How to sister joists

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:

1. 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.

2. 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.

3. 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.

4. 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!

I'm really fascinated by your question and suggested solutions. I am glad to hear that you question the wisdom of some of the suggestions. There are a few factors that are important to consider before picking a solution.

What type of hardwood flooring are you thinking of using? Nail down, staple down, glue down or floating? With any type but a floating floor, SLC is definately the wrong product to use underneath, as it will shatter when nailed, and the thinner areas will probably separate from the underlayment when glued to. I also expect SLC will not adhear well to the old vinyl flooring or even the plywood subfloor. Any flex of the plywood subfloor will result in cracks and broken chunks of concrete.

I would explore the possibility of jacking the lower end up one inch. This would of course depend on what type of foundation it is resting on, the roof structure, if the existing ceiling is now level, or does it also slope the same one inch, and finally, if there are any windows in the side walls, as it would effect the level of those as well if jacked.

Assuming jacking is not an option, the next easiest way would be to install "purlins" or surface shim boards every 12 to 16 inches on center across the floor in decreasing thickness and install a new 3/4 inch subfloor over these, screwing it down all the way into the old floor.

The most work, but successful method would be to remove the old floor completely and sister the original floor joists. Using full size sister joists would not be necessary. 2X4s or 2X6's would be fine as long as a good bond was maintained to the existing 2X8's. The custom wedge idea would work fine, but potentially difficult to rip long lengths for each joist and have them all consistent without a good ripping template, but could be done with a little forethought and clever rip guide.