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Probably most of us have heard the pitch of an ambulance change as it’s passed us, but that’s from a nearby sound source. If a sound source is far away, does the Doppler shift change? I find it hard to believe that the sound wave compression would be the same over a mile as it is over a couple of feet. At the same time, astronomy uses Doppler shift over distances of many light years, but that’s also through space, where we don’t have air molecules bumping into each other and losing energy.

(I heard a Doppler shift from an ambulance that passed me while I was driving yesterday. When I heard another ambulance late at night that I suspect was about a mile away, I started wondering if the pitch shift would be the same.)

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  • $\begingroup$ Are you maybe thinking that the Doppler shift is the difference in pitch that you hear between when the ambulance is approaching and when it is moving away from you? The Doppler shift refers to the change in frequency of what you hear compared to the source frequency. In the case of the passing ambulance you notice two different Doppler shifts - on increasing the pitch and then another lowering the pitch. $\endgroup$ – M. Enns May 4 '20 at 17:32
  • $\begingroup$ @M.Enns If it's not that, then how would you define it? $\endgroup$ – Dave May 4 '20 at 17:33
  • $\begingroup$ Okay, I see the subtle difference. No, I just mean the shift in pitch from, say 440Hz emitted, to what I hear at 450 Hz as the ambulance hears me. If the sound source is nearby, sure, that's what I'll hear. If the source is far away, do the waves colliding over the course of a mile before they reach my ear lessen the shift up? $\endgroup$ – Dave May 4 '20 at 17:39
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Ideally the doppler shift does not depends on the distance of sound source from the observer, so the doppler shift should not change. But there is a phenomenon where higher frequency sound wave scatter more than a lower frequency sound wave (for electromagnetic wave it is called Rayleigh Scattering). So if a sound source is far away, the high frequency wave got scatter mostly and one can only hear to the lower frequency waves.

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