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Old 10-14-2011, 08:51 AM   #1 (permalink)
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booming-wind noise

I have been interested in air flow considerations for quite some time.

SITUATION: Many recent cars develop considerable booming/air pressure variations(large vortex generation?) when the rear side windows are down.
I own a Honda fit 2008
OBjECTIVE: reduce the considerable wind noise and booming when rear wind is open
SOLUTIONS examined:
solution #1. "A" pillar window slightly open-
Result : no booming but loud wind noise.
solution # 2. Fitted a vertical deflector 30cm X8cm at 45 degrees at front edge of open rear window-
Result: much less booming but loud wind noise.
solution # 3. Purchased Honda MUGEN grooved wind visors ( waiting for delivery) which promises of less wind noise /less booming
Result: not known yet

Below : a photogragh of the Honda after a drive in a snow storm.

REQUEST, with regards to booming AND wind noise:
#1. Explain the observed deposited snow , in air flow terms (or by drawing it directly on photo on my Honda FIT '08
#2. Relate experience with HONDA MUGEN GROOVED side window wind deflector

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Old 10-14-2011, 12:16 PM   #2 (permalink)
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Try blowing the top edge of a bottle and hear the low & loud sound it creates. Helmholtz resonance is happening. The moving volume of air is going over that small opening of the rear window and starts an oscillation with the inside volume of air, the corresponding frequencies creates a Helmholtz resonance & painful eardrums. Helmholtz Resonance
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Old 10-14-2011, 12:41 PM   #3 (permalink)
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Open both rear windows at once and see if it still happens? On my xA, I have to limit the rear windows to an inch or two down, to avoid this -- and opening them a lot will add drag anyway. The noise is very annoying, but what are you gonna' do?
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Old 10-14-2011, 02:23 PM   #4 (permalink)
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I can create almost an "air hammer" effect in the CARDIS. Very powerful, very low frequency (maybe subsonic - you almost feel it more than hear it), a few hertz repetition rate. It happens if the back windows are rolled down, the front windows are rolled up, and speed is in the 40s or better. (No, I don't drive that way -- discovered it by accident one day after being parked with all four windows down and forgetting to roll the rear windows up.)

But here's the thing: I never experienced it before I replaced the rear seats with a load floor (and I' ve forgotten the rear windows were down more than once).

I agree it's likely a resonant cavity issue, and my experience hints that INTERIOR changes might also help you. Try some expanded foam or fluffy blankets in the back seat. Not a solution, but it may give you clues to help solve the problem.

Me, I figure it'll come in handy if someone I don't want riding with me insists on "shotgun." Five minutes tops and they'll be begging me to let them out at the next corner.

Let us know what you find out, please; I'm curious.

Cheers,
Rick

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