Why does the pitch of an airplane sound like it goes from high to low when it flies overhead?
In: Physics
If I throw a ball at you while I’m running at you, the total speed of the ball is how hard I threw it + however fast I was running. If I throw a ball at you while I’m running away from you, the speed of the ball is how hard I threw it – however fast I’m running. As I run towards you, me throwing a ball makes the ball faster, and if I wait to throw the ball until after I run past you, it will make the ball slower.
Sound wants to do something similar, but there are two key differences. The sound is a wave, not a single object like a ball. Secondly, sound can’t speed up or slow down in air. (It can if the air gets thinner or colder, but for practical demonstrations we will say the air between you and the thing making the sound is all roughly the same). Since the sound can’t speed up, the sound wave itself actually squishes together and there is less distance between the peaks and valleys of the wave. This is known as an increase in frequency, which shifts the pitch higher. Inversely, when the thing making the sound is moving away, the sound wave is being stretched out and the pitch is lowered. This phenomenon is known as the Doppler effect.
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