Why do concert halls have smooth plastic curved plates on the roof?
In rooms with flat, parallel surfaces (opposite walls, or ceiling & floor), you will hear an annoying reflection called “slap echo”. The sound waves are reflected back and forth between the two walls, and it is difficult to hear speech or music clearly. Slap echo can be eliminated either by placing sound-absorbing material on one of the parallel surfaces, or by making one surface curved, so that the reflected sound is dispersed. In a concert hall, we don’t want to absorb the sound, but to reflect the sound back to the audience. Therefore a curved, hard (reflective) surface is used. This is analogous to using a curved mirror in optics. The optimum “reverberation time” (time for a sound to die out) for a concert hall is typically 0.6 seconds to 1.1 seconds. Note: Concert halls designed for classical music (e.g. La Scalla) do NOT have curved plates on the ceiling made of PLASTIC — because a plastic surface will resonate at certain frequencies and thus will “color” the reflected sound. Th
