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From digest.v7.n561 Sun Nov 9 17:07:25 1997
Date: Sun, 9 Nov 1997 10:50:55 -0500 (EST)
Subject: Low resonance - reply

Brad Franklin wries about a low frequency resonance when his rear seatback was folded to carry skis:

When you open any window or sunroof air wants to flow into the car, and in balance, what comes in must go out or you would keep building pressure from the added air.

When you open only 1 item the air that enters must exit from the same opening. When the trunk is opened to the passenger compartment the inside of the car has enough volume that quite a bit of air must enter before the pressure is great enough that air forces it's way back out. So, the slow air in/air out is the resonance you were hearing.

When multiple windows are open the in/out sequence is random, and you don't get the resonance. When the trunk is closed, the volume is so small that the frequency is higher, and the amplitude (sorry to be technical, volume) is low, so you don't notice it. In a Touring, there are vents in the tailget to let the air out, because they know you'll have the resonance otherwise.

Sorry to be so technical. This is a pretty universal occurance for open trunk vehicles. It's rare enough, and the trunk is "supposed" to be closed, so BMW doesn't bother to engineer a solution.

Scott Adare
BMw CCA 120672

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