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Question about Doppler shift

Typhoid MannyTyphoid Manny Registered User regular
edited April 2012 in Help / Advice Forum
So this has been bugging me for a couple days.

If I understand it right, the Doppler effect means that when something is coming towards you, the pitch of the sound it makes (and the light that comes from it, but that doesn't really matter for the speed I'm concerned about) gets higher as it gets closer, because the waves compress.

So why in old war movies does the whistling a bomb or a shell or whatever makes as it falls to the ground get lower in pitch the closer it gets? Is there something I'm not understanding, or is the whistling a thing that doesn't actually happen?

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Posts

  • wonderpugwonderpug Registered User regular
    edited April 2012
    It's not that it gets higher as the object gets closer; it's that the sound is higher pitched in front of the object than behind.

    doppler2.jpg

    The pitch is related to how close together those lines are wherever you're standing. If you're directly in the path of the object, the pitch will be the same high tone until the exact point it moves past you, at which point it will be at a lower tone and will remain at that tone.

    The reason you hear a gradual change just as a firetruck or whatever zooms by with its siren is because you're not standing right in its path, you are off to the side. If it's a ways away, the sound change is minimal, but as you can see in the above picture, the distance between the lines changes the most rapidly right around the point perpendicular to the object and its movement.

    Good question about the bomb dropping sound, though. My guess is that the classic whistling sound drops in tone because the bomb is dropping off to your side, just as when a wailing fire truck passes by your side. If the bomb is going to land on your head, all you'll hear is the constant highest pitch. That's my take, at least, I'm curious to hear if anyone else has a different theory.

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  • InfidelInfidel Heretic Registered User regular
    I am going to guess that people that observe the high to low pitch whistle are more likely to survive to relay the observation.

    As opposed to those that heard a constant high pitched whine, cause well... :rotate:

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  • InfidelInfidel Heretic Registered User regular
    Thinking about it a bit more, the sliding pitch would probably be better explained by drag?

    I was picturing a bomb falling at a relatively constant velocity until impact but they are released at high altitude so do they slow down (and pitch goes down) noticeably due to increased air density (drag)? I am not a huge ballistics expert but that would explain the slide as opposed to the acute change in pitch like a vehicle passing.

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  • SpongeCakeSpongeCake Registered User regular
    So why in old war movies does the whistling a bomb or a shell or whatever makes as it falls to the ground get lower in pitch the closer it gets? Is there something I'm not understanding, or is the whistling a thing that doesn't actually happen?

    Could be a throwback to the Stuka bomber which had a "Shit-You-Right-Up-Son" siren attached in WWII.

  • romanqwertyromanqwerty Registered User regular
    This could just be one of those movie sound effects that doesn't reflect reality. I'm not sure you'd even hear whistling from dropped bombs like that.

    The alternative explanation is that that is actually the sound that the pilot would hear and that the appearance of a drop in frequency is just volume attenuation as the distance increases combined with the increased penetration of lower frequencies.

  • tarnoktarnok Registered User regular
    So.

    I worked out the math here. The equation that determines the angle of the straight line between you and the moving sound source is A(t) = arctan(a/[b-rt]) where a is your distance from the source perpendicular to its path, b is your distance from the source parallel to its path at time 0, r is the speed of the source and of course, t is time.

    As was observed earlier, the angle of that line determines the frequency of the sound you hear. If you graph the equation provided you'll find that the angle starts out near zero (dead in front of the sound source, highest pitch), increases very slowly for a while, then suddenly leaps up to ninety, flipping back around to negative ninety and quickly increasing back to almost zero then leveling out again. In real life terms this would correspond to a slow drop in pitch that picked up speed as the object came nearer, then as it receded a continued drop in pitch but with a falling rate of change.

    To visualize this I put it into my graphing calculator with the values a = 10, b = 500, and r = 73.33. This models an ambulance travelling at 50 miles per hour approaching a pedestrian who is walking on a sidewalk ten feet from the road who first hears the ambulance when it is 500 feet away.

