Common acoustic defects

Four common acoustic defects are as follows.

1. Echo: When a reflected sound reaches the listener more than about 0.1 second (some textbooks say 1/15 s) after the direct sound, it is heard as a separate repetition. It occurs in large halls with parallel hard walls far from the source. Remedy: cover the reflecting surface with absorbent materials such as acoustic plaster, curtains or panels; make the walls irregular.
2. Reverberation: The sound persists in the room after the source has stopped because of multiple reflections. Excess reverberation makes speech unclear and music muddy. Optimum reverberation time is about 0.5 to 1 s for speech halls. Remedy: raise the absorption (carpets, curtains, acoustic ceiling, upholstered seats). Reverberation time by Sabine is $T = 0.16V/A$.
3. Sound foci and dead spots: Concave surfaces, such as domes and curved walls, concentrate reflected sound at some points (foci) and leave other places with very low sound (dead spots). Remedy: avoid concave surfaces, or cover them with absorbing material; use convex diffusers.
4. Resonance: Some objects or air spaces vibrate at the same frequency as the sound, increasing certain tones. Remedy: damp the panels with absorbent backing, and avoid hollow spaces of resonant size.

Other defects are echelon effect, sound shadow, and external noise.