Why does wrapping rockwool in plastic not diminish performance?

[claz]

In an earlier thread, Hugh observed that wrapping rockwool in plastic only minimally affects performance.

(1) Why? (From a physical perspective.)
(2) I suppose high frequencies are the most negatively-impacted?
(3) I suppose the tightness of the wrapping affects the performance -- but is tighter or looser optimal?
(4) Would a few pinprick airholes improve performance (like the small cutouts in a sail)?

Hugh: or even fully wrap the battens with a thin and light polythene (eg. Clingfilm etc). Although counterintuitive, this doesn't appear to affect the performance significantly

(My curiosity about this is tangential to the previous thread, hence a new post.)

[Hugh Robjohns]

It's intriguing, isn't it?

I have found, first hand, that wrapping seems to have negligible effect on bass trapping. I presume that if the membrane can move it stimulates the movement of air inside the rockwool and thus the frictional losses still occur and the acoustic energy is absorbed more or less as normal.

It has been reported to me from a few different and independent sources that the idea still works adequately for broadband traps in practical applications, but I've not seen any scientific measurements to confirm or deny the claim.

Logically, I'd expect decreasing hf absorption and increasing hf reflection... with tighter membranes being worse than looser ones. But that is only supposition.

Pin pricks would negate the aim of sealing in fibres and off-gassing.

For those with deep concerns over air quality I suspect any minor reduction in efficacy would be outweighed by peace of mind and the inevitable improvement in overall acoustic performance in the room, even if it's not ultimately as good as it might be in a more conventional configuration.

[ef37a]

Then, even a very thick rockwool slab is only going to be a fraction of a wavelength at say 60Hz so low frequencies will pass through it (as they do for light walls) losing energy as they go?

Dave.

[Bob Bickerton]

The bass traps I made myself, using a design specified by Marshall Day after measurement of my room, are constructed of a wooden frame filled with dense rock wool but having a thin metal plate on the front surface both suspended and sealed on a rubber gasket. On top of the metal plate there’s another layer of acoustic material, less dense than rock wool and around 20mm thick.

I presume the metal plate acts as a membrane which enhances the efficiency of the rock wool absorption. That being the case wrapping your rock wool in a plastic membrane may actual improve things …………. An uneducated guess of course!

Bob

[ManFromGlass]

A colleague who needed bass trapping was told to leave the Roxul in it’s unopened thin plastic wrapping.
*I’m assuming Roxul over here is the same as your rockwool over there.

[claz]

Makes sense. It would be interesting to measure room response at a given position with/out covering!

[Martin Walker]

Logically, I'd expect decreasing hf absorption and increasing hf reflection... with tighter membranes being worse than looser ones. But that is only supposition.

Pin pricks would negate the aim of sealing in fibres and off-gassing.

I've often thought of introducing extensive pinpricks into the Real Traps 4-inch thick Minitraps that I've placed at my first reflection positions, as when I bought them after writing the SOS review I had no idea that they incorporated an outer layer of foil.

Their designer Ethan Winer was concerned about ending up with a dead-sounding room with lots of his thicker basstraps fitted, but omitted the foil in the thinner 2-inch thick Microtraps that he intended to absorb side reflections.

I've currently got an acoustic foam tile balanced on the side Minitraps, and my stereo imaging sounds noticeably more focused, but setting to with a pin to the side Minitraps themselves would look a lot more elegant

Martin