Resonance physics.

Train horns typically sound three or four frequencies together to produce a dissonance., unpleasant sounding) chord that easily gets a listener's attention. A train approaches an observer at 32.0 m/s on a straight section of track. Assuming a speed of sound of 344 m/s, and a single frequency when stationary of 155 Hz, calculate the frequency detected by the stationary observer a. as the train approaches. [171 Hz] b. after the train has passed. [142 Hz]