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Let's say you're driving forward by car and when you press the horn, the sound is reflected from the building in front of us. If we mark the frequency of the sound sent by $f_1$, frequency of the sound received by an observer located in the building by $f_2$ and frequency of the reflected sound which is received by the observer in the car by $f_3$,

What relations can we show for these frequencies?

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  • $\begingroup$ How do we describe such wave behavior? is it an example of double doppler-effect? Should any of the frequencies be equal to each other here? $\endgroup$ – Undergraduate Wannabe Oct 11 at 14:20
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The perceived frequency of the sound is dependent on the frame of reference. If the frame of reference is the car, the barn can be assumed to have a velocity (vector). If you take the projection of the barns velocity vector, onto the vector from the car to the barn at the instantaneous moment the sound hits the barn, this will give you the correct velocity with which to calculate the Doppler effect. In this way it is not really a "double" effect. It is just a single reflection in the stationary frame of reference that is the car, with a moving barn.

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  • $\begingroup$ So what are the frequencies relations, would you like to elaborate? $\endgroup$ – Undergraduate Wannabe Oct 11 at 15:10
  • $\begingroup$ @UndergraduateWannabe, f2 and f3 will be the same. The relation between f1 and f2/3 will be a function of the velocity vector and the location vector. $\endgroup$ – TallBrianL Oct 16 at 22:19

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