peaky" than a constant depth one.

You can get some second-hand confirmation of this effect by going to Len Morgan's thread

http://www.johnlsayers.com/phpBB2/viewt ... 88&start=0

I suggested such a trap for one of his live room's walls, he did it, and absolutely loves the result. Think it's somewhere around page 8 or so... Steve