Acoustic foam reduces reflections by trapping sound energy in small open pockets of air.

The longer version

Open-cell foam is mostly air, so an arriving wave pushes into a maze of connected pockets. Viscous drag and friction along the countless internal surfaces turn organised motion into a small amount of heat, and little of the energy is reflected back out. Wedges and pyramids also present a gradual change in density, which reduces surface reflection.

Absorption is strongly frequency dependent: thin material barely touches long wavelengths, which pass by almost unchanged, while short wavelengths are soaked up easily. That is why thin treatment dries out a bright room without removing low rumble, and why thickness or an air gap is needed for the bottom end.

Where this entry sits

Sound and Music gathers facts about how vibrations move through air and water, how ears turn them into meaning, and how instruments and notation shape what listeners hear. Expect echoes, pitch, volume, acoustics, and the odd silence in between. The full list sits on the Sound and Music page, where every entry is listed in order.