# Observed timing of sound of airplane flying overhead

Discussion in 'Physics' started by tjohnson, Jan 29, 2015.

1. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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If I'm outdoors and an airplane flies overhead, it seems like the time during which I hear it and the time during which I see it are staggered. I don't understand why this is, since the planes flying over my yard certainly aren't supersonic.

To explain what I mean, suppose I started a stopwatch at the same instant that a plane began approaching. At 0 sec, I would begin to hear the plane, but I wouldn't see it until 5 sec. Then at 25 sec, I would stop hearing the plane, but continue to see it until 30 sec. (These aren't real measurements, but just for an example.)

Any ideas as to what causes this?

EDIT: At first I didn't think the Doppler effect would have anything to do with this, but maybe it does. Perhaps by the time the plane has almost passed over the frequency of the sound is too low to be audible to humans?

Last edited: Jan 29, 2015
2. ### MikeML AAC Fanatic!

Oct 2, 2009
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The noise radiated is not omnidirectional with respect to the direction of flight. Stealth aircraft are designed to make almost no noise while they are approaching you...

3. ### Glenn Holland Member

Dec 26, 2014
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Sound travels at about 1100 feet/sec. at standard conditions of temperature and pressure.

There is a site where you hear the sound of the foghorns on the San Francisco Golden Gate Bridge from a nearby microphone and determine the time for the sound to reach your location.

http://goldengate.org/exhibits/exhibitarea2b.php

At my location about 3 miles east of the bridge, I get an average of about 1000 ft./sec.

Last edited: Jan 30, 2015
4. ### wayneh Expert

Sep 9, 2010
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Suppose you're in a well and can only hear the plane when it is directly overhead. The plane passes over your well at ~21,000', or 4 miles up. The sound from the plane that is directly overhead travels to you on the ground, taking about 5 seconds per mile. So, you hear the plane 20 seconds after it was overhead, and by then it is long gone from your view up the well shaft. At 600mph, it would travel 3.3 miles in 20 seconds.

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5. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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Thanks, that makes a lot of sense. I must have been remembering wrongly, because I thought I starting hearing the airplane before I saw it, not the other way around.

6. ### amilton542 Active Member

Nov 13, 2010
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So what are you suggesting that the velocity of sound is the new universal speed barrier?

7. ### tjohnson Thread Starter Active Member

Dec 23, 2014
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I'm not sure what you mean by that?

8. ### wayneh Expert

Sep 9, 2010
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You will hear a plane before it passes directly overhead, because you're not really in a well. Let's say you can hear a plane 5 miles away. More than that, and it doesn't register. If a plane is flying at 4 miles elevation, you will detect it when it is still 3 miles from you (3,4,5 right triangle). Now, it will actually have gotten still closer by the time you hear the sound.

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9. ### #12 Expert

Nov 30, 2010
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It is for sound.

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10. ### WBahn Moderator

Mar 31, 2012
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And possibly even more relevant is that the sound content is very directional. If you stand in front of most jet aircraft you will get a lot of high frequency content from the compressors and if you stand behind it you will get a lot of low frequency content from the turbines and the exhaust. High frequency content is absorbed more heavily with distance than low frequency content, plus the noise level from behind is usually much higher than in front. The end result is that you can hear an aircraft that is pointed away from you from a lot further off than you can hear one that is pointed toward you.

11. ### sirch2 Well-Known Member

Jan 21, 2013
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You also need to consider reflections. Sound reflects of tall buildings, hills and the like and this is why you often look in the wrong direction when hearing a aeroplane, particularly low flying fast jets.

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