RESOLUTION OF THE EAR
Depending on the temporal separation between direct sound and its reflection, the result might either be perceived as a phantom source or, at very high temporal separations, as an echo. In this case, the reflection is perceived as a separate sound event. This is very irritating and must therefore be avoided in room acoustical planning. Due to a variety of influences, there is also a wide scattering range for the timeshift of echo perception. For speech, 100 ms is usually taken as the echo limit. Only the sum of many slightly time-delayed reflections is finally perceived as reverberation.
