Hello Folks:
I do read what is going on, but my writing time these days is limited by
the grim financial facts of my life - I've gone back to work. Yesterday I=
flagged about a mile of wetland edge in Rindge, NH.
RE: the sound - wind tradeoff
This discussion has gotten to its usual point, here, as it seems to every
year, with Walt sounding the most like he understands the actual physics,
and everybody verbally dancing around the causal aspects at various
distances. (ok, I'm a snob about saying the same thing over and over! But
it needs to be resaid.)
Sound vs wind is all related to laminar vs. turbulent flow -- fluid
dynamics. I will make this short, and in outline form, in the interest of
time - yours and mine:
No turbulence, no wind noise! Turbulence is produced for a given rigid
structure above a critical velocity, which is why there is, for any setup,=
a certain wind speed above which we give up and stop recording. If you want=
a demo of this, try your water faucet: turn it on very slow and increase
the speed gradually - watch (and hear) the turbulent point.
A. Modifications of wind protection hardware to reduce wind noise (aka: YOU=
COULD MAKE OR BUY):
Walt is exactly right - it is not the structure that reduces the noise, it=
is the slower air. To be specific - air flowing in straight layers
produces no noise and conducts sounds well.
Large smooth objects that reduce wind SPEED without incurring turbulence
are the best among which to hide your mic.
But every rigid object outdoors has some wind velocity at which it will
begin to produce turbulence and thus sound. Larger objects produce infra
sound so we ignore them. [Sailors on inland small lakes know that
mountains can produces a turbulent switch in wind direction every fifteen
or twenty minutes - now that sound is LOW!] Small wires and twigs are the
most objectionable to most outdoor recordings. At high winds trees and
cars howl (white noise extending to lower frequencies).
"Wind screens":
1. Turbulence produces high frequency noise around the smallest objects -
which is why wire mesh hisses in the wind. Bad bad bad. Fake fur is
self-streamlining - it changes shape to follow the wind direction - at low=
velocity it lies open; at higher it shapes itself to produce the least fine=
turbulence - hence the least additional noise. I suspect that is the
evolutionary WHY animals use REAL FUR - they can run fast and still be able=
to hear their prey (or pursuing predator).
2. A series of streamlined cages, each smaller than the previous, with the=
mic inside would be ideal. BUT getting the streamlining, like airplane
wings, presumes you know exactly the direction the wind is blowing /
flowing. So each setup would work perfectly for exactly one wind direction=
and small velocity range. NOT PRACTICAL, usually. Not to mention HUGE.
3. The largest streamlined cage practical is the fake-fur screens - they
are small enough to move with the mic (i.e. be attached and transported)
but big enough to trap (slow) air gradually. The main thing is not to have=
rigid fine structures around which the wind creates hiss - why the fake fur=
.
4. The one situation I know of in nature of a wind screen exactly designed=
(by trial and error over millions of years) where the direction and
velocity of the wind has always been utterly predictable: the auricular
feathers covering most birds' ears. Study them! Here's a little wren with=
a 1.5 mm ear opening, going at 12 meters per second yet hearing everything=
just fine. Try that with your Panasonic capsule!
B. Modification of human recording behavior to reduce wind noise (aka: YOU=
SHOULD MAYBE TRY):
This is a whole category not even touched on (unless I missed something)
and is very important. In fact I OWN NO WIND SCREENS - except what "came
with" my Sennheiser mics. So what do I do?
1. Get yourself as far away from other wind-related noise sources as
possible (see next point, C below).
2. Get yourself in the lowest wind velocity area - behind a tree or car
even works.
3. Orient your mic so the long axis is parallel to the wind - but you can
hear this for yourself and it becomes second nature. Of course that may be=
easier said than done, where you must point the mic AT the sound you want,=
so it may mean moving to a different perspective on your subject, or
approaching from upwind or downwind direction as possible.
4. Get low. The earth itself has a very slow layer of air next to it, where=
the sound you want MAY be still recordable - this depends on each
situation, but it is always worth a try. Try it even without wind, some
sounds are louder and some quieter near the ground.
5. For those subject to our preoccupying passion who like to set the mic on=
a stand and leave it - a lot of this category goes unexplored -- another of=
my twenty reasons I gave up on parabolas (which I used from 1956 until
recently) in favor of easily-walkable rigs.
C. Wind noises that cannot be reduced:
There is a long list, a few:
Hissing and banging among plant parts. Dried oak leaves are the worst I
found. Solution - get far away from them.
Flag pole rope banging and flag snapping. (yccht. I just threw that in ;^0 =
)
Make your own list . .
All the best, Marty Michener
PS Sorry about my tone. Mister Mayor: We NEED a FAQ for this list!
At 12:39 AM 2/24/2005, Walt Knapp wrote:
>There is a misunderstanding that's common, that the structure is the
>protection. The wind protection is almost entirely due to the layers of
>still air between the layers of structure of the windscreen. It's the
>still air that primarily absorbs the energy of the wind to get it low
>enough by the time it reaches the mic. The primary function of the
>layers of material is to smooth the wind to a more consistent average
>speed. And to allow the formation of the still air. If it actually
>stopped the wind, it would also stop the sound.
>
>The biggest problem with a big tent is keeping it from flapping in the
>wind and making it's own sounds. Plus it has some real portability
>problems and is very hard to hand hold while walking through the brush.
>
>The biggest problem with foam is it's structure. Structurally it's a
>whole pile of tubes that connect all the way through it. It can act as a
>smoothing layer if it's away from the mic far enough, but the foam
>"windscreens" sold with mics are right on the mic. Pipes from wind to
>diaphragm.
>
>Foam does also have the problem that wind blowing along it's outer
>surface will produce new sound.
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