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  • Platypus BeirutPlatypus Beirut Registered User regular
    edited April 2012
    This could just be one of those movie sound effects that doesn't reflect reality. I'm not sure you'd even hear whistling from dropped bombs like that.

    The Stuka sound is notorious for being used as the sound of an airplane diving or crashing, even though the sound was produced by a siren, as SpongeCake stated above. Just an example to confirm that movie sound effects don't always reflect reality.

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    oldsig.jpg~original
  • Platypus BeirutPlatypus Beirut Registered User regular
    So why in old war movies does the whistling a bomb or a shell or whatever makes as it falls to the ground get lower in pitch the closer it gets? Is there something I'm not understanding, or is the whistling a thing that doesn't actually happen?

    My guess would be that this kind of noise would be the one dropping or firing the bomb or shell would hear. Provided the bombs or shells whistle. Fireworks sometimes do.

    oldsig.jpg~original
  • a5ehrena5ehren AtlantaRegistered User regular
    This could just be one of those movie sound effects that doesn't reflect reality. I'm not sure you'd even hear whistling from dropped bombs like that.

    The Stuka sound is notorious for being used as the sound of an airplane diving or crashing, even though the sound was produced by a siren, as SpongeCake stated above. Just an example to confirm that movie sound effects don't always reflect reality.

    Huh. Didn't know that.

  • InfidelInfidel Heretic Registered User regular
    Bombs do whistle, it is not just a movie thing. How exactly the pitch behaves might be in error though.

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  • Dunadan019Dunadan019 Registered User regular
    Infidel wrote: »
    Thinking about it a bit more, the sliding pitch would probably be better explained by drag?

    I was picturing a bomb falling at a relatively constant velocity until impact but they are released at high altitude so do they slow down (and pitch goes down) noticeably due to increased air density (drag)? I am not a huge ballistics expert but that would explain the slide as opposed to the acute change in pitch like a vehicle passing.

    high altitude unguided bombs reach terminal velocity fairly quickly depending on their size. As the bomb gets closer to the ground the air will get denser which should slow it down but it would be by a very small amount compared to its velocity.

    the sound that comes from an object traveling through air is made by the air separating from the object at certain points. for a dropped object that would be any corners and also the rear of the bomb including fins. the sound protrayed in movies is usually innacurate but could be explained by the angle explanation above. since the bomb would be dropped from a moving airplane it would come in from an angle and have a trajectory with a lateral component where the velocity vector would not always be pointing straight at you. I would expect that to result in a pitch increase as it approached as opposed to the standard movie decrease if you were about to be hit. if it were going to miss it might sound like it does in a movie to a degree.

  • tarnoktarnok Registered User regular
    Unless you are directly in the path of the object generating the sound you will hear a decrease in pitch as it approaches you because the angle between the line connecting you to the object and the object's path is increasing. If you _are_ directly in the object's path, ie, about to get hit by the bomb, there will be no change in pitch because the angle between your line to the object and the object's path will remain constant.

    The changing pitch is both predicted by the model and easily testable. Go to an empty parking lot late at night and have your friend lean on the horn as he drives by you at about 30-40 mph. You will hear a higher pitch dropping to lower as he approaches.

    The only way you can hear an increasing pitch due to doppler effects is if you move into the path of the object or the object changes course so that you are in the path.

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  • Dunadan019Dunadan019 Registered User regular
    tarnok wrote: »
    or the object changes course so that you are in the path.

    which is what happens for anything with a trajectory from the perspective of the impact point.

  • RaekreuRaekreu Registered User regular
    Infidel wrote: »
    Bombs do whistle, it is not just a movie thing. How exactly the pitch behaves might be in error though.

    My granddads both told me that the go-to policy for falling bombs and other heavy ordinance was 'if you can hear it before you see the actual explosion, you are too close'. The followup to that is 'if you can't hear it, it may be mortar fire'.

    I think that you got it right on your other post, I'm pretty sure that if the pitch doesn't shift or shifts very little, then the impact point will be quite nearby. If it shifts from high to low, then it should be moving away, and low to high pitch would be approaching.






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