> 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
|