Mineral Wool Density ?

[Michael Jones]

A local supplier of mineral wool has 4" 8lb/cu.ft. available.
This seems much more dense than what I thought 4" mineral wool was - I thought it was around 3-5lbs/cu.ft.

Are there any problems going with the denser materal? The 8lb?
Its for an exterior, staggered stud wall frame.

[SHEPPARDB.]

Mike,I dont know if it will be a problem or not.(I would'nt think so,
but you should ask John or Ethan.)I used some in certain areas
of my studio and it is not a problem as far as I can tell.I am not
finished with my studio yet,but I tested the live room with an
acoustic guitar before and after the rockwool.The rockwool
definately absorbs sound well.I only did two walls with the 4"
8lb. density,the rest I did with 2" 8lb. density.I would use the
2" where you are able to because you can get twice as much
of it for the same price as 4".

[Michael Jones]

Thanks Sheppard.
The supplier has 4"-4lb and 4"-8lb.
The 8lb he has in stock @ $0.90/sf.
The 4lb he has to order, and its $0.78/sf.
2-3 weeks for the 4lb.

I'll have to search John's forum. Seems like I remember this issue comming up before.

[Michael Jones]

Here's what I remember:

"When placed INSIDE a sound wall, the rockwool helps act as a trap and improves sound attenuation. The recommended density when used inside a wall is 3 PCF, same as 703. About half the air cavity taken up with insulation seems to be optimum. Heavier than 3pcf improves bass loss slightly, at the cost of higher freq's getting through. Lighter than 3pcf and the opposite happens (bass performance worse, highs better)"

-Knightfly

Hmmm.....

[knightfly]

Michael, that's another one of my late-night, polarity reversal brain farts - I hope it's the LAST, since I've gotten most of my references cataloged well enough to double-check - here's a quote from the USG study I THOUGHT I remembered -

"The best overall acoustic performance was achieved with a mineral-fiber blanket in the 2.5 to 3.0 Ib/cu.ft. density range. Densities above 3 lb./cu.ft. provide more sound attenuation at high frequencies, but low-frequency performance starts to suffer at higher densities. Mid and high-frequency performance goes down significantly as insulation density falls below 2.5 lb/cu.ft. The conclusion to manufacture standard THERMAFIBER Sound Attenuation Fire Blankets (SAFB) with a nominal density of 2.5 lb/cu.ft. was based on the acoustic research, cost-to-benefit ratios, fire performance and handling characteristics." -

Note the polarity reversal in the second sentence...

And here's the link to the entire article FYI -

http://www.usg.com/Design_Solutions/...lationperf.asp

For your exterior, staggered stud frame, weaving through the studs will help in another way - usually, this causes the insulation to contact the panel in the middle of its normally free span, which will contribute to panel damping. Damping reduces the Coincidence Dip in TL, normally between 2 kHz and 4 kHz with most drywall panels. This improves TL at those frequencies (less Coincidence Dip, better TL) Damping also helps on the low end TL - HOWEVER - if you were to use the higher density insulation, it's likely that its increased stiffness would cause you to get more than just damping, more like flanking. This would REDUCE the wall's performance, nearly back to the same as using non-staggered stud frames.

Here's a page that explains this better -

http://www.domesticsoundproofing.co.uk/tloss.htm

For that reason, as well as worse bass performance, I'd stay away from the higher density stuff. STC ratings are already ignoring bass and giving us a false expectation of good isolation with bogus ratings, there's no sense in exacerbating the problem by biasing insulation performance toward treble frequencies.

Sorry about the brain fart, maybe if I tried to get more than 4 hours sleep a night... Naah, sleep is 'way over-rated... Steve

[Michael Jones]

I guess I don't understand the premis of the observation:
If we all agree that Mass and Density are the only effective methods of sound attenuation, then the higher density material would seem to be more beneficial.
Additionally, as the density of a material increases, so too should its sound attenuating qualities; particularly in the lower end of the sonic spectrum.

However, the article seems to suggest the opposite.

[laptoppop]

I think there are two things happening here - mass/density and coupling. If you couple two surfaces so that they both vibrate at the same time, they will act as a big drum and will pass sound very efficiently. So you end up wanting both mass and a lack of coupling. If the insulation is too stiff, it couples the surfaces. If its not dense enough, it won't do much attenuation. If it is too dense, the high frequencies suffer. Oh man! My head is hurting!

-lee-

[Michael Jones]

"...If the insulation is too stiff, it couples the surfaces. ..."

[knightfly]

Shit, you guise don't need me anymore, I'm goin' fishin'...

[Han]

You might find this interesting: http://www.recording.org/ubb/ultimat...4;t=000782;p=1