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
    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
    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


    Dec 26, 2014
    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.


    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


    Sep 9, 2010
    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.
    tjohnson likes this.
  5. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
    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
    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
    I'm not sure what you mean by that? :confused:
  8. wayneh


    Sep 9, 2010
    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.
    tjohnson likes this.
  9. #12


    Nov 30, 2010
    It is for sound.
    sirch2 likes this.
  10. WBahn


    Mar 31, 2012
    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
    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.
    tjohnson likes this.