Why Do Those Pounding Noises in the Car Occur When You Open Your Windows?

Have you ever tried blowing across the the top of an open bottle? If you position your lips correctly (and at different angles), and then blow at the right strength, you'll probably be able to make a couple of tunes there. And heck, if you're good, you might even do the entire Philippine national anthem while waiting at the gas station.

That phenomenon is called the Helmholtz Resonance, which was discovered by German physicist Herman von Helmholtz.

Helmholtz Resonance happens when air passes over the edge of an opening—like a car window. When this occurs, it starts to create small, tiny air swirls--like a small cyclone or tornado.

These "swirls" come in and out of the car cabin, compressing and decompressing the air inside. Consequently, it creates wave-like pressures within the interiors, creating that bassy "whomp, whomp, whomp" sound you hear when you open your car windows.

According to Helmholtz, the smaller the hole and the volume of air inside the container, the higher the sound's pitch. That's the reason why you create sharper, higher-pitched tones with bottles, and low, knocking sounds with bigger spaces like the car cabin.

Factors that affect it

The sound tends to get worse when you drive recent car models. Oftentimes, the modern ones are created to be more aerodynamic, so air moves and passes closer to the vehicle's side. This increases the chances of air to come into contact with the edge of an open window.

The sound becomes more pronounced when you open the rear window--and lesser at the front. Incoming air tends to disperse when it hits the car's nose and side mirrors, passing over the front windows and lessening the air's strength. Then it smacks right back (in full force) as it moves toward the rear windows.

How to lessen the noise

One good way to lessen the sound is to open another car window. Doing this will lessen the pressure inside the cabin, lighten the air waves' intensity and the sound it creates.

Another good way is to drive slower, because the faster you drive, the more it would make the sound louder and more regular in consistency.