Analysis of Rumble Strip Generated Sound

Rumble strips are parallel, evenly spaced grooves that have been ground into pavement, often with a trough-to-trough spacing of 12 inches. They are typically found along the shoulders of limited access highways and are designed to produce steering wheel vibrations to warn drivers that they have drifted off the road. One model for sound production by cars moving across rumble strips is that the fundamental frequency is determined by the number of troughs driven over each second. Assuming 12-inch spacing, the sound produced would have a frequency, in hertz, that has the same numerical value as the speed of the vehicle, measured in feet per second. This project set out to verify the proposed model and provide guidance on how undergraduate activities can be developed to explore sound production by measuring the speed-dependent pitch heard when driving over a rumble strip. Sound was recorded using a smartphone in a car driving at a range of speeds over rumble strips. The audio files were then loaded onto a computer for spectral analysis. Two freely available spectral analyzers were put through a calibration check and then used to analyze the recordings. We found good, easily reproducible agreement with the model. Possible applications for undergraduate activities will be discussed.