Old School soundproofing still believes that batt insulation is a good soundproofing agent. There are some good sound batt insulations out there such as Roxul AFB and Bonded logic's bonded cotton insulation and they work well for both soundproofing and thermal protection, however no batt insulation is strong enough to stand alone as the sole soundproofing material in a wall ceiling or floor assembly.
If you can afford a good batt insulation I always encourage it, but if you are soundproofing as ceiling and are not really concerned about the thermal aspects of the ceiling then I feel adding drywall pieces cut to fit perfectly into the joist cavities and screwed to the sub floor above would be much more effective than any batt insulation. Now to super charge this application I recommend that before you screw the pieces of drywall to the sub floor you apply Green Glue sound dampening compound to the back of the drywall pieces prior to screwing them to the bottom of the sub floor.
Green Glue is a visco elastic damping compound that is taking the soundproofing industry by storm. It comes in caulk tubes and is applied using a large quart size caulk gun. Basically you would cut the pieces of drywall to fit snuggly between the joist cavities. You would the liberally apply the Green Glue to the backside of the cut drywall and then screw it to the bottom of the sub floor. The Green Glue will be sandwiched between the drywall and the bottom of the sub floor. The tighter you screw the drywall down, the better the soundproofing will be. In essence, the pieces of drywall sporting the Green Glue in their backsides with become a most effective sound dampening system that will dampen the impact noise from above long before it is able to be transmitted into the joists and ultimately into the ceiling below. Now, is more better in this case? Absolutely, if you are able to double or triple the Green Glued layer of drywall you will indeed increase the dampening ability of the drywall and the sub floor and add greatly to the soundproofing endeavor.
Once you have installed your layers of Green Glued drywall pieces into the joist cavities, now would be a good time to install your sound or thermal batt insulation.
Now for our finished drywall ceiling, I recommend that we spend the money for 2 layers of 5/8" drywall. You will screw in your first layer of drywall directly to the ceiling joists. Now your second layer will be installed directly over top of the first layer however, you guessed it, I recommend that you apply 3 tubes of Green Glue to each 4' X 8' sheet of drywall that you put up. Once you have finished with the double drywall using the Green Glue, you should immediately detect a lack of annoying noise coming from the people above. You have soundproofed your ceiling and your neighbors don't even have a clue that there was a problem there. How awesome is that? Read more about Green Glue and soundproofing walls, ceilings and floors in subsequent articles, for now this is Dr. Bob....Out!!!

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