Why Does Your Car Make a Terrible Sound When You Roll Down the Back Window?

We’ve all experienced it at some point. It always starts innocently enough. You’re in a car moving at a relatively fast clip, and then a single passenger rolls down their window.

Suddenly, a highly irritating pulse of wind and pressure shifts envelop the car’s cabin, instantly killing the mood until either the offending window rolls back up or another passenger rolls their window down in solidarity.

But what causes this annoying phenomenon that many casually refer to “wind buffeting?” Is it a defect in car design? Or does nature simply abhor a single lowered window? It’s time you got answers.

It’s officially called Helmholtz Resonance

Science, in all its wonder, formally named the phenomenon the Helmholtz Resonance after the German physicist Hermann von Helmholtz.

Among many scientific discoveries, Helmholtz developed a unique device called the Helmholtz Resonator to help separate and identify the individual pitches and frequencies contained within complex sounds with multiple tones.

It’s the same effect that produces sound when you blow into an empty bottle

Photo by Henry Phillips

Have you ever blown into an empty bottle to generate a low sound? That’s Helmholtz Resonance at work, too. In this scenario, the terrible sound we hear in cars features the same forces at work; the car body acts as an empty bottle.

As Neil Lewington, Ph.D., technical specialist and aerodynamics supervisor of Ford Australia, explains it:

“The air inside the bottle, or vehicle, is compressed and the air rushing past an open window, or the open bottle, creates little vortexes at the opening that cause the air inside the vessel to compress and decompress rapidly. We hear that action as sound – a horrible sound that sounds and feels like someone banging on your eardrums.”

It sounds and feels awful because the tone is so low we can actually feel it

The bigger the bottle, the lower the tone produced, or so the science says. A car with a single or partially open window acts like a huge bottle. 

According to Lewington, the low tone produced in cars in this condition via Helmholtz Resonance is at or around 20hz, a figure that audiophiles might recognize as the point at which our human hearing range stops at the low end. It’s also right on the edge of so-called infrasound, which refers to sound below 20hz that we cannot hear, but can still perceive via vibrations in parts of our body. 

In other words, what makes Helmholtz Resonance so memorable in cars is that we can in some cases feel the sound in our body and hear it.

Modern car designs are making the effect feel even worse

Pagani

The leaps car manufacturers have made in reducing aerodynamic drag to improve fuel efficiency and sealing cars tighter for noise reduction have each contributed to making Helmholtz Resonance worse. 

As Jim Zunich, GM’s global vehicle performance chief engineer for wind noise, explained to CarandDriver: “We want nice, smooth attached air for aerodynamics, but that’s worse for buffeting.”

In a nutshell, air flows more closely to the bodies of today’s sleek modern cars than their blockier ancestors. So when a window is opened, the air rushing by is in a much better position to invade the serenity of the cabin. 

Air-tight cabin designs also leave little room for air pressure to dissipate or leak out of the car.

The effect is less noticeable with front windows because of one crucial design detail

The design and position of side-view mirrors can enormously impact the air flow direction across the immediate windows behind them. As a result, car designers and engineers specifically design mirrors to help limit annoying air buffeting for the front two windows.

A vehicle’s rear window airflow remains untamed by comparison, making these windows more noticeable in generating Helmholtz Resonance.

The go-to solution to the problem still remains the same

While some design details, such as pop-up deflectors on sunroofs, can help mitigate the effect, the only real solution to ending Helmholtz Resonance while driving is the one you already know: roll up the damn back window to prevent air from leaking in; or roll another window down to create a way to relieve air pressure inside the car cabin.

And remember, window locks technically work for passengers of all ages, not just kids.