system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the helmet microphones that a helicopter or small plane pilot uses only pick up voice and not the helicopter engine?
gpt
The use active noise reduction. There are two microphones, one pointed towards the mouth and one pointed away. One picks up speech + noise and the other just picks up noise. Subtract the 2nd signal from the first and you are just left with the speech. 

Similar technology is used in telephones, noise cancelling headphones, fetal heart beat monitors, etc.