Sealed sound

[Sound22Card]

Would containing some of these loose fibre sound insulation products....within a sealed plastic bag, defeat the sound absorbing qualities?

 

[mushroommunk]

The answer, as with everything engineering based, is: it depends. The higher frequencies will reflect off then, but some of the lower frequencies will still get absorbed. At what point it transitions from absorption to reflection will depend on the thickness of the plastic. If the bag is completely sealed it may act more like a limp membrane trap and show a bit of a q to its effectiveness, but I'm not experienced enough to predict more.

 

[Sound22Card]

Thanks for that, mush. I thought of the limp film effect of dissipating low end too. I might try a couple of flat-garbage bag sized ones, but with a thicker plastic film, filled with just enough air to allow easy movement of the plastic by an imagined bass wave. I'd assume they should go on the far back wall for best effect?

 

[mushroommunk]

Kind of hard to say, corners could still work quite well. I think this is just something you'd have to play around with. I should say if you do decide to try this, don't pack the bag, looseish is still probably preferred.

 

[hollowboy]

I'd assume they should go on the far back wall for best effect?

Bass trap - Wikipedia

"porous traps are most efficient at the points of high particle velocity like 1/4 desired wavelength away from the wall"

Sound-Absorbing Drapery: Theory & Application

"more effective at absorbing longer sound wavelengths (lower pitches) if it is spaced several inches from the wall"

...but IMO, speaker stuffing doesn't matter much either way for low frequencies. When I tested a solidly built speaker, I found that almost everything below about 500Hz will simply leak out through the cone.

See post 127:

Measuring sound output from speaker cabinet walls

...it shows that low frequencies go straight through a moderate amount of stuffing, and straight through the speaker cones, with barely any attenuation.

Even in the "overkill" example, there was only ~10dB of attenuation in the 200-400Hz octave.

 

[Richidoo]

Someone (I don't remember who) that was related to Feastrex japanese drivers way back when, once described to me a tweak among Japanese single driver enthusiasts. They put an acoustic absorbent material, like fluffed wool, or silk or goose down, or the most exotic absorbent materials imaginable inside a small paper bag, and hang it by a string inside the center of the speaker behind the (single) driver. The absorbent material provides some mid freq damping but supposedly has less negative effect on high frequency. I never saw or heard it, but do remember hearing about it. fwiw...

 

[RevMen]

In noise control it's pretty common to have bagged absorption. Sometimes you have to protect your absorber from dust or microbes, like in some factories or in the HVAC system of hospitals. If the plastic is thin enough it doesn't have a meaningful effect in any audible frequencies.