Aus, you got at least a 98 on that :=) - actually, though, walls/windows (as far as isolation goes, NOT acoustic tendencies) are the same reduction both directions. The real thing that matters with glass is to at least equal the mass of each leaf with the mass of its respective glass - so, if you had a slightly heavier leaf on one side of a mass-air-mass wall, that's the side you'd put the heavier glass.
Glass weighs roughly 3 times what an equivalent volume of gypsum wallboard weighs, so theoretically if you had 3 layers of 1/2" wallboard on a wall that side's glass should be 1/2" to balance out the mass. In practice, slightly heavier is closer to working the best. If the other side of this fictitious wall were to have, say, two layers of 5/8" wallboard (15mm) then divide the total wallboard thickness of that side (30mm) by 3, and use at least 10mm glass on that side.
In practice, using 1/2" and 5/8" would be the best choice. And, as Ausrock mentioned, laminated performs better than solid glass.
Jeff, don't feel like the Lone Ranger here; nearly everybody just getting started in acoustics assumes the same things. Here's the basic reasoning - if you were to take an 8" long spring, and tried to compress it, it would take a given amount of pressure to do so, right? OK, now take that spring, cut it in half (two short springs) and try to compress it - takes more pressure because you have less length of spring material so therefore less "give", right?
OK, in acoustics, air is a spring (even if it's filled with porous material, like insulation) - so, when you put two pieces of glass in a window with an 8" air space, you have effectively two masses separated by an 8" spring. Now, add a third piece of glass centered between the other two - now, you have 3 masses separated by twice as stiff springs (half the length) so you get better coupling between the two extreme sides of the assembly - great for coupling, SUCKS for isolation...
In actual practice, there are times when a "triple leaf" barrier is used intentionally - what happens when you add that third leaf is an improvement in midrange Transmission loss, with a loss of up to 20 dB or more of isolation at lower, bass frequencies - so, the only time you would want a three-leaf wall or window is when you're trying to block human voice (or similar frequencies) and don't give a rat's patootie about bass and drums.
Things that improve isolation, whether in a wall, window, door, floor, ceiling, whatever -
Using only two centers of mass (leaves)
De-coupling the two leaves, either with Resilient mounting or separate frames.
Sealing every joint with real Acoustic-rated caulk.
Each leaf can be any number of layers, just no separation between them.
Air space can(should) have fiberglass or mineral wool insulation - 2.5 PCF is ideal for music.
More mass per leaf is better. Unequal mass in opposing leaves is better.
More air space between leaves is better. At around 12", you reach a point where little improvement can be had without serious loss of floor space.
The second layer of wallboard on each side is a large change in performance -
The third layer is quite noticeable, especially for bass.
The fourth layer may take instrumentation to see the difference.
Don't glue ANYTHING in a wall, with the possible exception of the first layer of wallboard when using slightly untrue wood studs - then, the glue may help eliminate possible rattles.
Guess that's about it in a nutshell - there's a lot more, but you only asked about windows, sorry if I rambled... Steve
