Why do air compressors make sounds?

[aiop]

Why do compressors for example in the fridge make that vibrating sound, I've heard a similar vibrating noise in fans which are compressing air as well... i think. Why does compressing air make sound? Can air hitting air create sound waves?

 

[Algr]

Well you really can't move air without moving parts. (Except maybe a nebula in space being pushed by solar wind?) It is usually a motor and some kind of piston that you are hearing. Sound waves ARE air hitting air, so I'd think that yes, even if no solid objects are involved, moving air would make sound.

 

[jack action]

Anything that vibrates within a medium will produce a sound wave.

If the frequency of the sound wave is between 20 Hz and 20 000 Hz and its pressure is above 20 µPa, then it is in the hearing range for humans.

More info:
https://en.wikipedia.org/wiki/Sound#Physics_of_sound
https://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level