Doppler Effect and Environment

In class today the topic of the Doppler Effect came up, and I asked students to explain it for those unfamiliar. A very bright student said correctly that it's when a wave frequency is altered for someone who is in motion. He said an example is how the sound of a police car changes from a higher pitched sound as it approaches to a lower pitched sound when it drives away. I found that example fascinating, because in Nebraska schools I always heard it explained in relation to train whistles. I guess there are more police sirens than train whistles in New Orleans.