Rally,
thanks for the input and detailed explanation of issues of wind in
the tripod.
I've got a few ideas about making the rig side boards a lighter
structure, and reducing vibration transmission from the tripod into
the rig. I definitely need to pay much more attention to lead
"dress" on the rig as that is another area for vibration transmission.
go use the at3032's - they will be fine.
cheers
Paul
On 05/03/2008, at 9:00 PM, ozrall wrote:
> 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
>>
>>
>>
>
>
>
> "While a picture is worth a thousand words, a
> sound is worth a thousand pictures." R. Murray Schafer via Bernie
> Krause
>
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