The pinna (outer ear) is not just a passive funnel — its complex folds create frequency-specific amplifications and cancellations that change with sound direction. A 6 kHz boost might indicate 'sound is coming from above'; a 10 kHz notch might indicate 'sound is from behind.' This is how you can tell if a sound is in front of or behind you — even with one ear covered.
Pinna gain is essential for HRTF-based spatial audio. The pinna's effect is why in-ear microphones (which bypass the pinna) sound different from over-ear recordings. For headphone listening, pinna activation (the sense that sound is external rather than inside your head) depends on accurate HRTF reproduction of pinna effects.