"Helicopter Acoustics"
04-10-2008, 10:30 AM
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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.
"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.
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.
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
04-10-2008, 03:35 PM
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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?
I wonder why older cars never did this...is it because the cabins weren't sealed as well as they are now?
04-10-2008, 04:31 PM
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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.
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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