Ownership: Explained: That Weird Throbbing When You Open One Car Window

This 1936 Ford coupe is the barn find that got away … but not for long

  This 1936 Ford coupe is the barn find that got away … but not for long There’s not much worse than getting scooped on a deal. Honestly John Foster didn't know he wanted a 1936 Ford three-window when he took delivery of a 1940 Fordor in beautiful downtown Burbank, Washington. But the coupe, it was beyond his means. So while he never totally forgot about the car, he kind of put it out of his mind for the time being anyway.

Explained: That Weird Throbbing When You Open One Car Window© John Pearley Huffman Explained: That Weird Throbbing When You Open One Car Window From the December 2016 issue

It can be as loud as standing alongside a Boeing 767 at takeoff. And it’s about as irritating as having someone thumping on a bass drum in the back seat. Whether you call it wind throb or buffeting or just plain annoying, it happens when someone in the car opens a single window at speed and it stops when a second window rolls down.

The phenomenon that produces this noise is the Helmholtz Resonance, the same principle that makes a bottle hum when you blow over its open top. It’s the interaction of the gas in a container with a single orifice and the other gases that are, um, passing over that orifice. In this case, the container is the car. The interaction between the two masses of air produces vortexes that compress and decompress the air, producing the throbbing effect. Hermann von Helmholtz, the German physician and physicist who described this interaction, died in 1894 and was thus unavailable for comment.

Check Out the Amazing Stories Behind These Barn Finds & Hidden Gems at the 2016 MCACN Show

  Check Out the Amazing Stories Behind These Barn Finds & Hidden Gems at the 2016 MCACN Show There are always a lot of very interesting cars at the Muscle Car and Corvette Nationals, but regardless of what brand or make you personally appreciate, it seems everyone ends up in the Barn Finds & Hidden Gems section of the Donald E. Stephens Convention Center. Author and Automotive Archeologist Ryan Brutt gets credit for the legwork and persistence that creates this popular display each November. While “the rarer the better” is a good MCACN creed even here, with these cars it is often the stories that make each one special. How did it get from the past to the present? Where it was found? What will be done to it later? Mr.

How intrusive buffeting can get depends on the car’s shape and the size of its windows as well as the speed the vehicle is traveling. Modern cars and trucks are more subject to air thump because they’re so aerodynamically efficient and well sealed against wind intrusion. Jim Zunich, GM’s global vehicle performance chief engineer for wind noise, explains: “We want nice, smooth attached air for aerodynamics, but that’s worse for buffeting.”

  Explained: That Weird Throbbing When You Open One Car Window © Provided by Car and Driver

Gimme Buffet

See the top image for how the side-mirror shape can affect air movement around the front windows, and the middle image for how not much can influence flow around the rears. A pop-up deflector would shift airflow over the open sunroof rearward, mitigating those annoying vortexes.

2017 Chevrolet Camaro ZL1

  2017 Chevrolet Camaro ZL1 Chevrolet sends the Camaro to finishing school and ends up with an impeccably behaved monster.It’s a curious truth of automotive engineering that, in general, the lower the volume target for a new vehicle, the more effort is invested in perfecting it. Engineers seem to sweat the nuances of a Ferrari far more than they do those of a Fiat. The same goes for performance models of mass-produced vehicles. With the tedious stuff already taken care of—say, making the car come together as easily as a SnapTite model on the assembly line—the performance guys are free to spend months toying with bushing stiffness.

When the car was designed with little concern for aero, the haphazard mass of air boiling around it only coincidentally adhered to the vehicle’s surface and only occasionally allowed the Helmholtz Resonance to generate its vortexes. And even when the problem did arise, the car’s other windows and doors were hardly airtight, so the air leakage around them relieved any pressure differences. But a modern cockpit is a well-sealed drum from which only minimal air pressure leaks. In this small, particular way, a modern vehicle is too good for its own good.

But why is buffeting so much worse when just a rear window is down? Two words: side mirrors. They’re one of the last things developed in a vehicle’s design, and they’re placed and shaped ­precisely to direct airflow in a way that minimizes buffeting at the front windows. But there are no easy tweaks to be made to tune the airflow around the rear windows.

Rare Mopar Swap Meet Gold From The Spring Fling!

  Rare Mopar Swap Meet Gold From The Spring Fling! There’s not a Mopar guy on the planet who hasn’t at one time spent massive amounts of time looking for a new project or parts for one. You might even argue that guys don't finish projects because they're having too much fun digging through swap meets. There is an art to spotting a rare widget in a huge box of junk, but there are telltale signs that you might be looking in the right place. © Steve Ridlon mopar-car-show-cpw-2017-spring-fling-photo-highlights-20.

Buffeting is likely to get worse as vehicles get more aerodynamic. “Obviously, if we could resolve it free of charge, it’s something we would do,” concludes Zunich. “But because it comes with styling, aero, and noise penalties, it’s a trade-off engineers have to make.” Well, whether or not we understand how buffeting works, most of us have figured out a solution: Crack another window.

Powering Up

Software supplier Exa developed PowerFLOW, the simulation software that generated the images on this page. PowerFLOW is a computational fluid dynamics (CFD) program. CFD generates a detailed mathematical model of a fluid flow, in this case allowing engineers to observe and tweak how air flows over, around, and through a car on a molecular level. It’s just one of the tools that allows automakers to design a complete car before a single part is stamped or cast. Suck on that, Instagram. —Jared Gall

Follow MSN Autos on Facebook

Breaking Records at the 'Ring: Subaru Steps It Up With a WRX STI .
Taking our favorite pair of Boxers on a few laps of the Green HellBut in order to even come close, the weather needs to cooperate, which it's not doing. As if a mind-boggling 73 turns weren't enough of a challenge, the Nordschleife ups the ante by featuring different weather systems at the same time around its generous 12.9-mile expanse. One section might be sunny and dry while another is foggy or hailing. Or wet. Right now, it's all wet.

See also