[Harp-L] Re: Waxing Rivets for Overbends [plus!]

[Bobbie Giordano <bobg@xxxxxxx>]

Ken "Mojo Red" posted:
>I've had very good luck with using the "Turbo Tape"
>idea to stop overbend squeal on LO harps and on
>Hering 1923 harps (not all of which squeal). 
<...>
>Here's a graphic from the TurboLid Website:
>http://www.turboharp.com/graphics/company/Turbotape.TIF

Interesting... some years ago, I met James Antaki at the original Harmonica
Summit in Chapel Hill, North Carolina, and my fascination with acoustical
properties and other dynamics of free reeds was enhanced by some very good
conversations about them with Jim, who had been studying them for some time
in great depth.

I did some more thinking and research after that, and later, at the Summit in
Minneapolis, as I recall, he and I had another discussion with my partner
Douglas Tate in attendance in our hotel room.  I strongly proposed the
possibility that the activity of vortices caused essentially by the Karman
vortex street phenomenon was at the root of what induces the squeal or
ringing of reeds one sometimes experiences when bending or overbending reeds
of a harmonica.  Jim countered quite firmly that he doubted that was
involved, but when later still I saw his computerized animated image of the
air flow as it moves past a reed, I was even more convinced there might be
something to it.

Now, I see at the website Mojo Red directed us to that a description is given
with the computer model image of "HOW IT WORKS (We Think)" which is explained
thusly:

>Air flow in direction causing reed to close creates instability 
>downstream (known as "Karman vortices")  TurboTape interrupts this 
>flow pattern, dampening the instability, resulting in smoother air 
>flow.  (We Think)

Hmmm... Kind of gratifying to learn I may have been on the right track after
all. :)  I still think there is more to it, though; mainly because the Karman
Street qualities would logically happen with any brand of harmonica, and
indeed, I've meddled with this phenomenon so long that I can make virtually
any harmonica exhibit this annoying trait... even the Renaissance!  There are
definitely harps it's easier to make it happen on, however, I have to agree.

The usual consideration of the Karman Street action is in reference to a
stationary cylinder placed in the middle of some steady gas or fluid flow,
but any shape of object can cause something similar.  The interesting aspect
to this is that the vortex shedding [the alternating vortices downstream of
the blocking object] induces vibration of the blocking object as it flows
past it.  And, as I understand, if the flow [liquid, gas, air] is such that
it creates a frequency in the vortex shedding that matches that of that
object in the flow, resonance can occur.  A prime example of this is the
singing of telephone lines... or an Aeolian Harp!  So, why not the ringing of
harmonica reeds???

Here's a link I put up that illustrates the phenomenon.  Had it so long, I
can't remember where I got this GIF... sorry!  [And apologies to the source!]

http://www.tfn.net/~bobg/KarmanVortexStreet.htm

Aside from this effect, I have to wonder what else is going on, because with
a free reed mounted over a slot, you have a flat object in the air stream,
not round; it is also flexible allowing it to wag back and forth in line with
the air stream and through the slot as the air flow drives it; and the
differences in reed width, profile, material, flexibility, plus reedslot
geometry, etc., all add their own influences.  But, it could also help
explain why the ringing noise always seems to be a pitch relative to the
squealing reed's natural pitch... a reed vibrating in resonance with an air
stream, that we manipulate in terms of velocity, volume and pressure, is what
produces what we hear when we play harmonica notes.  Alter those values
slightly, adjust the resonance, and a new note is available from the same
reed, ala bending or overbending.

And remember, it is apparently only the closing reed which has this irksome
property... ie, it's not the overbending reed squealing, it's the "choked"
reed crying!  That is, in an overblow, it's the blow reed, in an overdraw,
it's the draw reed.  It's this matter that makes me think the reedslot itself
may play a part in the squealing problem, but maybe we'll never really know.

Ken was answering Jonathan Metts, saying:
>I got the idea from Dr. Antaki, the TurboLid guy.
>Basically, you place a very small bit of tape in
>the center of the squealing reed (in the center so
>as not to alter the tuning). This stabilizes the
>reed during overbending and, voila! no squealing.

AND during regular bends as well, as they begin with the closing reed action
which is involved in the squealing.

>Personally, I simply bought some very very thin
>black tape from the local art supply store. Works
>great.

I suspect that is the stuff used to line out walls and such in floorplans.
[Visual Arts interior design courses.]  ;)

>Jonathan Metts wrote:
>> Tinus recommends waxing the rivet ends of reeds
>> on his website.  I would like to try it on my old
>> Lee Oskars (which I have gapped well for
>> overblows but they squeal like a mofo!), but I
>> have not found any pure beeswax to do it.

Sort of fun covering all these subjects in one post, as I actually coined the
word "overbends" and first discovered and proposed the "wax fix" for
squealing harmonica reeds years ago on Harp-L.  However, I initially used
plain white paraffin, not bees wax, to calm the ringing reeds, while around
the same time, BTW, Pat Missin described his method of using nail polish for
the same purpose.

As for the vortex properties, I got to thinking about that because I've never
really accepted a commonly purported notion that harmonica reed sounds are
created the way "siren sounds" are made, because the chopping action is quite
dissimilar between the two modes of sound production.  My idea is that the
air is "sliced" across the air flow in a siren situation, but a reed's
movement cuts it in a to and fro manner, against and with the air flow,
pinching it in the slot ["valve-like"] twice every cycle of the reed's
oscillation, and physically creating a compression wave on the downstream
side of the reed's action... not simply chopping the air stream into a series
of basically equal packets, but a series of compressed and rarefied packets
of air.

However, the fact that the squeal in reeds can be mitigated by applying wax
or polish where the reed root and pad combine, or by applying the TurboTape
idea, makes it seem possible that the cause might be a combination of
effects... vibrational reed movements, flapping or torsional, etc., and
vortex shedding action.  Regardless, it does seem the annoying noise actually
emanates from the basal area of the reed not the tip.  [Touching a pointed
prob, like a pin, to the reed's root while it's squealing will tend to
silence the noise.]

Another point to note is the ringing sound doesn't happen simultaneously with
the offending reed's normal note-sounding... either one or the other but not
both.  [If you do hear another tone, it is the opposing reed's sound, not the
squealing reed making double noises.]

Sorry if I got on a tear, there!  Just fascinates me, though.  Here's a
website on measuring the Karman vortex street with a bit more info on the
phenomenon:

http://www.efunda.com/designstandards/sensors/flowmeters/flowmeter_vtx.cfm

Happy De-squealing!! ;)

Bobbie