Doppler shift for approaching source is positive, so the formula is f' = f (v / (v - v_s)). With train at 20 m/s and sound speed 340 m/s, it's f' = f (340/320). Sign convention? Approaching means higher frequency, positive sign in denominator for observer at rest. But honestly, all these signs I remember from the whizzing sound of my ceiling fan in my study room—it’s the same frantic hum as my thoughts before an exam!
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