What causes the weird throbbing sound when you drive with only one window down?

The short answer

When wind blows over just one open window, it pushes air into and out of the cabin in a rhythmic pattern, which produces a super low frequency sound. It's the same principle that produces a sound when you blow across the top of a bottle.

The long answer

You’re driving down a highway when your backseat passenger opens their window to get some fresh air. WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP WHOMP...

Ugh. That weird, throbbing sound is the airy equivalent of nails on a chalkboard.

After you yell at them to close their window, you wonder, "What causes the weird throbbing sound when you drive with only one window down?" Let's dig into the science.

What is Helmholtz resonance?

Have you ever blown across the top of a bottle? Well ​maybe not as impressively as these guys​, but you'll quickly notice it produces a sound. This phenomenon is called the Helmholtz resonance effect.

Source: ​University of New South Wales​

The bottle is a container with a narrow neck and a larger cavity filled with air at atmospheric pressure. When you blow across the top, the airflow drives the column of air in the neck down, compressing the air inside the bottle. That compressed air pushes the neck-air back out to return to normal pressure, only for your breath to push it back in, and the cycle repeats.

Source: ​University of New South Wales​

In essence, the trapped air acts like a spring going up and down, and this steady movement of air produces a sound. This is the Helmholtz resonance effect.

What does this have to do with the weird throbbing sound in cars with one window rolled down?

The same principle that makes a sound when you blow on bottles also makes a sound when you open a window.

In this scenario, the car is a big bottle. When you open a window while driving, the wind blows across the top of the opening. This pushes the air inside of the car, and the interaction between the two air masses creates vortexes that alternately compress and decompress the trapped air, producing the throbbing sound.

The sound produced when opening a window is so uncomfortable because it's an incredibly low frequency. At around ​20Hz,​ it's at the threshold of what someone would say they "hear" versus what they "feel." When it's produced in a car, the large volume of air makes this sound extra "throbby" and aggravates your ear.

Why is the throbbing sound worse when the rear window is open?

Cars are designed to be aerodynamic to reduce drag, but there's a key feature that needs to stick out: side mirrors.

Side mirrors: The bane of car aerodynamics

While side mirrors are designed to minimize the throbbing effect when a front window is open, there's not much that can be done to redirect airflow away from the rear windows, so the sound is worse.

Why does opening another window stop the throbbing sound?

If you open another window, you're providing another outlet for the compressed air inside the vehicle. So instead of oscillating in and out of the one open window, the air can now travel through another opening.

Why is the throbbing sound worse in newer cars?

Over the years, design has evolved to produce highly aerodynamic cars, much more so than older cars. The more aerodynamic and well-sealed a car, the closer the air can travel along its body.

Source: ​The Autopian​

This better design makes the throbbing sound worse when you open a window because the wind can more easily disturb the air inside the cabin, and the cabin air has fewer places it can escape to relieve the pressure build-up.

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Sources

Badar, B. (2022, May 4). What’s That Weird Sound That Happens When I Open My Car Window? Erie Insurance. https://www.erieinsurance.com/blog/weird-sound-car-window-wind-buffeting

Ford Media Center. (2024, January 25). Helmholtz Resonance: How Ford Engineers Tackled the Dreaded Window Throb Across Ranger & Everest. Ford. https://media.ford.com/content/fordmedia/img/me/en/news/2024/01/helmholtz-resonance--how-ford-engineers-tackled-the-dreaded-wind.html

Pearley Huffman, J. (2016, November 28). Explained: That Weird Throbbing When You Open One Car Window. Car and Driver. https://www.caranddriver.com/news/a15344069/explained-that-weird-throbbing-when-you-open-one-car-window/

Torchinsky, J. (2013, October 21). Why do slightly opened car windows make that awful sound?. Jalopnik. https://www.jalopnik.com/why-do-slightly-opened-car-windows-make-that-awful-soun-1447498738/

Wolfe, J. (n.d.). Helmholtz Resonance. UNSW Music Acoustics. https://newt.phys.unsw.edu.au/jw/Helmholtz.html