Singing Pavement

With all the highway travel I've been doing, I've started noticing little differences in the roads.  One aspect that has always interested me is the way the sound of the car changes depending on the type of pavement.  I decided to try to get a quantitative picture of the changes on our way out of Boston today.  I grabbed a frequency analyzer for Steve's iPad and took some samples when we were driving on a recently paved road, an old worn road, and a soon to be paved, milled road.  The milled road wasn't very long, so I only managed to grab one sample.

In order to account for the varying speed of the car, I decided to define a characteristic length: the car speed divided by the dominant frequency of sound.  I'm not sure what this length means exactly, but it's useful as a way to compare.  Here are my results:

Pavement Type	Car Speed (m/s)	Dominant Frequency (Hz)	Characteristic Length (cm)
New	29.5	235.7	13
New	30.8	126	24
New	31.7	334.6	9
New (Average)	30.7	232.1	15
Old	31.7	150.2	21
Old	30.8	289.9	11
Old	31.3	161.7	19
Old (Average)	31.3	200.6	17
Milled	27.3	137.1	20

It seems the characteristic length increases as the quality of the pavement decreases.  This makes me think the length may be related to the average distance between bumps or pits in the road, but I'm sure it's more complicated than that.