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Re: Question About mics + windgags

Subject: Re: Question About mics + windgags
From: "Rob Danielson" danielson_audio
Date: Mon Oct 25, 2010 10:49 am ((PDT))
At 4:12 PM +0100 10/25/10, Avocet wrote:
>  > Some seem to have had some success with using a bit of cardboard on
>>  their pocket recorders to get a bit more directionality.
>
>Cardboard is not the best thing to use unless it has a shiny surface.
>If it is absorptive you will lose high frequencies.
>
>The other important factor is the distance between the surface and the
>mic diaphragm. Most mics are small nowadays, but when this distance
>becomes comparable with half the wavelength of the high frequencies
>you want, the frequency response goes to pot. In practice you are
>talking about 17mm (3/4 inch) at 10KHz I've been recording birds at
>10K but judge the effect with your ears.
>
>A large sheet behind the mic will simply baffle out the mid and high
>frequencies but not the rumble from behind. You will get roughly a
>cardioid response rather than omni.
>
>However, there is an effect I used professionally which is the
>"surface effect". Sound will run along a surface giving a small omni
>mic a strong boost, for instance with an interview across a table.
>With a directioal mic, it actually sounds better than putting the mic
>on a table stand which picks up an interfering reflection. Forget foam
>"mice" etc, just lay the mic flat down hard on the table. Most people
>are surprised. Try it on the roof of a car. I had an inelegant guide
>for my assistants - imagine your interviewee throws up violently and
>if the puke would get into
>the mic, it'll work.
>
>This is how you can considersably increase the directionality of a
>small stereo pair. Even if it means using two sheets for instance with
>built-in mics, and get these touching the mics as above. I'm thining
>rounded off sheets of pexiglass or vinyl How you keep all this in
>place is another matter. :-) I'm suggesting sheets the size of the
>lower wavelengths you want to record, ie, a foot  or two. I've never
>tried combining this with a back baffle, but that should work too, as
>ever, keeping it as close as possible to the mic diaphragm. This may
>even work batter than a parabolic reflector, which doesn;t do stereo.
>I have several thoughts on that subject as well.
>
>As for choice of mics, my advice is to go for low noise first. Most
>modern mics have a reasonable frequency response. Ideally the mic
>noise should be at or below the mic input stage of your recorder or
>mixer. Check that and the impedance matching (use a transformer if
>necesary) first before you spend money. I've got expensive MKH mics
>but they sound hissy into an mp3 recorder.
>
>As for high noise limits, buy any cheapo dynamic mic and it will
>record a drum. If it distorts call that an applied effect. 90dBA is
>about when hearing damage starts and you should be wearing ear
>defenders. You won't find much wildlife above that.
>
>I invented the original windsock that goes over gun mic windgags,
>which was then taken over by Rycote. Most thin cloth materials will
>transmit sound (listen through them), and all a wind gag does is to
>present a smooth surface to the wind. When you're trying a mic on the
>roof of your car as above, throw a piece of cloth over it as well.
>Invent your own windgag on a wire frame or plastic basket. The cloth
>thickness limit this time is about 1/10th the wanted wavelegths.
>
>David
>
>David Brinicombe
>North Devon, UK
>Cogito cogito ergo cogito sum - Ambrose Bierce
>

Hi David, James & Ben--
If we discussing improving the stereo image of the PCM-M10 built-in
mics that Ben has purchased,  the drawbacks I encountered were poor
separation under 500Hz and lacking high frequencies in the center of
the stereo field:
<http://tinyurl.com/2ahex2c>http://tinyurl.com/2ahex2c and more
generally, http://tinyurl.com/2bm2ug6. Using one or more boundaries
is a clever idea but, as David suggests, they'd need to be 14" or
larger to affect low end cross-over.

The capsules being buried in the body presents a substantial
challenge in getting the diaphragms of the built-in capsules into the
pressure zones of the boundaries to affect/shape HF response/angles.
If one could free-up the capsules with a hack, using small parallel
boundaries with the capsules facing forward tends to do a pretty good
job (See Curt's arrays http://tinyurl.com/y55p29w)

Still there might be a way to improve the imaging of the built-in
mics. Its not clear from the images, but the omni capsules seem to be
oriented at about 90 degrees.  Here's Sony's plot for the M10
capsules [pdf]  http://tinyurl.com/ybrpjan -- though it doesn't say
whether the capsules are in the recorder housing or not. There's a HF
boost from 3-4Khz at 90 degrees which should help at bit, but
considering the 90 degree angle, I'm not sure why the HF in the
center of the field isn't better. Maybe the housing is affecting the
omni patterns. I suppose someone could make a pink noise localization
test with the recorder's built-in mics to get a better sense of what
is happening, but before going to that length, its clearly easier to
buy the same higher quality capsules Sony is using and work with
known, exterior arrays.  Rob D.


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