Bass trap material suitability

[Arksun]

The more I read on this topic the more I realise the importance of gas flow resistance.

My question is, if both specific acoustic performance data is lacking, and there's nothing on gas flow resistance in the technical specs either, are there any other pointers to the suitability of a material for bass trapping?, or is one left completely in the dark.

There's some hemp/cotton insulation I've been looking at, 50/50 mix. It's density is 35 Kg/m3, but there's no frequency specific acoustic data, nor can I find gas flow resistance information.

I am guessing it would be fine at 4" thick, but for anything greater I'm concerned about it having a high resistance, dont really want it starting to reflect at say 8" thick.

[Ethan Winer]

It's tough to guess how well a given material will absorb. My general rule is that if a material has no absorption data available, you might as well skip it and find something that does have data.

--Ethan

[Brainchild]

Yeah, it's a total bitch. Since you know the materials and ratios of the material (half hemp, half cotton), and you know the total density, if you can research the average fiber diameter of the hemp and cotton fibers, you can calculate the flow resistivity from there. Kind of. To a certain extent.

[Arksun]

Originally Posted by Brainchild

Yeah, it's a total bitch. Since you know the materials and ratios of the material (half hemp, half cotton), and you know the total density, if you can research the average fiber diameter of the hemp and cotton fibers, you can calculate the flow resistivity from there. Kind of. To a certain extent.

The Thermo Hemp which is almost all hemp has resistivity of 6000 Pa-s/m2. Pretty damn good for making very thick stuff. (Someone on this forum made great use of this stuff. Was going to get this myself but there's big delay in new stock)

On the flip side UltraTouch Cotton product , whilst not having gas flow data, seems to perform poorly at lower freq with greater thickness according to their acoustic data.

Perhaps this Isonat hemp/cotton lies somewhere in the middle, but again its pure guesswork which is a bummer :/

I've fired off a couple emails, hopefully I'll get a response back.

*edit* would looking at its thermal performance figures give any additional clues i wonder?

[avare]

Originally Posted by Arksun

would looking at its thermal performance figures give any additional clues i wonder?

In absolutely no way what so ever.

BTW, it is difficult to judge the acoustics of hemp/cotton without knowing what type of binder is used. Adding to Ethan's "without acoustical data, don't use it.."

Andre

[Brainchild]

Guess how bored I am.

Here's a hint: I just spent some time looking up the fiber density and diameters of hemp and cotton [very similar to one another: ~94 lb./cu.ft. and ~21-28um].

Given that there's a lot of missing information - and a range of likely values covering roughly an order of magnitude - , I still think it's safe to say that the material probably has a pretty severely low flow resistance.

[ebon]

Hey,

the following will help finding any data around:

There is norm which explicitly defines a Echo-Room and therefore lets you find a Attenuation coefficient for any material: "DIN EN ISO 354"

The Room:


Information about the Room:
German: Hallraum - Wikipedia, translated to english
English: Echo Chamber


Just google for any material, add "DIN EN ISO 354" and "pdf"

Google Search Query


A Sample PDF of a company that does those tests officially and independent


Attenuation coefficient for 160mm Thermo Hemp - this one:
...
1,00 until 200Hz
0,85 until 100Hz







It comes from Mother Earth.




For our Bass Trapps, we use big jute sacks filled with straw, hemp and wood chips to fill up the volume, framed.

Our wide band absorbers are two 14mm hemp segments = 28mm depth, build up in a wooden frame for different absorbtion sufrace sizes.

Also Helmholtz resonators can be build with those material.

MDF wood is cheap


We don't really need any foam, folks.


greetz,
ebon

[Adam_Weishaupt]

Is it as goood as owens corning 705?