Avalon Acoustics AVALON INDRA - Manual (Page 24)

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Avoiding Early Reflections
The speed of sound is approximately one foot (30 cm) per millisecond.
Therefore, to preserve the natural soundstage on your recordings, there
should be no reflected sounds arriving at the listening position with a path
length less than ten feet (3 meters) longer than the direct path from speaker
to listener (see Figure 8.1). This means that if the speaker or listener is placed
closer than about 5 feet to a wall or other surface, that surface should be
covered with sonically absorbent material.
Reflected Sound
Direct Sound
Figure 8.1 - The reflected sound must travel further than the direct
sound, and therefore reaches the listener at a later time.