Concealed corner bass trap ?

[msikio]

Hello all,

I'm thinking of using "Gyptone Line 6" gypsyum board to built concealed corner bass trap. Those boards come in 2400 mm x 1200 mm size (8 ft x 4 ft), the perforation size is 6 x 80 mm (0.24" x 3.15"), and the perforation rate is 13%. It looks like it can be cut every 600 mm (2 ft) without affecting the nice look created by the perforation arrangement.

Those boards can be caulked and painted exactlly in the same way as regular gypsym boards, achieving a very smooth transition between the original wall and the bass trap. I've seen those used in a commercial recording studio and in a TV studio.

I'm planning to use two layers (80 mm and 40 mm) of 70Kg/m3 stone wool (equiv. to 4" O/C 705) to fill the cavity behind the boards. I think I should have a small gap (1/2" ?) between the 705 and the Gyptone to allow the mineral wool to "breathe".

I already posted this thread on the 3D Audio forum, I hope this cross posting is not considered as bad manner Ethan Winer nicely replied saying the perforated gypsyum board might reflects too much sound even in the frequency range concerned by a bass trap.

Any more thoughts ?

Thanks,

Michel

PS : I did a simple Google search, and the Gyptone Line 6 does'nt seem to be available in the US. There might be an equivalent product ?

[John Sayers]

Yes - I know the product Michel. It's not a bass trap material, it's a helmholtz resonator material for low mid frequencies.

the pre- cut slots are set so you can't tune them other than varying the distance off the wall but you'll never get them down to the frequencies of a corner bass trap.

Check out slot resonators under acoustics at the SAE site.

http://www.saecollege.de/reference_material/index.html

cheers
john

[knightfly]

This type trap would behave as a Helmholtz resonator; for those to be effective, the absorbent (usually with a cloth cover) needs to be in contact with the front panel for a couple of reasons - one, this damps the panel so it doesn't tend to vibrate at resonance, which can return un-wanted sound to the room. Two, the Helmholtz formula is based on all air movement being forced to travel THROUGH the absorbent instead of AROUND it.

Also, most of these formulae (including the one found in the slat resonator calculator THIS forum, which is one of the FEW that doesn't use the WRONG FORMULA) are set up to do a "mouth correction" of 1.2; this is based on OC 703, and using different absorbents will change the gas flow resistance and also the resulting Fo.

However, when building these as corner traps, the varying depth kind of "muddies the water" anyway - it's not generally known how much, but what little data is available on this type trap indicates that a variable depth one, in order to be predictable, needs to have individual septums between panel openings.

The GOOD news; lots of our members build this type or similar, and LOVE the way their rooms sound. If yours ends up too bright, you could mount some 1" absorbent in FRONT of the trap, stood off by an inch or so - this would bring the reverb time down in the highs a bit more without negatively affecting what the original trap is doing... Steve

[msikio]

Thanks you very much Steve and John for the replies. I though I could use the Gyptone in a alternative way, for elegant and easy covering purposes.

I actually like the wood slat resonator design, I will do my first experiment with it it shortly... at the moment trying to get rough slates professionnally planed down for free

I already built a "Helmholtz resonator cupboard" using louvered shutters as doors. Will post pictures when finished.

[msikio]

This type trap would behave as a Helmholtz resonator; for those to be effective, the absorbent (usually with a cloth cover) needs to be in contact with the front panel for a couple of reasons - one, this damps the panel so it doesn't tend to vibrate at resonance, which can return un-wanted sound to the room. Two, the Helmholtz formula is based on all air movement being forced to travel THROUGH the absorbent instead of AROUND it. Steve

Steve,

1. If the panel moves, it acts as a membrane absorber as well ? Can't we drop too birds in one shot ?

2. I read elsewhere the damping of an Helmholtz resonator cavity is only there to broaden the resonant frequency range and thus lower the "Q"? BTW what the "mouth correction" would be for the 704 type fiberglass ? (I'm using 70kg/m3 Rockwool).

Thanks,

Michel