fiberglass n hemp, absorption coefficients HELP

[clusterchord]

hi guys. compelled by the DIY Broadband Absorber - pictures posted thread, and wonderful work some guys have done here, i decided to go that route as well. now, since i already have some allergies, i was very concerned about health issues, and soon as airmate told me about hemp as a safer (but more pricey) alternative, i contacted the manufacturer.

im rather unexperienced in acoustic measurement, and i wondered if u comment on the following absorption figures; i compared Hock thermo-hemp to OC 700series, to find out if it could be as good in the LF region..


so,here are the overall figures i extracted from both pdf's:


Hock thermo-hemp (plain, no paper membrane, density is 35kg/m3):

40mm (1.575inch) 0.20@125Hz
160mm (6.3inch) 0.85@125Hz


Owens Corning 700series:

703 plain 2inch 0.17@125Hz 0.86@250Hz
705 plain 2inch 0.16@125Hz 0.71@250Hz
703 FRK 2inch 0.63@125Hz 0.56@250Hz
705 FRK 2inch 0.60@125Hz 0.50@250Hz


if i'm reading this right, OC seems better when used with FRK, but plain panel figures are slightly better for hemp. i knew FRK membrane makes a difference, but wasn't aware it was this big. could this mean i could get FRK figures if i place insulation paper on hemp as well? what do i think of this?

and if i do, what kind of material, or insulation paper (thickness/density) should i be using?



entire Hemp measurements doc is here: SCHALL_Englisch.pdf
and full OC info is here: Fiberglas700Series.pdf


thanks

[Ethan Winer]

Originally Posted by clusterchord

i knew FRK membrane makes a difference, but wasn't aware it was this big. could this mean i could get FRK figures if i place insulation paper on hemp as well?

Maybe, but you'd need a way to bond the paper to the hemp. I've never seen that stuff in person so I can't comment on how easy or how difficult that might be.

--Ethan

[clusterchord]

Originally Posted by Ethan Winer

Maybe, but you'd need a way to bond the paper to the hemp. I've never seen that stuff in person so I can't comment on how easy or how difficult that might be.

hi Ethan.

well they do offer both the insulation paper and special glue that comes with it to connect it to hemp. is glueing okay? i suppose i'd only cover the panel with glue towards the edges, not the whole surface ?


btw, hemp does look very much like rigid fiberglass - there are some shots of it in this pdf: http://www.thermo-hanf.de/upload/pdf...t_2004_ENG.pdf


thanks

[Ethan Winer]

> well they do offer both the insulation paper and special glue that comes with it to connect it to hemp. is glueing okay? i suppose i'd only cover the panel with glue towards the edges, not the whole surface ? <

Again, since I've never even seen the stuff in person I'm hesitant to tell you much one way or the other. I do like the concept of hemp though - a lot.

--Ethan

[clusterchord]

understood. ok, if i may, let me rephrase the question - in which manner is FRK (as i never held one in my hand) connected to 705 ?


is the paper glued to the whole panel surface, or not? im afraid putting glue on the whole 120x60cm could this possibly affect the acoustic properties (i.e. close the "porous" material).


any suggestions what type of material could i use for DIY "FRK" membrane?

or if u r familiar with thickness and density that Owens Corning used?





my plan is to do filled-corner bass traps, but portable with reactangular triangle frame made of wood, but wth hollow sides. (cutting the panels in 4 wedges, and stacking one on top of another to go all the way to the ceiling).

i was considering making this bass trap in two pieces, lower with a plywood membrane and a bit of spacing (like your old "tuned" absorber approach, tuned to a lowest room node area), and upper half just straight broadband with "frk" style membrane. what do u think f this approach?

then id place the plain hemp broadbands to create RFZ left-right, a cloud above, and put 3 skylines in the back. room is 4.6 x 3.4 meters.

apologies for the abundance of questions


regards
Tom

[Ethan Winer]

Tom,

> is the paper glued to the whole panel surface, or not? <

Yes, and it's a really sticky spray glue.

> im afraid putting glue on the whole 120x60cm could this possibly affect the acoustic properties (i.e. close the "porous" material). <

Right, you don't want contact cement applied with a paint brush, but rather a thin film sprayed on.

> or if u r familiar with thickness and density that Owens Corning used? <

See this:

www.ethanwiner.com/density/density.html

> i was considering making this bass trap in two pieces, lower with a plywood membrane and a bit of spacing (like your old "tuned" absorber approach, tuned to a lowest room node area), and upper half just straight broadband with "frk" style membrane. what do u think f this approach? <

I tihnk you should go with one type or the other.

--Ethan