Why are sound waves from a moving object compressed?

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Curiosity Science Physics Why are sound waves from a moving object compressed?


  • Why do sound waves emminating from the front of an object moving forward become compressed for the observer also standing at some point in front of the object?

    I mean, I get that this is responsible for the Doppler effect, etc, but why do the waves actually *compress*? Is it because the observer sort of hears a layered effect of simultaneously hearing a more outer-ring sound wave that the object gave out when it was farther away, on top of more inner-ring sound waves as the object moved closer?

    If that was the case, wouldn’t that indicate that as a sound wave moves out from an object (moves from being a more “inner ring” sound wave to being a more “outer ring” sound wave), the pitch drops? Ie. Taking two unmoving observers and an unmoving sound source, the further away observer would hear the sound as a lower pitch. But that’s not the case, from what I understand. For still sources and observers, only the volume will drop if the observer is further away, not the pitch. So I don’t see how “layering” sound waves of identical pitch could be responsible for the compression.

    So how do sound waves ACTUALLY become compressed, or is it just something we know that they do?

    I hope my question makes sense! :-)

  • The waves are nothing but a physical vibration of air molecules. So when an object moves through the air, it interacts with the air molecules like how a boat or a submarine interacts with water.

    In your case of a moving object, these air molecules get compressed in the direction in which the object moves. This is because the object has a velocity and this keeps pushing the air molecules as it moves. And like you said, the frequency of the waves will be higher at the front of the object. But at the opposite, the frequency of the waves will be lower as they are not packed up together (imagine the same boat and wave analogy). Yep! This is the doppler effect.

    And in your second case of a stationary omnidirectional sound source and a stationary observer, the frequency of the sound will be constant in all the directions. Why? Because the object is stationary and it’s not compressing the sound waves in any direction. The sound waves have their original frequency. The volume at the observer’s end will be less audible as the amplitude of the sound wave is inversely proportional to the square of the distance – aka inverse square law.

    And here is an interesting case! When the object starts travelling closer to the physical maximum limit of the molecules could travel (speed of sound), they keep stacking up together like a barrier. And when the object travels faster than this limit, it breaks that barrier and causes a sonic boom. Read more on how sonic boom works.

    • Ohhhh, okay – so, if I’m understanding correctly, the source of the sound is sort of “catching up” to the vibrating air molecules it put in motion? It pushes the sound in the air, the same way it pushes the air – the same way you’d feel wind from a passing plane.

      Could something else theoretically compress the sound waves of another object? Say, if a plane is flying a few feet above a train at the same speed (as if that were safe), and the train was making no noise of its own (for some reason…), and then the plane stopped moving forward (in midair…no one was injured teehee), could the train have some effect on the sound waves created by the plane?

      And…there’s no such thing as a “sound wave”? It’s **just** air, just the atmosphere we know, but vibrating? There’s nothing new being “released” into the air, unlike light?

    • And…there’s no such thing as a “sound wave”? It’s **just** air, just the atmosphere we know, but vibrating?

      Spot on! Yep. Sound is just the air molecules vibrating at different frequencies. It’s a pressure wave. No, nothing new is released into the air, because it’s the air that is vibrating in the first place. If there’s no air, there won’t be any medium for this vibration (sound) to happen. This is why you can’t hear sound in space.

      Also, in the case of light, it is the oscillations of electric and magnetic fields — one of the fundamental properties of the universe and it does not need a medium to propagate. Whereas, the sound is a compression wave, oscillation of air particles. It is just a property of the medium (air). In other words, for sound waves to exist, you need a medium like air or water. Electromagnetic waves exist because the universe fundamentally has it.

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