A commuter train passes a passenger platform at a constant speed of 40.0 m/s. The train horn is sounded at its frequency of 320 Hz. What wavelength is detected by a person on the platform as the train approaches?

1 Answer
Dec 7, 2015



I'll assume 20 degC for which the speed of sound is #343.7"m/s"#

This is an example of the Doppler Effect.

For the person standing on the platform, more wavefronts are arriving per second so he or she will detect sound of a shorter wavelength and higher frequency.


If the source is approaching the observed wavelength is given by:


Putting in the numbers:


You can compare this to the note which someone would hear on the train:


So you can see that the person on the platform detects a slightly shorter wavelength and a slightly higher frequency of note.