How do planes make so much noise after they have flown over / away?


I live near some flight paths so a lingering sound is normal but recently somewhere else a military style jet flew over head and the sound lasted much longer at a very high volume, long after we could see it anymore – why?

In: 1

Same reason a motorcycle is louder after it passes by. The exhaust and the majority of the noise is directed rearward.

Military jets move very fast compared to other planes and they’re much louder. When a moving object generates sound waves the waves are compressed near the front of the object and spread out as they travel around and behind the object. These compressed sound waves have more energy than other waves and in turn create a louder noise. If the plane does hit the speed of sound you’ll see it and won’t hear a single thing anything until it’s way past you. On top of all that military planes don’t have sound dampening measures like commercial ones do because nobody is reading a magazine and taking a nap on a military plane.

Because airplanes are very far away and very fast. Sound takes time to travel, and by the time the sound of the exhaust reaches you, the plane has moved much further away.

If you are trying to compare to a commercial jet, the military jet you saw was much more powerful (faster and louder) and likely flying much lower (closer to you). All of these effects combined produce a louder, longer-lasting sound.

2 reasons: they emit sound in all directions, and they’re also very high up.

Because of the speed of sound, by the time the sound from a plane has reached you it’s already moved farther on.

Even if sound moved instantly, a plane’s huge dustance from you means moving laterally doesn’t change their distance by a ton. A car on the street might be 5 feet from out at its closest (and loudest), and 500 feet from you when its out of sight. 100x the distance. A plane is maybe 30,000 feet from you at closest, and 42,000 feet from you when its 45⁰ away and halfway to the horizon. Not even 1.5x the distance.

Sound travels pretty slow relative to light

Sound moves at just 343 m/s, but planes tend to cruise up at 10,000+ meters while moving at 800 km/h.

If a plane is up at 10,300 meters(~34k feet) it’ll take 30 seconds for the sound to reach you all the way at ground level. In that time the plane has travelled 6.7 km so you can probably still see it. If that same plane was flying at just 3400 meters then it would take 10 seconds for you to hear the sound and the plane would have traveled 2km but it would have passed across your field of view significantly quicker.

Fighter jets can also make the sound go on long after they’ve passed because they’re engines are just big and noisy and project that noise behind them so you’re not even really hearing noise generated by the plane but instead by a giant speaker built into its tail. The sound from its passing may have been over fairly soon but now you’re in the noise cone of its engine so you can still hear it.

Commercial jets have noise restrictions on how loud their engines can be, military planes do not have to follow those restrictions