DIY & Home Improvement

As a result, what you'd to help with sound insulation is radically different.

When it comes to houses, the main places you need to do something different is on top of the floor, and below the ceiling.

Normal construction:
Joists are put across the walls to form the structural support for the floor and ceiling. Then 1-1/8" or thicker plywood is nailed on top of those joists. The finish flooring material is put on top of that plywood.
Below, drywall is directly screwed to the joists to form the ceiling.

Ways to make it quieter:
-Use ring shank nails when nailing in the plywood. They prevent squeaks. Squeaky floors are caused by the wood moving up and down on a nail that has become slightly loose. Ring shank nails don't come loose. In fact they're really damn hard to pull out if needed.

Upgrading the nails is the easiest thing to do, and something you can easily retrofit if you're changing out the old finished flooring.

-Put insulation between the joists - since it's an interior space, it isn't usually insulated. Adding insulation means you won't transmit sound through the air between the joists. But most of the sound goes through the solid materials.

-Use a flexible adhesive to attach a second layer of drywall to the ceiling. This absorbs some of the vibration, and thus the sound. Again, can be easily retrofitted to existing houses - most people won't notice the ceiling being about 1/2" shorter.

-Instead of screwing the ceiling drywall directly to the joists, you attach a furring channel to the joists, and then attach the drywall to the channel. The channel is able to absorb some of the vibration instead of transferring it to the drywall.

Also can be retrofitted, but significantly more work.

-On top of the plywood floor, put on a layer of high-density foam insulation. Also line the bottom ~2" of the walls with this insulation. Then pour lightweight concrete on top of the insulation. Put your finished floor on top of that concrete. The mass of the concrete reduces the movement, and the insulation helps isolate the concrete from the plywood, absorbing what noise does pass through.

This requires the house be engineered for the extra weight of the concrete, and the stairs have to be built to account for the extra thickness of the floor.

-Once you've done the concrete thing, you can add sound insulating material between the concrete and the finish flooring. The top is fabric, the bottom is a mesh of plastic. Again, flexible so it can avoid transmitting vibration.