"Helicopter Acoustics" - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 04-10-2008, 11:30 AM   #11
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Quote:
Originally Posted by imidazol
Helmholtz resonance:
"When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time."

Consider it the springiness of the air inside the car being bounced high and low on pressure. If air weren't so spongy, this wouldn't happen.
Thanks. Much clearer explanation than mine!

Quote:
Originally Posted by trkelly
Well I guess that about answers the question then, it appears to be quite common and must be due to the design of the car, I have had plenty of other vehicles and this was a first for me.
Yes, it has everything to do with the design of the car and how the air flows over the body. Some will be worse than others.

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Originally Posted by statusplease
my 93 ssei, does it even if both rear windows are completely down, with the sunroof and front windows closed. Also, my honda pilot sounds like a 15" sub with the lowest note going through it with just the rears down
Yeah, I should have been more clear about opening another window. Ití* not surprising that opening both rear windows might even make it worse. Youíre still creating a pressure differential within the cabin. If you crack a front window you reduce the pressure difference to a negligible (or tolerable) level.
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Old 04-10-2008, 03:19 PM   #12
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It doesn't hurt my ears, but I can definitely feel it in there, plus it makes my eyes water.
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Old 04-10-2008, 04:28 PM   #13
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My car does this too, if I open the rear windows before the front ones.
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Old 04-10-2008, 04:35 PM   #14
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One would think that there should be a way to stop this with some kind of airfoil gizmos at the leading edges of the windows.

I wonder why older cars never did this...is it because the cabins weren't sealed as well as they are now?
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Old 04-10-2008, 05:31 PM   #15
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Actually, a better seal in the cabin will create a worse effect. The problem is that air pressure is rapidly increasing then dropping, causing the rumble. like I said, in a friend'* brand new car, it was TERRIBLE but fun to play with

It'* just the design of the cars, I'd imagine the angle at which the windows hit the oncoming air has a lot to do with the rate at which it floods into the car and whether or not it becomes trapped.

The air is blowing out of the car because of the windspeed outside, the lower air pressure outside sucks it out. Then all of the sudden your car has become somewhat of a vacuum. It fills this vacuum with air, which in turn gets blown out again. This creates the rippling pressure effect, and consequently, a rumble in the part of your body which senses pressure changes the best -- your eardrums.

And those air deflectors that you can put on the cars would help if the rears were designed as the fronts are. There is no deflector on the leading edge of the window in the rear, only the top. If there was one on the leading edge, depending on your speed and stuff, the air would likely pass right over the window at a distance large enough to not suck out any air inside the car.
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