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Date: Sun, 22 Apr 2001 19:25:37 -0400
From: Lawrence de Martin
Subject: Re: [CB] ocarina car explainedI have observed this phenomenon, and the pitch is fairly consistant over a range of window openning sizes, and also remains similar in frequency with the sunroof opening. This disproves the Helmholtz resonator theory, since Helmholtz' spheres were tuned like an Ocarina - the formula shows the relationship between frequency and opening.
This is what led me to the aeolian turbulence theory, in which the airstream speed and volume enclosed determine the frequency. Try speeding up or slowing down to see if it changes key. BTW, I also observed that this only happens with a crosswind.
I also had a very frightening experience with a crosswind and aeolian resonance. I was driving a pickup with a U-Haul, both of which had slow leaks in the right side tires. I had loaded the heaviest items in the tail of the truck and the head of the trailer, thinking it would stabilize the load. When the northwest wind whipped up, the rig started fishtailing wildly. Just like with the wndow resonance, the frequency (about 1/2Hz) was constant over a wide range of conditions - and if I counter-steered, it got worse!
I couldn't slow below 45mph on I65, but I managed to get to an exit by steering into the oscillation. The proper pressure fixed the suspension problem, but then the pollution compressor froze and threw the fan belt. Motorized vehicles are evil machines! They're bad for your hearing, even if they aren't in aeolian resonance.
Larry de Martin
Michael McNeil wrote:
> <The car has become a Helmholtz resonator...
> The equation for the resonant frequency is:
> f = velocity * square root (3r/(8pi^3*R^3))---------------------------------------------------------
Date: Sun, 22 Apr 2001 17:04:07 -0700
From: bitwise
Subject: Re: [CB] ocarina car explainedDave,
The sunroof on this 2000 wagon has 2 or three 'settings'
along a detented range from closed to fully open, but none is
specially marked for noise reduction. I did see something about
it in the manual, though - guess I'll have to read it again.
How do you like your Audi? (Off-topic, I know)Larry,
You make some valid points. Looks like I'll have to experiment
some more, after all. Fortunately, bees aren't common around
here. :-) I could wish I had some ear protectors that were
effective at subsonic frequencies, but what would a contrabass
maniac be doing with those in the first place.
Agreed that a jointed vehicle on a freeway is one place where
low-frequency oscillations definitely are not wanted.Craig
---------------------------------------------------------
From: "Jean Adler"
Date: Sun, 22 Apr 2001 21:11:23 -0500
Subject: [CB] emailGrant,
I am having the oddest messages whenever I try to post. Which email is
correct? I have several addresses in my address book. Also, I will get
messages that the size is too large.
Jean---------------------------------------------------------
Date: Sun, 22 Apr 2001 23:51:05 +0000
From: Robert Howe
Subject: [CB] Selmer Super 80 Action Series 2 soprano saxophone for saleHello guys and gals,
I've got the Bb soprano saxophone of your dreams for sale. It is a like-new
Selmer Super 80 Action Series 2, two years old and hardly used. I am
selling it for $2500. The list is $3900 and the discounted price $2900.
Gold lacquer in great shape, a little wear on the case, serial 450984,
single neck, has a front high F and keys to high F#. Plays like a dream,
but I am happy with my Yanagisawa and I need the cash.After April 15, boy, do I need the cash.
Serious inquiries only. Unlike most of my sales, this price is firm.
Thanks,
Robert Howe
---------------------------------------------------------
Date: Sun, 22 Apr 2001 23:24:14 -0700
From: bitwise
Subject: Re: [CB] ocarina car explainedHere are the results of my latest, rather limited experiment.
I took advantage of a trip to the store to check the frequency
vs. speed, as Larry suggested. Below 40 mph I heard nothing,
unless the sunroof was open. Between 40 and 45 (about as fast
as I dared go with Murrieta's finest lurking about), I observed
little effect on frequency, but a marked increase of volume with
speed. Varying the size of the window opening produced a similar
result. There seems to be an 'optimum' range of window opening
sizes. The test was conducted on a road that makes a big loop,
so crosswind should have been canceled out.
No noise was heard with only the front windows open.Craig
---------------------------------------------------------
Date: Mon, 23 Apr 2001 14:48:18 -0700
From: Grant Green
Subject: Re: [CB] ocarina car explained
>I have observed this phenomenon, and the pitch is fairly consistant
>over a range of window openning sizes, and also remains similar in
>frequency with the
>sunroof opening. This disproves the Helmholtz resonator theory,
>since Helmholtz' spheres were tuned like an Ocarina - the formula
>shows the relationship
>between frequency and opening.
>
>This is what led me to the aeolian turbulence theory, in which the
>airstream speed and volume enclosed determine the frequency. Try
>speeding up or slowing
>down to see if it changes key. BTW, I also observed that this only
>happens with a crosswind.What are the frequency equations for aeolian turbulence? I'd expect,
under the Helmholtz equation, that the amount of window opening won't
change the pitch very much, because it depends on the fourth root of
the area. But, I'm always willing to examine other theories :-)Grant
++++++++++++++++++++++++++++++++++++++++++++++++
Grant Green http://www.contrabass.com
Professional Fool -> http://www.mp3.com/ProFools
++++++++++++++++++++++++++++++++++++++++++++++++
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