A sound diffuser
is not to be mixed up with sound absorber. While a sound absorber absorbs the audio-waves, the sound diffuser sends the audio-waves in many different directions. This is good if you don't want an entirely dry room, but want some sound reflections for recording, say, drums.
The more expensive diffusers sends audio waves in different direction based on frequency.
- Everest, F. Alton. The Master Handbook of Acoustics, McGraw-Hill, 2000 (ISBN 0-07-136097-2).