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Bass trap mystery fail / bad rockwool?
Old 18th July 2020
  #1
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Bass trap mystery fail / bad rockwool?

So after reading a ton here, I tried my luck at building a very simple bass trap - porous absorbers are so simple, it can't go wrong, right? Using Rockwool RockSono Solid panels (45kg/m3), 70mm thick per plate, I stuffed a box (600x500x280mm) with it, 4 layers deep. Not sure what the Pa.s/m2 is of this stuff, but I heard something about 20k, which does okay in simulations.

Okay, so the box is stuffed and wrapped with a thin curtain (leftover Ikea Hannalena, very open/breathable, and tested transparent at by throwing it over the sub), and now it's time to test, but how? I know, maybe I can run REW, and do a before-after comparison, with the box pushed flush against the thing making noise so all the sound has to go past it. Or around, if it reflects off the rockwool and manages to sneak out via the 5mm-ish speaker grill gap. Run REW, do a sweep, and what I measured was a broadband absorber that works only above 300hz. Okay, what. This needs more investigation.

Putting the bass trap in front of the speaker, with 10cm of space left, and grabbing a heavy laminated high gloss white wooden panel, I did a series of test. Put REW on tone generator mode, put the UMIK-1 on the tower speaker, aimed forward, and tested reflectivity. As absorption is 1-R (if R is expressed in power), we can assume what isn't reflected is absorbed and indirectly measure absorption at near field.

1000hz, 500hz: absorber vs blocked: -4dB difference. Rockwool this deep works very well at 1kHz, so -4dB is basically the maximum this test method will show.
300hz: -2 dB difference
200hz: ~-1 dB
100hz: unmeasurable

Right behind the absorber it's at least 10dB quieter so it isn't leaking sound through it; but I don't want bass to just bounce off it, I want it to go through it. It is almost as if these Rockrool plates just bounce the sound off instead of absorbing it.


Next test: repeating it with the leftover Rockwool. 100x60x28cm, in foil (taped the package back together). Same result. Oi, it is supposed to go through it and get absorbed.

Has anyone seen anything like this before? What now, would shredding the rockwool by hand / using a bread knife to create fluff to fill a box work better? Or what brands/types of insulation worked (measurably) well for you?

I'm located in the Netherlands, so anything sold in here, or in Europe would be preferred.
Attached Thumbnails
Bass trap mystery fail / bad rockwool?-img_20200717_213204.jpg   Bass trap mystery fail / bad rockwool?-img_20200717_233008.jpg   Bass trap mystery fail / bad rockwool?-bass-trap-test-2.jpg  

Last edited by Booinky; 18th July 2020 at 04:44 PM.. Reason: REW chart got lost
Old 28th July 2020
  #2
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akebrake's Avatar
 

Quote:
Originally Posted by Booinky View Post
... 70mm thick per plate, I stuffed a box (600x500x280mm) with it, 4 layers deep. Not sure what the Pa.s/m2 is of this stuff, but I heard something about 20k, which does okay in simulations.
Sorry, I don't understand your bass trap experiment.

Are you trying to "block" sound from the speaker with a 280mm soft absorber?

Best
Old 29th July 2020
  #3
Here for the gear
 

Same here, what is it about ?
The efficiency has to be measured when on a wall, at a strategically identified location, by measuring from the listening point....
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