During an experiment with moving sound sources, students notice a change in pitch. As a train approaches, the whistle's pitch appears higher, and as it moves away, the pitch seems lower. This change in pitch is explained by three potential factors: the Doppler effect compressing the sound waves when the train approaches, resulting in higher perceived frequency; the elongation of sound waves due to the train's motion away from the observer causing a lower perceived frequency; or the constant pitch of the whistle regardless of the train's motion as the Doppler effect is negligible in this scenario. Which factor best defines the observed change in pitch? Discuss.