[Ansteorra] helm resonance

[Catie Clark]

>
> Does anyone have a tried and true method to reduce the 
> ring in a helm.  I have heard that you can use car 
> undercoating to reduce the ring but haven't tried it.
> thanks
> Garreth

Helms that ring tend to be good resonators with insufficient damping and
nice "unclamped" (in the engineering dynamics sense) edges capable of free
vibratory motions.  To stop the damping, disrupt the physical factors that
cause or abet the vibration.  These are:

a. a favorable combination of shape and mass to create a good resonator
b. the unclamped edge of the helm which makes the "ring" possible
c. poor damping

Screw up any or all of the above and you degrade the resonance.

Caveat: there is no magic cure and no "one true way" of fixing that ring.
Every helm is different and there's no way to avoid the trial and error
process of fixing the resonance on a sight-unseen helm over the internet.

So what follows is a list of ways to attack resonance and why they
may or may not work for any given helm (or grill - sometimes it's the grill
that's the problem).

There are a variety of physical systems involved in stopping vibration:

1. Vibration Absorbers:  leather strips, painted helms, glued in foam

You can "send" the energy of vibration (ie the resonance or "ringing" in
your helm) somewhere else, to another vibrator which will preferentially
"eat" that energy through its own vibrations. This is how simple vibration
absorbers work, like the spring-and-lever gizmos in shavers and the springs
in your car. The vibration of the absorber doesn't have to be efficient, 
it just
has to preferentially suck up the energy of the resonating helm.  This 
is how
glued-in spongy foam and leather edge strips (and ablative coatings) 
work. If
you have taped-over foam that pulls in and out of your helm, consider trying
gluing some of it into the helm instead.  Or add a leather strip like 
bias tape
around the bottom edge of the help (it's the edge that resonates so that's
where this damper should go) or paint the inside of your helm (use black,
helps cut down on glare too) or all of the above. Depending on the helm,
sometimes you need two or more absorbers.  Note that the leather strip
damps just the edge but paint and glued-in foam damp vibrations before
they get to the edge.

You might think that a little paint isn't going to do much for helm ringing,
but sometimes that's all it takes!  It's not all that different from the 
fancy
paints they use on submarine hulls.  I've had good luck going the
leather-bias-tape route too - it's a preferred method for basinets since
you can use it to add a cool way-period look to your helm.  I'm currently
am using both paint and glued-in high-damping-constant foam on my
ringy sallet.

2. Change the Resonator: add mass, drill holes, disrupt the edge

You can try to change the resonator itself.  This may or may not work
depending on the helm.  It's not that the method is bad - it's just that the
physics may not be optimal.  The first way to try to change the resonator
is to add or remove some mass - like adding some metal to the welds,
adding a line of big burgundian rivets to a sallet, or adding/enlarging 
air holes
in a closed face helm.  The problem here is that the added/subtracted mass
may not be enough to change the behavior of the resonator.  If it's a 
ringing
grill that's your problem, add some additional welds.  This will add some
mass AND decrease the amount of unclamped grill edge that can vibrate.
Drilling holes decreases the resonating mass but strategically placed, can
also add additional vibrating edge to resonate away some of the energy.

It's almost counter-intuitive that adding more edge to vibrate will help
lessen helm ringing - until you recall that small holes will vibrate at 
higher
frequency which attenuates vibration more efficiently.  So one hole is 
good,
three new air holes are better, and a row of holes with decorative rivets
rocks!  If you do holes and decorative rivets, there's an added benefit 
that
each of those rivets becomes an additional vibration sink.  One rivet may
not be a great absorber - but several holes and several rivets may be what
the doctor ordered!

You can attack the edge directly too.  This is a little more tricky than 
other
methods since too much disruption of the bottom edge or face opening
can compromise the structural integrity of your helm.  One way to disrupt
the edge is to saw two to three itty bitty _thin_ cuts not more than 1/4"
deep along the bottom edge of the helm.  Now you're just broken your
resonator's resonating edge, and as any trombone player who's cracked a
bell can tell you, it does work right anymore once the edge is broken.  (End
each saw line with a drilled round hole so the sawed line does not turn into
a propagating crack).  This is a method of last resort for obvious reasons.
But I fixed a really ringy barrel helm this way once.

3. Life with grills

Do you have a grill?  If yes, is it the grill that's ringing?  You have 
less options
with a ringing grill, especially since you can't decrease mass or physically
disrupt the edge with breaks and still be legal to fight.   Basically, 
you can add
some mass and disrupt the edge by adding some welds.  Or you can get rid
of the welds and make the grill-helm attachment system into a complex (ie
many parts) vibration absorber by riveting the grill to the helm instead 
(adds
holes and rivets to attenuate vibration).

Basically, there are many ways to attack the ringing problem.  The 
solution is
often highly-helm-design dependent.  But once you approach the problem
from the physical causes perspective, you should be able to identify what
part of the resonance problem may give you results for the helm you 
currently
own.  To sum up, you can:

paint the inside (or outside if you're being Germanic)  (vibration abosrber)
use glued-in foam (vibration absorber)
glue leather "bias-tape" strip on vibrating helm edge
add some welds
drill new or enlarge current air holes
add holes and rivets

This is not an exhaustive list.  I just think I've babbled on too long 
already.

Going to Three Kings?  I consult (bribes of good coffee are not mandatory
but greatly appreciated)  I'm really really good at fixing armour problems;
have physics degree, will travel...

And just for the record, I spent a couple of years doing vibration analysis
for a living.

ttfn
Therasia's inner nerd