Paul,
>From my long lens photography experience (400-2000mm lenses)It is near
impossible to create a stable platform in any wind at long focal
lengths. Not sure of the actual vibration frequencies.
I don't profess to be an expert in this field at all.
This is a rehash of something I wrote up for photography, but it is
likely applicable here as well.
If strong wind is the problem, and without being sure what your
proposed isolated base is comprised of, wind on any lightweight tripod
is a difficult problem.
Any vibration (energy) that gets into your tripod will ultimately
generate a resonance in your tripod system.
The resultant induced vibration on the tripod legs/column is a
function of the vortex shedding frequency which is a 'Cross Wind'
induced vibration and this pushes and oscillates the legs sideways (at
90=B0) to the direction of wind. Add some light gusts (or any wind
velocity changes) and you have two directions of movement (vibration)
in the legs.
If you have ever seen big industrial chimneys they often have
spiralling flutes or appendages that wind their way up the outside of
the chimney - these help reduce vortex shedding.
Structurally this effect can cause a total collapse that would
initially seem to be impossible just from a bit of wind !
A similar thing could be done with a tripod, wrap and secure something
uneven around the legs and or the column, reduce the height as much as
possible.
You can only go so far with the reduction of the vortex induced
vibration, the rest is now dependent on the physical properties of the
tripod system.
This is a function of design, elasticity and mass (in simple terms)
Since we have no control (or understanding) over the design and
elastic properties of a tripod system (once you have bought a given
tripod), you're left purely with the ability to deal with mass.
The heavier the mass, the lower its natural frequency (resonant) of
vibration (or a harmonic of this frequency).
The induced vibration of the tripod by the wind (at the vortex
shedding frequency) called the "Forcing Frequency" is now vibrating
the tripod.
To avoid maximum vibration, what you want to do is to reduce the
natural frequency (resonant frequency) of the tripod system such that
it is a lot lower than the natural or resonant frequency of the
tripod. If it is the same, then things will be worse and maximum
amplitude of the vibration will occur - (this is how bridges collapse !)
You add mass - if your tripod is light - its resonant frequency will
be closer to the vortex shedding frequency.
Gitzo carbon fibre tripods probably have elastic properties that aid
this, but they dont have mass - but what I think they have done is
create a tripod which is lighter but has the same stability as their
heavier tripods.
Problem is even the heavier tripods are not necessarily heavy enough
to satisfy the requirements of a big lens setup and probably not
necessarily any help with the sound recording either - I dont know the
answers to this but I am sure that other members have more practical
experience.
Adding mass is not exactly the most convenient thing to do either, but
you may have a back back that can be secured (but don't let it swing)
Also, loose cables, straps or anything that can move, flap or quiver
in the wind should be fastened or removed entirely
I guess you can evaluate adding mass or some sort for spiralling
material just by listening.
The explanation is not perfect but it's as I was told and researched
at the time.
Anyway - its more food for thought !
Hope you solve the problem and let us know, then I can safely use my
own pair of AT3032's !
Cheers Rally
>
> I'm still convinced that there is some resonances being transmitted
> into the rig in winder conditions. When using the same tripod with a
> scope in windy conditions the vibrations from the tripod are visible, =
> so I assume this vibration would be passed into the rig. I'm going to =
> try building an isolated base for the rig to eliminate this
> potential noise source.
>
> cheers
> Paul
>
>
>
|