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.
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