Decorative stretched fabric over empty walls turns a room acoustically dead.

Evidence

In custom listening rooms it is common to cover large wall surfaces with stretched fabric to cover what might be otherwise an unattractive collection of diffusers, absorbers and loudspeakers. This is more than a visual alteration to the room because fabric has flow resistance; if you cannot easily see daylight through a fabric, or blow through it, it will not be acoustically transparent. Such fabrics, with nothing more than empty space behind them, become absorbers, and experience shows that many listening rooms end up being unpleasantly dead because of it. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1379