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I How far do vibrations from a vehicle emanate?Doppler effect?

  1. Feb 18, 2017 #1
    So I have a strange question. I study snakes, which are of course sensitive to vibrations in the ground. A common technique for finding snakes is to drive roads at night. A constant debate that comes up among herpetologists is whether or not the vibrations of the vehicle are causing the snakes to flee, or, if the snakes were already moving, and you just happen to find the animal when it is moving.

    So my questions, and I realize this will apply on a specific basis (probably every single vehicle is unique). #1, Is there any general knowledge out there about shock waves/ vibrations from an idle engine (let's say a 4 cylinder gasoline engine) on asphalt.

    #2 - I'm not sure if the doppler effect only applies to sound waves, and if so, I'm not sure what the equivalent is, but in my mind, I'm picturing a speed boat, which leaves a wake behind it, and of course immediately in front of the boat, the water has yet to be disturbed. So this seems like an equivalent to the doppler effect, but with waves (literal, liquid waves). Would there not be something similar with a vehicle driving down the road?

    At idle, vibrations should radiate in 360 degrees from the vehicle. As it moves forward, the vibrations emanating from the front should get more compressed as the vehicle moving forward is covering the ground that those waves are propagating from. I know sound moves faster in water than air. But what about water vs land? For sake of simplicity, I'm suggesting a substrate made up entirely of asphalt. I'd assume it would be slower, because there are still plenty of air pockets in asphalt. But asphalt is also less flexible than water....

    So the last question, and the main one, #3 - at what speed would a 4-cylinder gasoline engine have to travel at, on asphalt, to stay "ahead" of the vibrational waves it is creating?

    I appreciate any insight, or suggested reading anyone could point me to. I know whatever answer comes out of this will not be 100% precise as there are too many variables and considerations (the type of asphalt, the distance of the engine from the ground, the size of the tires, how well balanced/ old the engine is, and plenty of others) - I'm just looking for an approximation, something with a ~75-80% certainty.

    Thank you,
  2. jcsd
  3. Feb 18, 2017 #2


    Staff: Mentor

    You need to ask the same questions for two cases. Sound emitted in air by the vehicle. Sound in the road and the ground excited by contact with the tires. So the answer to #3 is the speed of sound. But the speed of sound in air is not the same as in the ground. Suffice it to say that cars can't drive faster than the sound barrier in either case.

    Your question reminds me of my youth living near train tracks. By putting one's ear to the rail, trains could be heard approaching from many miles away.
  4. Feb 18, 2017 #3
    Ok, but of course snakes are deaf and don't easily pick up sound waves - they do pick up vibrations in the ground. I figured the physical vibrations of the ground would be greatly slower than the speed of sound. Yes, that's basically the idea, putting your ear to the ground, at what distance can a vehicle be noticed. Hmmm... maybe I could just do a simple curiosity experiment. But ultimately, yeah, I'm pretty sure you can feel the vehicle approaching from quite a distance. I would imagine a train would be easier to sense because of the steel or iron in the rails (much more dense) and the fact that the train is vastly heavier than 1,500 lbs.
  5. Feb 18, 2017 #4


    Staff: Mentor

    Not greatly slower, 17895 mph.
    Last edited by a moderator: May 8, 2017
  6. Feb 18, 2017 #5
    lol. Nice, I like having straight up values. Ok. A friend just started discussing the topic with me. In my boat analogy, I was mentioning the fact that if you're on a raft, you don't really sense an approaching speedboat (yes you can hear it, but let's pretend you're deaf). It wouldn't be until after the boat passes, that you feel the waves generated by the boat. Even if underwater, sure you'd be able to hear the sound of the engine (the sound wave), but you wouldn't be able to "feel" it, until the boat passed. He mentioned that that situation had to do with a surface wave vs a compression wave vs a sound wave. I guess these are the sort of waves I was thinking of. But he was saying that on land you wouldn't produce a surface wave.

    Regardless, thanks for the conversation. I think I have my answer - no matter how fast, or how slow, a snake would be able to sense a vehicle approaching at essentially the same distance, whether the vehicle was driving at 15 mph or 70 mph. Thanks!
  7. Feb 18, 2017 #6
    For some highly sensitive physics experiments such as LIGO and LHC, it's very important that as much as possible background noise can be eliminated.
  8. Feb 24, 2017 #7


    User Avatar
    Science Advisor

    Sure, the Doppler effect will apply. But as stated above, the source needs to be moving awfully fast to make much difference.
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