REFLECTION DELAY TIME, ms the effect of reflections for various amplitudes and delay times, as simulated in an ane-choic environment. Below curve B the echo increases the perception of spaciousness, while below curve A the reflected sound is reduced to inaudibility. Above curve C the reflection is perceived as an echo.

Echo and reverberation are not the same thing. Echo is a repetition of the original sound that is distinctly perceptible, whereas reverberation is a prolongation of the sound through multiple reflections, which is frequently beneficial for music. Long-delayed reflections are like echoes, but have a somewhat shorter delay time. They are not perceived as separate sounds, but blur the understanding of the original sound. Flutter echoes are sounds that persist locally due to multiple reflections between parallel planes, concave, or chevroned surfaces. They can be caused by two, three, or more reflections. Figure 17.13 gives several examples of acoustical defects.

Coloration is the emphasis of certain frequencies or frequency bands over others. It can be caused by room-mode buildup or by absorptive materials that only absorb in certain frequency ranges. Focusing is the buildup of sound energy in localized regions of a room, due to concave surfaces. Shadowing is the blockage of sound traveling from the source, or from a significant reflecting surface, to the receiver. Each of these defects can detract from the overall acoustical environment in a room and each can be avoided with careful design.

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