How do you reduce the sound transmission through a floor? Using "green glue" between a couple layers of plywood subfloor seems to work well. Are there any other tricks or suggestions?
How to reduce floor sound transmission
floor
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Best Answer
Recording studios have to solve this problem all the time. You may find good information by hunting under that aegis.
One thing to take note is that transmission of high frequency and low frequency have very different solutions. High frequency it's mostly a matter of stopping the air motion; since you're taking about floors, you want to damp out low freq:
Large mass helps by converting vibrations to lower (subsonic, inaudible) frequencies. The extra layer of subfloor you added is extra mass. The canonical solution to blocking lowfreq is a big bookcase full of heavy books; think about that on its side.
Physical isolation with a sound-dampening substance also helps--reducing the amplitude of the transmitted vibration. For example, rigid foam strapping over the joists before nailing the subfloor down might assist with the noise transmission through the wood. This sounds like your 'joist isolators'. Or a thick pad under a carpet (if there is a carpet).
Ironically by tying your subfloor tightly to the joists, you have increased sound transmission.
Spoiling the emittance of the listening side also helps. The ceiling of the floor below is acting as a big, taut speaker, magnifying sounds from above. This is what QuietRock does--it absorbs energy as it flexes, turning it to heat instead of sound. Putting irregular structure behind the wall might help alter its resonance, prevent booming noises from transmitting.
Sound diffusers help break up the sound. Because the sound you are creating is things like footsteps, this is harder to do--but be aware that the upper level can be amplifying the sounds by aiming them at the floor (think of the extreme case of a dome, which will reflect noise straight back at the floor). Acoustic tile in the upstairs room might help with high freq reduction, or baffles in the corners--or just more bookcases and sofas upstairs.
You can also add diffusers to the downstairs room, but you run the risk of making it sound 'dead' or making it quiet enough the muffled noise from upstairs becomes more annoying. ;)
Hope this works out!