[Christof]

I'm planning to insulate the walls in my new loft to help absorb sound transmission between rooms.  I'm not sure which would work better, fiberglass or blown in cellulose.  Anybody know?  I should also add that these will be 2x6 walls and I'm robbing from my bathroom allowance to pay for the extra insulation.  I would like to keep costs minmal such that I don't have to end up bathing in a used 5 gallon drywall mud bucket, but an audiophile does have his priorities

[warnerwh]

If you're trying to isolate sound in your room you'll have much better luck using resilient channel. It's cheap too. Just fiberglass or blown in cellulose won't do any good. Resilient channel will let the sheetrock vibrate so the sound doesn't transfer through the wall.  Doing a layer of sheetrock before the resilient channel will also help.  If you use it make sure the drywall screws aren't so long as to go through the resilient channel and then into wood or you'd be losing any benefit.  Another thing I did was to place the resilient channel somewhat asymmetrical.

I did this on my ceiling as the living room is right above me and am very pleased. It's ok to rock out down here and I bother nobody. My ceiling did cost about 400 bucks (triple layered plus the R.C.) to do not to mention many hours of back breaking labor.

[Red Dragon Audio]

... Just fiberglass or blown in cellulose won't do any good.

That's not quite correct.     A wall with no insulation will pass much more sound than a wall that is well insulated. Blown-in type insulation is quite a bit better than your standard pink fiberglass bats.  This is due in part to their spray in nature which is very good at sealing off any nooks,holes and gaps that let air pass and thus sound.
(NuWool has a very good "spray-in-place" cellulose insulation product that reduces sound transmission tremendously. http://www.nuwool.com/products/celluloseinsulation/wallseal.html">WALLSEAL®)


Take the Movie Theater for example.   We've all been to a movie where the scene we're watching is rather quiet but the theater next to ours is showing a Jerry Bruckheimer movie with crazy action going on.

What sounds do you typically hear in that situation?

Low booming noise from explosions and things of that nature…
However you typically don't hear any mid/high frequency information because the walls are usually insulated with 2" thick fiberglass and thick velvet material from top to bottom on either side of a cinderblock/concrete wall that is sealed off (it is even likely that there are two cinderblock walls with a 6” gap between them for fire code which would help reduce sound transmission even more).

If that wall were just sheet rock and 2x6's with no insulation you'd hear significantly more mid/high frequency information and it would be rather annoying.

Now the opposite situation...
 a cheap hotel or apartment where you can hear the conversation from next door.  Usually this is because the builders cut corners and didn't insulate the walls or there are huge air gaps under the adjoining doors or perhaps the ventilation system connects the air volumes of both rooms.  

This brings us to the purpose and use for Resilient Channel…

Resilient Channel
Resilient Channel works better at minimizing low frequency energy transfer between rooms by isolating the sheetrock from the structural frame work.  This reduces sound transmision from vibration between rooms caused by low frequency sound waves.


....so...Blown-in cellulose or well stuffed pink fiberglass will be very beneficial for sound absorption.  Combine this with Resilient channel and you’ll get even better reduction in sound transmission between rooms.  Also warner’s suggestion of using more layers of sheetrock will also help reduce sound transmission.

 
The easiest thing would be to buy your wife and family a set of these Bose QuietComfort noise canceling headphones and save yourself time, trouble but maybe not money  

[warnerwh]

There is apparently cellulose insulation designed to help isolate sound that I'd been unaware of and is very effective.  In my experience regular R 19 insulation in the walls wasn't as effective as I'd hoped so I'd go with an insulation designed for sound isolation.  Wish I'd learned of this stuff BEFORE I put up the walls.

[JLM]

The "special" acoustically fiberglass offers little improvement over the garden variety fiberglass.  BTW fiberglass can be blown like cellulose too and in fact our new house used a newer method of blowing into the open cavities with wetted fiberglass.  I agree blown should be a bit better than batts.  If you live in a termite region or have mold, or respiratory concerns use the fiberglass (doesn't support a critters or need pesticides to prevent).

If your DIYself careful installation of batts will work fine as you can caulk gaps and take care with the batts to get good coverage (don't over pack).

With a 6 inch cavity the best option (used it on my current room) is to build two sets of 2x4 stud walls, one flush to each face of the 2x6 top and bottom plates and staggered to each other so they don't touch each other.  This eliminates the solid (sound transmitting) connection between rooms.  Then you can insulate.  

I also used a weather sealed/insulated fiberglass exterior door, insulation above the drywall ceiling, and insulated flexible ductwork.  The only real sound that comes through (I'm in a basement with no windows to the room) is via the ceiling as the builder refused to use gauge metal furring strips (resliient channel) for fear of cracking the drywall.  It's almost weird quite in here.

[Inscrutable]

I am looking to create the dead end of a LEDE room.  Had planned on going with two layers of drywall sandwiching Green Glue.  Do you think RC would be more effective - either directly on the studs or in between the two drywall layers - in lieu of GG?

[ctviggen]

If you have room for 2x6, build two 2x4 staggered walls with resilient channels and insulation (doesn't matter much which type; the "sound insulating" is only marginally better).  See:

http://soundproofing.org/infopages/soundwalls.html
http://www.sdsc.edu/~nadeau/Rebuilding/About/AboutSoundproofingWalls.htm

STC (sound transmision coefficient) is a reasonable approximation for music, though it only looks at certain frequencies.  Unfortunately, it's the only data that's typically published.

[Christof]

Thanks for the advise you guys.  My objective in insulating the walls of the loft(s) is two fold.  I want to have a better audio area for myself but since I am putting multiple lofts into this old building I would like to keep sound from traveling from unit to unit as well.  My unit will have deluxe treatment but the last thing I want to hear is someone elses Bose Soundwave pumping crap through my walls and I'm sure they don't want to hear what my gear does.