Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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If you have reflections that arrive at the listener more than 30 milliseconds after the direct
sound, you'll want to fix that because these reflections sound like echoes and destroy the
intelligibility of the sound. You have two options when dealing with late reflections. The first is
simply to absorb them by attaching to the reflective surface something absorptive like a thick
curtain or acoustic absorption tile (Figure 4.36). The other option is to diffuse the reflection.
Figure 4.36 Acoustic absorption tile
When reflections get close enough together, they cause reverberation. Reverberant
sound can be a very nice addition to the sound as long as the reverberant sound is quieter than
the direct sound. The relationship between the direct and reverberant sound is called the direct to
reverberant ratio. If that ratio is too low, you'll have intelligibility problems. Diffusing a late
reflection using diffusion tiles (Figure 4.37) generates several random reflections instead of a
single one. If done correctly, diffusion converts the late reflection into reverberation. If the
reverberant sound in the room is already at a sufficient level and duration, then absorbing the late
reflection is probably the best route. For more information on identifying reflections in the room,
see Section 4.2.2.3.
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