This was a student project for my Introductory Mechanics course. Students created a CD with a 500Hz sound signal. They then convinced a campus police officer to play the CD in his stereo and while holding his PA mic next to the speaker he drove past them. They used a digital camera to record the sound and created a wave file. Using my sound spectrum program and the Doppler shift they found the squad car's speed. Here is the result. The posted speed limit was 45mph.