The equation for the doppler shift of a sound wave of speed v reaching a moving detector is f ? = f v + vd v ? vs , where vd is the speed of the detector, vs is the speed of the source, f is the frequency of the source, and f ? is the frequency at the detector. 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 29 m/s blows a 310 hz horn. what frequency is detected by a stationary train? the velocity of sound is 343 m/s.