A stationary source directs an 800-Hz sound wave toward an approaching object moving with a speed of 25.0 m/s. What is the frequency shift of the reflected wave if the air temperature is 20ºC? (Hint: There are two Doppler shifts here. Why?) f = 800Hz the speed of sound = v = 331 + 0.6Tc = 343 m/s the speed of the object = vo = 25.0 m/s For the first doppler shift, f1 = (v + vo)/v * f f1 = 858 Hz For the second doppler shift, I'm guessing that we use the object moving as the source of the sound and the original sound source as the observer: f2 = v/(v - vo) * f1 f2 = 925 Hz But the answer is 126 Hz!