Noticed it today, rather than a gradual build up of noise you just suddenly hear an airplane like it appeared out of nowhere?
In: Physics
Generally speaking, commercial aircraft cruise at fairly close to the speed of sound (about 75-85%).
If the plane is flying toward you, this means that the sound waves can get all bunched up ahead of the plane. The result is that you don’t hear the plane as early and it gets loud very quickly when you do hear it. That means it can sometimes sounds like it just came out of nowhere.
If a plane is flying at the speed of sound, you won’t hear it until it is literally over you. If it is flying faster than the speed of sound, you won’t hear it until sometime after it passes you. That’s because it moves ahead of its own soundwave, leaving sound as a wake behind it.
How you hear a plane also depends on a lot of factors:
1. How far away is it?
2. How fast is it moving?
3. What direction is it moving?
4. What is the air doing — is it hot, cold, windy, cloudy, etc
5. What kind of environment are you in? A tall city? A flat plain?
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