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I recently drove a Nissan Versa rental car similar to the one shown above.
The Versa made a strong throbbing sound when I drove down a highway at 65 mph with the front side windows closed and the rear windows open.
Here is the sound recording that I made with my Samsung Android: Versa Sound
Please listen to the sound file using a headset with good low frequency response to hear the full effect.
The Fourier transform is shown above with its fundamental frequency at 21.2 Hz and integer multiples thereof.
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Dr. Stephen Granade explains:
That “whum whum WHUM WHUM” noise happens because the wind passing over the small window 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… 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”…
…As you drive faster, the rate at which the whums occur speed up and the loudness goes up.
Interestingly enough, Granade goes on to theorize that “It’s more noticeable in modern cars because they’re more aerodynamic,” the thinking being that cracking a window is more disruptive to a smoothly-tuned airflow. If that’s true, it would mean cars with boxy shapes would suffer less from Hemholtz resonance.
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A more technical explanation is:
Buffeting (also known as wind throb) is an unpleasant, high-amplitude, low-frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Vortex shedding in the shear layer over the cavity opening (side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure.
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- Tom Irvine