What makes driving on the freeway so loud?

Greetings:

What makes driving on the freeway so loud? Every time I get on the I-15 for the struggle home, I must roll up my windows because the noise of passing cars is nearly deafening. I have three possible answers: the engine noise of passing vehicles; the sound of the tires on the asphalt; or the most likely (I think), the rush of air around the vehicles as they slice through the atmosphere.

-- Dan Townsend, e-mail land

Since speeds on the homebound 15 commute swing back and forth wildly between 5 and 85 miles an hour, the road noise you hear comes from different sources. (I'm assuming here you're not driving some rattletrap that freely transmits engine noise and vibration into the passenger compartment.) At low speeds, it's engine noise from other cars and trucks. Or perhaps the grinding of your molars in frustration. But as speeds pick up, tire noise clearly dominates. Even when you stand on a lonely hill and hear the faint hiss and hum of a distant freeway, you're hearing mostly tire noise. Tires are like big resonators. Every time a section of tread hits the asphalt, the tire vibrates and creates a sound. Air compressed between the tread spaces and the road surface is expelled in a sort of tire fart. And as the tire turns and a section of tread is released from the road surface, it springs back into place and makes more noise. It's pure bedlam down there. A concrete road surface is much noisier than asphalt, which is porous and absorbs some of the racket.

Tread design is a big contributor to tire noise. Ever hear the sound of snow tires on a dry road? Or those huge off-road, mud-buggy tires? Their rugged treads are designed for work, not for sneaking past the Highway Patrol. A treadless tire would be most silent; but while your life would be quieter, it would be considerably shorter.