Hearing

The subjective human response to sound.

The ability to hold a conversation depends on the background noise level and the distance between the people.

1 dB deviation per critical band is noticeable in direct comparison. Even smaller deviations can be picked up by trained listeners.
+/-3-5dB deviations are not easy to perceive when there is no immediate reference, except for trained listeners.
Group delay differences of roughly 1ms are noticed in extreme cases.
Group delay differences of about 2ms become noticeable in listening tests. This equates to approximately 70cm difference in propagation length.
5 to 10ms group delay differences may be disturbing, however, 50 to 100ms group delay errors are tolerable depending on signal type.