What is the Best radar/Laser detector?

If the detector moves toward the source, vd is positive; if the source moves toward the detector, vs is posit?

• If the detector moves toward the source, vd is positive; if the source moves toward the detector, vs is positive. A train moving toward a detector at 27 m/s blows a 277 Hz horn. What frequency is detected by a stationary train? The velocity of sound is 343 m/s . Answer in units of Hz What frequency is detected by a train moving toward the first train at a speed of 22.7 m/s? Answer in units of Hz

• Answer:

  I believe this holds your answer: http://stephenzhang.com/answers/what-frequency-is-detected-by-a-stationary-train/