system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: You can locate an airplane when you hear one with reasonable accuracy. But why is it much harder to locate a helicopter?
gpt
You can make use of the Doppler effect to gauge the direction of incoming sound waves (changes in pitch correlate with the direction and motion of an object). 

Helicopters move a lot slower. Especially if they're just hovering, you lose one source of information about it's direction. So it's harder to locate.