Filling a stud cavity with fiberglass or mineral wool absorbs sound inside the wall and damps cavity resonance. In a single stud wall it adds about 3–5 STC; in decoupled walls it adds more, often 8–10 points.
- Use unfaced batts that fill the cavity without compressing.
- Density matters less than people think; full fill matters more.
- Spray foam is a poor acoustic insulator: it is rigid and can couple the faces.