We’ve all been there. Driving along on a fine summer’s day, feeling the warmth of the sun on our skin, thinking about how wonderful life is. But then you get a bit too hot and you think “I know, I’ll open a window to let some cool, refreshing air in,” only to be greeted by the sound of eardrum-popping bass.
You curse compromise and either sit there sweating like a Honda at a muscle car meet or sacrifice the serenity of the drive for some sweet fresh air, glaring at smug-faced drivers next to you pumping their air con.
But why does this buffeting happen? Why does it seem to be worse in some cars than others? Why does its severity depend on which window you open? And why does opening another window help? So many questions, and fortunately science has the answer.
It’s called the Helmholtz Resonance, and it’s the same idea as when you blow across the top of an open bottle to make a tune. It was discovered by German physicist Herman von Helmholtz, who explained that as air passes over the leading edge of an opening, such as a car window, it creates small vortexes – i.e. mini tornadoes.
These vortexes then compress and decompress the air in the cabin, creating waves of pressure, which your ears interpret as sound. So, the repetitive ‘whomp, whomp, whomp’ sound is the air in the cabin getting its butt kicked by the air passing outside the window.
Helmholtz explained that the smaller the hole and container of air, the higher pitched the sound. That’s why bottles make a tuneful hum and car windows, which open to a large cabin, make a low thumping sound.
There are a few factors that can affect how severe it is. The buffeting tends to be worse in modern cars because they are more aerodynamically efficient, meaning air passes closer to the side of the car. It’s the same reason it tends to be worse if you open a rear window, because the front of the car and wing mirrors disrupt the air passing over the front windows before crashing back into the rears.
You can mitigate it somewhat by opening another window, which alleviates some of the pressure and changes the resonance. Meanwhile, going faster will generally just make the buffeting louder and more consistent, but really keep your foot in and you should be able to reach a point where it smooths out and dies down – though this probably isn’t feasible away from the autobahn…