That aerodynamic "whump-whump-whump"

The Show
1

What can be done about that "whump-whump-whump" we all know from opening the window on an especially aerodynamic car?

That was Cindy's question this week on Car Talk. Cindy loves her Jeep Liberty-- except for the complete and utter chaos that ensues when her kids open the rear windows. The constant "whump-whump-whump" is driving their eardrums bonkers. (Want to hear Cindy's call? Listen right here.)

Tom and Ray didn't have any brilliant suggestions, but they figured that someone, somewhere, had invented some kind of solution to this aural drubbing. Do you have a fix for the modern ear-drumming caused by especially tight and aerodynamic cars? Let us know. Ears everywhere will thank you!

2

Depends on the vehicle.

On my 98 Pathfinder I would only get it if I opened BOTH windows. As long as it was just one window it didn’t happen.

On my 05 4runner…it happens if either or both windows are open. But if I crack open one of the front windows it goes away.

3

Don’t open a window.
And if you must, open a second window slightly, opposite the one you want open, to allow full air flow instead of packing the cabin with air pressure that can only ecape back out the one that’s open only to be packed with pressure again which needs to exit the only open window etc etc etc …whup whup whup whip whup.

4

The way I have been able to have a good flow of air at high speed without the issue is opening only half-way both windows on one side of the car only. You can open a little more the one in front than the one in the back and you can regulate the amount of airflow you want.
The only issue is that you create a flow of air on the opposite side of the car going in the direction of the car and your hair will be disturbed … but you have the windows opened anyway and you wanted lots of air … right?

5

You need to experiment, there is no answer that works for every car or SUV. Once you have the noise then try opening a front window a crack, then the sunroof, the other side front window, etc. You should be able to find a way to reduce this harmonic noise to a tolerable level. If not, just close the windows and drive on.

6

The whump whump is a Helmholtz resonance, a fancy name for the sound you get blowing across the top of a bottle. Several possible approaches to the problem:

  1. Disrupt or deflect the air flow over the opening. Easy but messes up the aerodynamics.
  2. A properly designed screen (acoustic resistance) over the opening will dampen the oscillation. (Makes the car look like a prison transport)
    Note: A closed window represents an infinite resistance. (Easy and cheap)
  3. Increase or decrease the static pressure in the cabin. Creating a steady air flow across the opening accomplishes the same thing as 1. (Add a cabin vent)
  4. Change the volume of the cabin to produce a less annoying frequency. (Not very practical)