My dog, Dylan, loves to ride with his head out of the window of the car. Even if it’s freezing out he still wants his head out in the wind. But if we’re going over about 25mph, I need to have more than just his window down to avoid that horrible throbbing that occurs when driving with one window open. This makes for a cold driving experience!

So what is that loud and unpleasant throbbing and why does it occur?

The phenomenon is called “Helmholtz resonance” and occurs when a container of gas (like air) has a single opening. When air outside the container passes the opening it creates small vortex pockets which cause the air inside the container to compress and decompress rapidly.

Helmholz resonance happens at different frequencies depending on the size of the container — the bigger the container the lower the frequency. The Helmholtz resonance is how a sound is created when we blow across the top of an open glass bottle. The whistling sound you hear is caused by Helmholtz resonance. Just like with a car window it is creating vortexes and a throbbing but due to the small size of the bottle the frequency created is at a high pitch and we don’t notice the throbbing. A car is a very big container, so the resonance happens at a low frequency which is the throb that we hear and feel.

Helmholz resonance only occurs when a container has one opening. A second opening allows an escape for the increase in pressure created by the vortexes that result from air passing by an opening.

Update — another thing.

The effect is more pronounced for the back windows than the front windows because the rearview mirrors create turbulence which interferes with the effect. Also – aerodynamics is why the Helmolz resonance is more pronounced in modern cars; vehicles of yesteryear had more turbulence.