My researches into windshields go back to the 1960's before Rycotes. I
invented the first soft cover "Brinisocks" and sold them to the BBC
Film Unit to put over the first Rycote baskets which tended to
whistle, as did also the original glass fibre Sennheiser blimps.
The key membrane is the one which cuts the low frequencies, including
zero frequency. Held to the lips, it needs to show quite a lot of
resistance to blowing through. This surface will of course cause
reflections, but in practice this is not a problem with a single
membrane or two layers close together as with a basket and close
cover.
Wind itself doesn't make a noise. It is when it hits an object that
turbulence frequencies are generated. Those frequencies can be quite
high with small diameter objects like mic stands and cables producing
Aeolian tones like wind in telephone wires. (remember them ?)
Putting one windshield inside another will change the frequency
response and the directional response - test this out. It won't
prevent the wind noise which has already been generated by the outside
layer.
With my original Brinisocks, I used brushed acrilan as they were
also intended for use in rain having been treated with a waterproofer.
With BBC budgets, filming often had to continue in the rain with the
script modified on the spot if necessary. :-(
The false fur came later, and what that does is to slow the windspeed
close to the bass cut membrane and reduce turbulence.
One idea I disagree with is that a larger windshield is better because
it is further from the mic. A larger membrane has a larger area, so
that tends to cancel out.
The benefit with size of a windshield is simply down to aerodynamics.
Wind noise is caused by air turbulence, and turbulence is less with
larger radii of curvature for the same airspeed. It is that simple. If
you want the best shape for wind at any angle, a spherical windshield
will give you that, but would be a bit clumsy to use.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
|