Does "velocity component" mean the direction the sound is coming from?
Peter Shute
From:
On Behalf Of Avocet
Sent: Tuesday, 27 September 2011 1:06 AM
To:
Subject: Re: [Nature Recordists] Re: Wind sensitivity
> I've seen the occasional reference here to some microphones being
> more sensitive to wind than others, and assumed it was because of
> something about their shape making them catch the wind more. But
> surely once they're inside some sort of wind cover, they're all
> equal in that regard?
Mike,
It is the outside shape of a blimp or sock or whatever windgag which
creats most wind turbulence. A false fur surface produces much less
turbulence and at a lower frequency than a smooth surface such as
stretched cloth or foam.
That's only a partial solution as the mic is still going to be hit by
turbulence from the mic mount, stand, hand or the body of the person
holding it. The killer frequencies are around 1 Hz to 20Hz which don't
sound as audio, but these freqencies can easily modulate or chop up a
recording making it unuseable. You can't filter this out later. You
won't find anything about sub-audio in the mic specs.
Any directional mic has an omni sound pressure function combined with
a fig-8 velocity function. It is the sensitivity to the velocity of
the sound which makes them highly susceptable to both air and mic
movement. It doesn't matter what gizmos are inside the mic body, there
is still a velocity component to its sensitivity. If that velocity
component has a sub-audio frequency response it can be a serious
problem.
Any mic input stage needs a high pass filter, and for field recording
it is useful to switch in a "bass cut" response. This is typically 6dB
per octave and a bass cut is well worth using to avoid any input stage
overloading. The bass can be equallised back in on replay with no
quality loss.
Putting an inner puffgag or soft shield inside a blimp where there is
no wind has the effect of adding an acoustic HPF which raises the
overload point of the mic itself. However, if you do frequency
response tests it will make a difference to the mic axial and and
directional responses. However, that's better than an oveload.
A well designed "yeti" sock over a blimp will act as a HPF/bass cut as
well as streamlining the blimp as above. However while this may
prevent the mic itself overloading, and the bigger the blimp the
better, the unquoted response region around 1 to 20KHz may well have
high sensitivities in some directions. You won't find this on any
polar diagram, neither will you find an off-axis frequency response
which is often very different from the axial response.
To sum up, use the biggest diameter blimp and a strong bass cut on the
input to avoid wind overloads. You should still hear wind on the
surroundings like grass, shrubs or trees or yourself, or even off the
ground, and as long as these dot overload your recording at low
frequencies, they are part of the environment you are recording.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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