Sound Absorbing Fabric
Fabric that will absorb sound frequency waves is thick and porous. The waves enter into the fabric, and some become trapped in the fibers and folds and convert from sound energy into heat. Sound energy that makes it through the cloth hits the wall or window 6” to 12” beyond.
It is reflected into the curtain again. Some more noise will be absorbed. The waves that pass back through the curtain into the room will be softened and dampened. Echo and reverberation are controlled, and listening enjoyment is improved.
The best fabric is thick, 32-ounce velour type cloth. Heavy blackout curtains are good too, especially multi-layered curtains. Some fabrics have an NRC (noise reduction coefficient), the higher the value, and the better at decreasing reverberation, reflection, and echo.
Acoustically Transparent Fabrics
Transparent fabrics are used to cover acoustic panels, bass traps, and diffusers that are made of absorbent insulation. They cover and contain the fibers of the insulation while permitting soundwaves to pass through as if they weren’t there. Many fabrics are transparent.
To check if a fabric is transparent, hold it up to your mouth and blow through it. If the air passes easily through it, it’s transparent. (...)