Re: sunroofs and booming

["Janet Scruggs" <scruggs@mbay.net>]

Mike, someone on the list nailed the problem a few digests ago, sorry I
don't recall who it was, when likening the situation to an organ pipe.  A
smaller example is that of blowing over the mouth of a soft drink bottle. 
Actually a flute is little more than a highly refined metal tube with
controllable openings which alter the volume, and therefore the resonant
frequecy. Influencing factors are volume of the enclosure (which doesn't
have to be completely enclosed, actually), area of the opening, and mass
flow over the opening.  My guess is that he aspect ratio of the enclosure
may affect how easily the resonance starts.  There is a mathematical
relationship between the factors but I don't know what it is. This is the
acoustic equivalent of an electronic 'tank circuit' in that when all
factors are proper the pulses are additive and a fundamental resonant
frequency is the result.  The bigger the volume, the lower the frequency.
Change any of the factors and the resonance goes away.  

To address the situation in our cars we can influence any or all of the
contributing factors; change speed, close the sunroof a little, or crack
open a window... or any combination of the three.  As Mike points out,
fiddling with the wind deflector will influence the phenomenon, but more
from a standpoint of detaching the airflow from the opening... like moving
the bottle away from your lips.  Most of the cars that I have had with wind
deflectors on the sunroof have had springs that are too strong for all
speeds, resulting in the deflector having too great an angle of attack with
the windsteam, creating turbulence and noise.  Someone with much
intellectual curiosity and too little to do could devise a mechanism that
would automatically adjust the wind deflector the a point which would
minimize acoustic resonance.  Anyone?  Hands please.

Regards, Gross Scruggs.



Recently Mike Veglia wrote:

Subject: Re: sunroofs and booming
Don't count on it. As I think I posted once before, that was a *huge* peave
of
mine with my then new (now long gone) '88 80q. I think it may have to do as
much with the resonant frequency of the body shell as the proximity and
location of the airflow over the front of the sunroof. (Like where you put
your lips on a flute makes a difference.) I have never expereinced it in an
(equaly low cd) type 44 chassis car, sedan or wagon...though I gather
perhaps
other listers have. (I am far from an aerodynamic expert...care to step in
Gross?) On my 80q one could reduce this simply by applying pressure against
the flap at the front of the sunroof opening. I showed this to an Audi zone
rep...to no avail. Granted it is very annoying IMO and I would think that
this
would have been "fixed" sometime in the evolution from 80/90 to A4 body
shapes.

Mike Veglia
87 5kcstq (sunroof closed with 14 days straight of rain in "sunny" Santa
Cruz,
CA.)