When the source moves around the head in the median plane, the changes
are more subtle. Arrival time is more or less the same, as one would expect.
The main changes are in the relative arrival times and strengths of the
pinna reflections. This
shows up in the frequency domain as a notch whose frequency changes with
elevation (see the HRTF
in the median plane). Note that the difference between front and back
shows up in the mild but clear lack of symmetry about a horizontal line
at 90 degrees elevation.
(In case you are wondering what causes the faint streaks sloping at about
a 30° degree angle at the bottom of the picture, they have been traced
to a floor echo. It is hard to get experimental data that is free from artifacts.)
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Horizontal 
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