Why Does the Air in Your Car Make that Strange Sound When a Window is Open on the Highway?
(3 minute read)
When I drive my dog to the park, I usually open one of the back windows of my car so she can sniff the breeze. But if I forget to close that window before turning onto the freeway, I’ll feel a rhythmic pounding on my eardrums as we pick up speed.
Why? Where does that noise come from?
The short answer
Two air masses (“outside” air and “inside” air) are rubbing against each other. The resulting friction causes a vibration, and we experience vibrating air as sound.
The long answer
Think about what happens when you blow across the mouth of an empty glass bottle. The moving outside air gently pushes on the inside air, temporarily compressing it. Air doesn’t stay compressed if it has somewhere to go, so it will try to expand back to its previous size. But as long as you keep blowing, the inside air will keep getting squeezed, then expanding, and so on. Soon it reaches a steady oscillation, or movement back and forth.
The constant oscillation of air pressure inside a rigid container is called Helmholtz Resonance.[1] The resonance frequency, or the speed of the air vibration, depends on the size of the container. If you repeat your experiment with the bottle half-full of water, you will hear a higher-pitched sound than when the bottle is empty, because the air mass inside is smaller. Since the inside of your car is a lot bigger than the inside of a bottle, its air vibrations make a low-pitched thudding sound. You might also feel a weird pressure in your ears. That’s because the inside air is being repeatedly squeezed, which in turn exerts a force on your eardrums.[2]
The fix
Want to enjoy a fresh breeze while cruising in your car but hate that annoying sound? Try opening more windows to give that pressurized air a way to escape.
Follow-up questions
Why don’t I hear the sound when the car is moving slowly?
Helmholtz resonance only happens when one body of air is moving across the opening to another body of air. In the case of the glass bottle, you move the outside air by blowing. But you won’t get a sound just by breathing lightly on it. You have to cross a certain threshold of force before the air inside starts to feel the squeeze. Same with the air in a car.
Why is the sound worse when a back window is open than a front window? And why is it more noticeable in some cars than in others?
These are related questions, and the answer has to do with the smoothness of air flow around a vehicle as it moves. On more aerodynamic vehicles (like sports cars), air slides evenly over the surface in straight-ish lines. On less aerodynamic vehicles (like freight trucks), air gets pushed in many directions, creating swirls and vortexes. The smooth flow will create a stronger Helmholtz resonance. Think about the bottle again: you get the best sound when you blow straight across the opening, not into it.
The same goes for front vs. back windows. The rear-view mirrors block some of the air flow around a car’s exterior, creating a vortex near the front windows. This means that when a front window is open, the outside air is not moving flat across the opening; it’s swirling in all directions. Hence, you get a weaker sound. Around the back windows, the air has somewhat smoothed out, so you get a stronger sound.
[1] Huffman, J.P. (2017, Nov. 2). 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/
[2] For a cool related video, check out: Larraza, A. [NPS Physics]. (2015, May 26). Helmholtz Resonator – Think you know how Helmholtz resonators work? [Video]. YouTube. https://www.youtube.com/watch?v=PoEyIJx3uM0
