Soundproof a Wall With Putty Pads, Mineral Wool, and Damped Double Drywall

The Problem

Sound finds the cheap paths through a wall: straight through lightweight drywall, through empty stud cavities, and — most overlooked — through every outlet box and wire penetration, which are literal holes in the assembly. Soundproofing that ignores the boxes is a bucket with holes in it.

The Technique

An ADU builder's best-for-the-price system, roughly $3.25 per square foot in materials plus putty pads:

1. Seal the penetrations first. Press acoustic putty pads around every outlet box and wire penetration — the material works like modeling clay: peel the backing, mold it carefully around where wires enter, and wrap the back and bottom of the box until it's sealed in. Bonus: the putty is also fire-resistive and doubles as a firestop.
2. Fill the cavity with mineral wool. Its density is what makes it work better than fluffier batts.
3. First layer of 5/8-inch drywall. The high-density stuff — mass is the point.
4. Damping compound between layers. Squirt acoustic damping compound across the first layer, then hang the second layer of 5/8 over it. The sandwich — mass, viscoelastic layer, mass — converts vibration to heat instead of passing it through. The second layer is your finish surface.

For ceilings — an over-under ADU, say — add resilient channel (RC) between framing and drywall to break the vibration path; for walls, the four steps above are the system.