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
It is a gradual buildup, but planes move FAST, so the gradualness happens fast. Commercial jets tend to fly at around 75% to 85% of the speed of sound. That means the sound from the plane isn’t all that far ahead of the plane itself. So the sound builds quickly, and then – bam! – the plane is almost right above you as the sound pressure peaks.
If you ever hear a slower moving prop plane approach, the sound buildup is much slower as the plane itself is moving slower and the sound wave gets pretty far in front of the approaching plane.
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?
sound is actually an after effect on the air around an object and moves much like water. Watch a boat as it slices through the water it pushes water to the sides and creates a wake. The waves up front are shorter and closer together and the farther back from the boat the longer and further apart these waves are. Air moves the same way it just isn’t seen it is heard. the differences is that the shorter waves are higher pitched (closer together) and lower pitched (farther apart)
TL:DR the doppler affect.
Other have commented about being near the speed of sound. Planes travel that fast, but they are usually not anywhere near their top cruise speed when they are close enough to the ground to be heard loudly. They are still fast though as in 150+ mph fast. They are also loud. Particularly during takeoff. The reason the sound is sometimes sudden is a combination of their speed, which even at low altitude can be as fast as race cars, and how they are maneuvering (turns / climbing out) but what I think is a huge factor is the plane simply flew into your rough line of sight (sound is a bit more bouncy and dynamic that that so that’s why I said rough) and so is suddenly no longer blocked by your surroundings.
Answer: you may be hearing a common commercial plane, the Airbus A220, which makes the famous “Whale Noise” when on approach to an airport.
When the 220s turbofan engines reach a certain RPM, it makes a sudden noise change which we hear as a whale howling.
So in addition to the other answers, you may also be in the right flight path for an airline that flies the A220 like Delta. Something like a 747 won’t have that distinct transition.
There are lots of great answers hear. Another factor is our brain is great at filtering background information like sounds. All day long we get inundated with noise and we don’t *really* notice it moment to moment. Our brain stops us from devoting energy to it since we don’t need to deal with it. So an airplane engine is just muted by our brain until something sets it apart. Might be that it crossed a sound level threshold, might be a different sound pitch than normal, etc. If you ever get out in the wilderness without noise around ( I live in a desert) you can hear airplanes much farther away than you think you could because your sense of hearing can focus on the sound.
You’re floating on the lake. A boat goes past you, really fast. A few seconds later, the wake from the boat hits you, makes you bob up and down a good bit. The airplane is the boat. The sound is the wake.
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