Have you ever been driving around and decided to let just a bit of that sweet outside air in? Maybe you just want to smell the pine trees, or maybe you're trying to clandestinely vent out a stealthy fart. You have your reasons. But as soon as you get that window down an inch, you regret it. Because of that sound.
It varies from car to car, and tends to be worse when it's one of the rear windows opened, but it happens to some degree on almost all newer cars. It's halfway between a sound and a rhythmic pulsating of pressure. An unending thrum thrum thrum thrum that, for most of us, drives you crazy.
Sure, opening the window more or sometimes changing your speed up or down will get rid of the offending pulsations, but the question remains: why does this happen?
This seems like one of those things that smug know-it-all science has the answer to. So, like I usually do when I have a question like this, I went to Jalopnik's tame physicist, Dr. Stephen Granade, who is on a first-name basis with science, to explain what's going on.
Here's what he told me:
That "whum whum WHUM WHUM" noise happens because the wind passing over the small window opening is like a bored drunk blowing over the neck of an empty beer bottle.
Air passing over an opening forms tiny tornadoes as it moves past the front edge of that opening. When those tornadoes, or vortices, reach the opening's back edge, they make a wave of pressure that pushes air into and out of the car. Since sound is nothing more than waves of pressure, this makes noise. If you're driving slowly the effect's not too bad, but if you drive fast enough, you reach a resonant point. Imagine I stand by your open car window and use my science powers to push on the air inside the car, compressing it a bit. The car air then springs back out, then back in, then back out, then back in. With each cycle of moving out and in, the amount of air movement gets smaller until it completely dies away. But if I push on the car air again just as it finishes springing back out and is headed back in, and I do that over and over again, the amount of air movement gets a whole lot bigger and doesn't die away. That's what happens when you drive fast enough. The vortices keep pressing on the air in your car just at the right time to make big pressure waves that we can feel and hear.
The technical term for this effect is the Helmholtz resonance, though car people call it "side window buffeting". Back in the 1850s, a scientist named Hermann von Helmholtz showed that the sound's pitch depends on the size of your container of air and of the opening. The bigger the container of air, the lower the pitch. The smaller the opening, the higher the pitch. If you blow over a bottle, you get a medium-pitch whistle. Since a car's a big container of air, you get a low throbbing noise.
Driving faster won't do much to fix the problem. That resonant point I mentioned isn't really a point, it's more of a smooth peak. As you drive faster, the rate at which the whums occur speed up and the loudness goes up. Eventually, if you drive a lot faster, you'll move past the resonance and the whums will die away, but that's not much help if you don't want to get a ticket. The only real ways to fix it are to open the window more, which changes the resonant frequency; open another window, which messes up the resonance; or close your windows.
This isn't a new thing by any stretch of the imagination. I can remember this happening with my buddy's old 1984 Mustang when he had the sun roof open. The earliest research on it I know about is from 1966. J.E. Rossiter, who was with the UK's Ministry of Aviation, investigated why airplanes with open bomb bay doors would have the same "whum Whum WHUM" problem that we have in newer cars.
It's more noticeable in modern cars because they're more aerodynamic. By trying to improve gas mileage as much as possible, car manufacturers are creating cars that have much smoother air flows over the windows. My 2007 Fit has side window buffeting like crazy, but my old 1966 Fairlane never did. In my Fit, like in a lot of cars, it only really happens with the back window. I've not worked through all of the math, but my instinct is that it's because the air flow doesn't hug the car as closely and isn't as smooth over the front windows. As the air passes over the front of the car, it spreads out a bit before coming back and hugging the car sides closely near the back window.