Can you hear This?
　　When something creates a sound wave in a room or an auditorium, listeners hear the sound wave directly from the source. They also hear the reflections as the sound bounces off the walls, floor, and ceiling. These are called the reflected wave or reverberant(反射的)sound, which can be heard even after the sound is no longer coming from the source.
　　The reverberation time of an auditorium is determined by the volume or interior size of the auditorium. It is also determined by how well or how poorly the walls, ceiling, floor, and contents of the room (including the people) absorb sound. There is no ideal reverberation time. Because each use of an auditorium calls for different reverberation. Speech needs to be understood clearly: therefore rooms used for talking must have a short reverberation time. The full-sound performance of music such as Wagner operas or Mahler symphonies should have a long reverberation time. The light, rapid musical passages of Bach or Mozart need a reverberation time somewhere between.
　　Acoustic problems often are caused by poor auditorium design. Smooth, curved reflecting surfaces create large reflections. Parallel(平行的)walls reflect sound back and forth, creating a rapid, repetitive pulsing effect. Large pillars(柱)and corners can cause acoustic shadows as the sound waves try to pass around the object. Some of these problems can be solved by using absorbers and reflectors to change the reverberation time of a room. For example, hanging large reflectors, called clouds, over the performers will allow some sound frequencies to reflect and others to pass to achieve a pleasing mixture of sound.
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This passage is mainly about_____

A.sound waves and their effects.
B.the types of music orchestras play.
C.walls of an auditorium.
D.the design of an auditorium.