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Another Auto Design Fail: The Helicopter Effect, a.k.a. 'Side Window Buffeting,' a.k.a. Helmholtz Resonance

By Rain Noe - October 23, 2013

in Cars

Earlier I griped about how the act of getting in and out of a car, a very basic automobile experience, is often neglected by designers. With the exception of the Tanto we looked at, there aren't many examples that demonstrate designers are really thinking about the problem and/or have the clout to address it.

Another lame car-based experience is the helicopter effect, a.k.a. "side window buffeting." You know the deal—you're driving at highway speeds when someone in the back cracks their window open, and suddenly your eardrums are assaulted with such a helicopter-rotor-like din that you can practically hear Wagner. Then you must open a window on the other side of the car to try to balance the effect. Or you can do what I do, which is to eject the offending passenger under a strict zero-tolerance policy for disturbing my automotive ecocsystem.

I warned you, dude

I warned you, dude

Two questions, and the first is: Why does this happen? Jalopnik asked physicist Dr. Stephen Granade to explain:

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. Score another plus for the Tanto.

Second question: Why do you think this problem hasn't been addressed by design? Do you think designers simply don't have the juice to incorporate a manufacturable solution—or that no one cares?

o

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Deepak Eapen

8 years ago

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Wind buffeting is a common trait of roadsters, which can only be culled by mounting a good wind deflector. I have retrofitted something called Backblade windscreen on my drop top and now there's not even the slightest trace of wind swirls or draught noise in my cabin.

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- Jamie Cox
  
  Deepak Eapen
  
  7 years ago
  
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  Yeah, the helicopter effect is rampant on most roadsters. It was almost unbearable on my ride. But after an additional draught-stop was mounted things calmed down. Thanks to the Zefferus wind deflector now my face and head don’t get whacked by the buffeting gust. Huh.
  
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- Jane Williams
  
  Deepak Eapen
  
  7 years ago
  
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  Yeah, Backblade wind deflector is
  what I have affixed on my ride too. Until a draught-stop was mounted, the wind
  noise and buffeting were too overwhelming that I was about to get rid of my
  drop top. Now I can relish the al fresco cruises even on motorways.
  
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Bradley Mcclary

9 years ago

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PROBLEM NOW SOLVED with,
VentAIRator Vented Visors

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Scott K

10 years ago

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Creating a vent in the rear deck of the passenger compartment and venting the trunk always solved the issue for me. I have had a few vehicles that did this Helmholtz resonance and the custom venting I did stopped it instantly each time you just need to make damn sure that the vents are screened to keep out critters. The ex learned that one real quick, lol.
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Tiby

11 years ago

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My 1979 Volvo 244 GL had vents on the rear parcel shelf just under where the bottom of the rear window is. They funnelled air to the boot or out underneath the car. Never had any problems with the helicopter effect, inability to close a door because of air pressure, or anything else of that nature. Simple solution.
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mipster

11 years ago

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Wouldn't a simple interlock that would open the other rear window if one is opened a small fraction solve the issue without redesigning. It could even be speed sensitive, etc.
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mzungu

11 years ago

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Effects depends on traveling speed, and how much the window is open, how many windows are opened.... You can't design around all that infinitely different configurations...
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Ken J

11 years ago

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My pickup truck has diffusers along the front edge of the sunroof opening to break up the air coming in. I hate the wind noise it makes and was always wondering if there was a better way to tackle the problem of Helmholtz resonance. The only thing I can think of is changing the internal volume of the cabin so as not to resonate. While you roll down that one window, a cylinder could expand and contract to change the cabin volume out of resonance. It would probably require some active smarts in the car to know what volume adjustments would be required.
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JamesS

11 years ago

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@Mike. I have those plastic nodules on my car and always wondered what they were for. If they are in fact there to combat the side buffeting, then I would guess they are some sort of vortex generator to create a boundary layer for the fast moving air to move over. But I wonder if this would defeat the purpose of opening the window in the first place?
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Rob

11 years ago

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This happens on my VW Golf TDI whenever someone cracks a rear window. So annoying. (and uncomfortable for the eardrums)
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Mike Parsons

11 years ago

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Had this happen on the new Audi A4, it was so bad you could see the glass was moving a good 5mm, it was actually quite scary!

I thought that this effect was combated by the small plastic nodules found half way up the front windows?

I have no idea what they are called, just that they are missing from modern cars!

Cheers,

Mike

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username

11 years ago

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because a good portion of "car designers" are sylists, not problem solvers
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Thijmen

11 years ago

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I think its a simple weighing of importance. Of course cardesigners know about every aspect of the cars they make and probably have one or more ideas how to minimize the helicopter effect. But I think this is just choosing aerodynamics over being able to comfortably open up the rear window at highway speeds because the first aspect is always relevant, and the second one only when you have more than two people in your car with the need to have air blasted to them at 130 kph (which is almost never probably).

This refers to getting into the car as well. Probably the easiest way to get into a car seat is being able to walk into the car standing up and sit down on a chair. However, this would make a car idiotically larger than needed most of the time. Just measure the time you are getting into and out of the car and compare this to the time you are sitting in it. However, there are many models which offer easier access (tanto like models as the suzuki wagon R+, smaller pick up trucks, vans like the VW transporter), but these models have greatly reduced aerodynamics.

Multi billion projects like car models are managed to extremes to minimize mistakes. Combine this with over 100 years of global car design knowledge and I think calling these downsides of design choices "auto design fails" is a bit short-sighted.
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Jim

11 years ago

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I would guess that with all the focus on interior climate control that's developed (for example, multiple climate "zones" for differing desired temps, at least in the front of the car), designers are not considereing the passenger in the back might open their windows. Also, it's not likely to be a big selling feature (can you imagine the adds for this - Now finally you can crack your window to get fresh air after your passenger farts, without going deaf).
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