RE: [Harp-L] Waxing Rivets for Overbends [oops!]

["MLeFree" <mlefree@xxxxxxxxxxxxxxxxxxxxxx>]

Vern Smith wrote:

> Karman vortices are a function of the velocity of the air moving past an
> object (like a reed) and the width of the object.
>
> If the object is a flat plate (and a reed is) then the frequency = 0.18 x
> velocity of the air / width of the object.

I just knew you'd come up with a simple closed expression that explains it
all...  8^)

> We can get an idea of the velocity of the air through the slot by the
> difference in pressure in the chamber from pressure in the room.   If you
> are really blowing/drawing hard, this can be as much as 10 inches of
water.
> This corresponds to a velocity through the slot of about 212 feet / sec.
> Plugging this velocity and the reed dimensions into the Karman frequency
> equation above yields a frequency of about 5724 hz which is far above the
> pitch of the highest reed.  Unfortunately, this picture is vastly
> oversimplified.
>
> The video is of a flat plate in a free air stream...but that doesn't occur
> in a harmonica.  When the reed is in the slot no vortices are shed because
> the flow can't pass the reed.  Then the velocity is constantly changing
> because on the downswing the relative velocity is lower and on the upswing
> it is larger.   I don't know for sure, but I think that it takes some time
> for the vortices to build and they are not suddenly just there when the
reed
> comes out of the slot.  I am with Turbodog that Karman vortices are
> unlikely suspects to cause squealing.

I was thinking that the narrow width of the gaps between the reed and
reedplate would have a heavy impact on Karman vortices...

> A more likely suspect IMO is vibration of the reed in a higher/different
> mode.  The next higher mode for the reed is with a node at the base, then
a
> loop, another node about two-thirds of the length out and another loop at
> the tip. This would produce a frequency at approximately 3x the reed
> fundamental.  Or the reed might vibrate in a twisting mode. In either
case,
> any nail polish, wax, or other gunk applied to the reed would absorb more
> energy at higher frequencies and tend to suppress the higher modes.

Makes perfect sense, especially if you've ever seen the many complex modes a
drum head can vibrate in.

> A way to test for twisting mode could work.  Polish the tip of a reed to a
> near-mirror sheen.  Reflect a laser pointer beam off of it onto a screen
> such as a white wall or piece of paper.  Blow the reed normally and
observe
> the pattern swept out by the beam.  Then make it squeal.  If the squealing
> pattern is wider than the normal one, then reed twisting is responsible.

I knew sooner or later I'd find a reason to buy one of those pocket lasers!

> One could use FEA to locate the next higher mode or twisting mode of
> vibration analytically.  Then if those modes were prominent in the
spectrum
> of the squealing reed, then that's  evidence pointing to higher-mode
vibes.

My thinking exactly.

> Another thing that could be done is to view a sqealing reed while
> illuminated by a stroboscope.

An harmonica timing light!  I wonder if Harold Edgerton ever photographed
the innards of a harmonica?

> Then there is whistling.  Air forced through a small orifice (the slot)
from
> a resonant chamber (your embouchure) can whistle.  In the immortal words
of
> Lauren Bacall, "You know how to whistle don't you? Just pucker up and
blow."

Seems like a whistle wouldn't be loud enough to account for the loud and
annoying screaching I read about when overblowing asian harmonicas (I'm not
an overblower).

Thanks for shedding the light on this interesting subject, Vern.  I knew ya'
would!

Michelle