Question 2: Why is the perception different for the two tones?
There are two reasons.
- The ITD (and, thus, the implied change in location) is smaller for the 2000-Hz tone than for the 500-Hz tone. So all else equal, the perception should be weaker for the 2000-Hz tone. However, the ITD for the 2000-Hz tone (0.0625 ms) is generally large enough for a human to perceive, so that cannot be the only explanation.
- The phase locking of auditory nerve fibers is strong for 500 Hz but weak for 2000 Hz. Without strong phase locking, the brain is unable to discern the ITD of the sound and, thus, unable to create a localized perception.