Re: sunroofs and booming

[Robert Houk - SMCC WorkGroup Server Firmware <Robert.Houk@East.Sun.COM>]

   Ok, remember that whole thread about (sometimes painful, always annoying)
   pressure changes with the sunroof open? I've found it's sensitive to the
   road!  For example, I go cruising down one road, big time rumbling.  Stop,
   turn onto similar road(but paved at a different time and hence a different
   composition) and no noise! Neither road is bumpy at all, it just seems
   like road surfaces matter.  Could explain why some listers got it, some
   don't...I'd be willing to be the suspension has a lot to do with it too.

I dunno...that's kinda hard to explain. I wonder what is going on?

I did some experimentation/data gathering with my A4...so...

First off, just to make sure we're all on the same page, this is
specifically limited to the A4, as that's the only model *I* have
access to, and that exhibits the behavior.

Specifically, we are not talking wind noise, nor "turbulence", this
is an "acoustic" resonance, very pronounced. With one rear window
open, at ~35mph, you just hear/feel buffeting that is trying to
resonate but hasn't really synced up yet. By about 45mph, it is
definitely resonating on and off. 50-65mph is the prime car speed
range to experience it. (I wholeheartedly encourage anyone with any
questions to borrow/test-drive an A4 to experience this in person!)
By about 75mph, it is falling out of resonance, and 80mph is back
to straight "wind noise/turbulence" that you would expect from an
open window at 80mph.  It appears to be symmetrical wrt which rear
window is open. The effect seems to be somewhat moderated by driving
just at the rear corner of a big truck, but is definitely still
present.  This resonance is very pronounced. It is distinct pressure
pulses at, I am going to guess, around 4 to 6Hz. You perceive them
with your entire body: each pressure wave whomps you upside your
head, pummeling your eardrum, rattling your eyeballs, and shaking
the fillings in your teeth. It is actively unpleasant in magnitude,
not quite painful in short duration. In 30 years of riding/driving
assorted multi-wheel vehicles, I have never experienced anything 
like it; it's not shimmy, it's not wind "noise", it's not suspension
vibration. The closest analog I can come to it is being some dis-
tance off from a .50 Browning heavy machine gun -- a repetitive
series of distinct pressure waves breaking over your body.

Driving with both rear windows open modifies the resonance somewhat.
It comes on a little later, and at a higher frequency (I'm gonna
guess around 10-12-15Hz -- almost a "buzz" [a *BUZZ*]). It lasts
until over 100mph where it starts breaking up (and I ran out of
open roadway...).

Opening a front window even just a little almost completely kills
off the resonance.

You can feel the whole inside of the car "pulsing", but it is not
coming from the steering wheel, body, transmission, etc. The roof
is particularly "busy", so it might well be that the actual source
is the roof panel being driven by waveflow on the top side, acting
as a piston to pump the whole car interior (as opposed to wind
flow acting on the rear window orifice), it's kinda hard to really
distinguish the two cases from the driver's seat. [Hey, it could
be both acting in concert! I dunno...].

Opening the sunroof (canting up the back, rather than sliding the
whole panel back into the roof) has a radical effect on the reso-
nance also, but whether it is changing the cavity (another opening
like the front window) or deflecting the airflow over the roof I
can't say.

I have tried this on several different roads, in different directions,
at different times of day. There is substantial variation in details
(first effect at 30mph, or 35mph, etc.) but the overall picture is
remarkably consistent within the those limits.

Blah humbug!

I hope they fix this before foisting the S4 on us!

					-RDH