Contrabass Digest

2001-04-22

Date: Sat, 21 Apr 2001 19:11:55 -0400
From: farfl
Subject: [CB] Gold Bass Sax
 

Grant wrote:
>  and a gold plated bass sax at
> http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1425597922 (opening
> bid = $9K!!).  I'll be surprised if anyone bids on the sax...
>
> Grant
>

I'd be afraid to take the damned thing out of the house! Out of the case, even! I'd be afraid of
scratching it, or denting it, or........
I think I'd rather have an old, semi-beat-up Conn workhorse!
Best Regards,
Steven

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Date: Sat, 21 Apr 2001 18:27:22 -0700
From: "Timothy J. Tikker"
Subject: [CB] Ocarina car explained

This from an engineer/organbuilder friend:

Date:
           Sat, 21 Apr 2001 10:10:40 -0600
From:
           Michael McNeil

The car has become a Helmholtz resonator. Helmholtz investigated the
resonance of glass spheres with a small tube attached. He would blow
across the tube to generate a sound. The car interior is the sphere (not
too accurately) and the open window is the opening, and the motion of
the car with just the right crosswind will generate a sound just like
blowing across the mouthpiece of a flute; the amount of the window
opening is analogous to pipe cutup and also the distance of a flute
player's lips to the mouthpiece.

The equation for the resonant frequency is:

f = velocity * square root (3r/(8pi^3*R^3)),   sorry for the messy
notation in an email format and my liberties with the Helmholtz original
format.

r is the radius of the (window) opening in meters
R is the radius of the sphere (car interior) in meters
and velocity is the speed of sound in air, or roughly 335.3 meters/sec.

If we assume that r=0.1m (the window is rolled down a few inches)
R=1.2m (the interior of the car is about 7 feet across on average)

then this car will resonate at about 9Hz. Increase the size of the car
or reduce the opening of the window and the pitch will drop. The window
opening will drive the car interior at a specific speed, just like a
flute works between lip pressure and lip distance. You hear this rarely
because the forward motion of the car does not have the right angle to
make the window work; it needs a crosswind of just the right speed
relative to your car's forward motion.

Helmholtz never did understand why an organ pipe sounds (it wasn't until
1971 that we understood that as a function of matched impedances), but
he would immediately recognize the resonance in your car if he were
alive today.

I trust this is what you were after.

Regards,

Mike
http://www.provide.net

---------------------------------------------------------

Date: Sun, 22 Apr 2001 00:05:52 -0700
From: bitwise
Subject: Re: [CB] Ocarina car explained

Works for me. Grant had already posted the Helmholtz idea, but
thanks for the additional info. I'm not sure about the necessity of
a crosswind - I have noticed this phenomenon in just about every
4-door vehicle I've been in, in varying weather conditions of course.
I've also heard it in some 2-door vehicles.

Further, I neglected to mention that partially opening the sunroof
on the car in question *really* gets things vibrating! When I first
noticed the cross-sectional shape of the sunroof opening (looking
across the roof from the side), I figured it would make a nice
fipple - except facing the 'wrong' way to the airflow.

I haven't tried various combinations of windows and sunroof.
In fact, I'm not much inclined to experiment further. The 'note'
produced is loud enough to be quite unpleasant (and open
windows allow entry to things better kept out - one bee-in-the-
car experience is more than enough).

Craig

---------------------------------------------------------

From: "Dr Guy GRANT"
Date: Sun, 22 Apr 2001 18:09:14 +1000
Subject: Re: [CB] Ocarina car explained

Gday
Glue your old harmonicas to the car!
Guy
 

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Date: Sun, 22 Apr 2001 12:56:04 -0700
From: David Richoux
Subject: Re: [CB] Ocarina car explained
 
 

bitwise wrote:
> Works for me. Grant had already posted the Helmholtz idea, but
> thanks for the additional info. I'm not sure about the necessity of
> a crosswind - I have noticed this phenomenon in just about every
> 4-door vehicle I've been in, in varying weather conditions of course.
> I've also heard it in some 2-door vehicles.
>
> Further, I neglected to mention that partially opening the sunroof
> on the car in question *really* gets things vibrating! When I first
> noticed the cross-sectional shape of the sunroof opening (looking
> across the roof from the side), I figured it would make a nice
> fipple - except facing the 'wrong' way to the airflow.

my new Audi wagon has a sun roof - if it is all the way open, especially when
the rear door window are partially open the beats are at about 5-10 per second
at anything over 40 mph (I have not actually counted, but it is helicopter
loud!)  But there is a setting (pre-set by the factory) with the sun roof open
about 80% -eliminates the noise altogether...  I guess too many non-bass
enthusiasts were complaining ;-)

Dave Richoux
***End of Contrabass Digest***

 
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