Bookshelves, drywall layers, acoustic panels and acoustic foam are all relatively inexpensive ways to soundproof an interior wall. Drapes and furniture also absorb sound and help reduce echoes.
Sound bounces off bare, hard surfaces, raising noise levels in a given space. Filling empty spaces with soft furnishings, such as beanbags or floor cushions, reduces sound and echoes. Bookshelves filled with books and layered drapes along walls and windows also help reduce sound.
Adding a second layer of drywall makes it harder for sound to travel through a wall. The drywall must be of a different thickness than the original layer or else the sound will simply transmit from one layer to the other. The two layers of drywall should be separated by 3/8-inch thick beads of acoustical caulk. Acoustical caulk can also be used to plug sound leaks, such as openings around ceiling fixtures, electrical boxes or door casings. Adding sweeps to the bottom of doors and weather-stripping to door frames also prevents sound leaks.
Acoustic panels and special acoustic foam absorb sounds before they can bounce off walls and ceilings, reducing sound transmission. Acoustic panels can be mounted on walls using Velcro or clips. Acoustic foam can be leaned against walls or installed between layers of drywall.