Doppler shift from a moving source

A freight train blasts its horn as it rolls toward a crossing. To Noll, waiting at the gate, the horn sounds higher than a parked train's horn would.

The horn emits one fixed frequency either way. For a moving source and a still observer, the shift follows one rule, set by which way the source moves.

The train is the source of the sound. Noll, standing still, is the observer.

While the train moves toward Noll, the frequency he receives is higher than the frequency the horn emits. The pitch he hears is higher.

Source moving toward the observer: the received pitch is higher than the emitted pitch.

After the train passes and moves away from him, the received frequency is lower than the emitted one. The pitch he hears is lower.

Source moving away from the observer: the received pitch is lower than the emitted pitch.

The rule for a moving source and a still observer: source moving toward the observer, higher received pitch; source moving away, lower received pitch.

A source moving neither toward nor away gives no shift: a parked train's horn arrives at exactly its emitted pitch.

Source moving neither toward nor away: a parked train's horn arrives at exactly its emitted pitch.

A police car drives away from Maren with its siren on. Is the pitch she hears higher or lower than the siren's emitted pitch?

The source is moving away from the observer.

Source moving away means a lower received pitch.

The pitch Maren hears is lower than the emitted pitch.

A race car roars toward the start line where Pim watches. Compare the engine pitch Pim hears with the pitch the engine emits.

The source is moving toward the observer.

Source moving toward means a higher received pitch.

Pim hears a higher pitch than the engine emits.

A garbage truck rumbles toward Ottoline, who stands still on the curb. Is the engine pitch she receives higher or lower than the pitch the engine emits?

A helicopter flies straight away from Pim, rotor thumping, while he stands still. Is the pitch he receives higher or lower than the pitch the rotor emits?

Maren says: 'A moving train's horn sounds higher to a listener it approaches, and lower to a listener it has passed.' Is Maren correct?

Your turn

In which case does the standing listener receive a pitch higher than the source emits?

A subway train passes Noll on a platform with its horn sounding the whole time. What does Noll hear?

A dirt bike circles Pim at a constant distance, engine buzzing, while Pim stands at the center. Compared with the emitted pitch, what pitch does Pim receive?