At 09:26 AM 2/14/2003 -0500, you wrote:
>My ears tell me that the barrier definitely improves stereo response when
>using two mono mikes. To prove this, one has only to put two omni mikes si=
de
>by side and listen to a broad soundfield using headphones. You will notice
>only a very minor stereo effect. Then slide a barrier between the mikes. T=
he
>stereo effect is hugely improved.
>
>It's as simple as that.
No argument, so far.
>When such a mike setup is placed in the parabola, with mikes pointing
>forward, one records a nice stereo soundfield that is independent of the
>parabola. The on-axis sounds concentrated by the parabola are then added t=
o
>the stereo soundfield. The result is quite pleasing.
>
>I don't think the same result could be obtained without use of the barrier=
.
>I base this on simple listening tests.
>
>Lang
Folks:
Again, I disagree with the direction of Lang's third paragraph. It is NOT=
so simple. Try using a flashlight without the reflector, sometime and tell=
me how useful it is. (This is not an irrelevant example).
We are comparing a virtual image and a real image. Remember from physics,=
(the names are a bit ironic): a REAL image you can't see, because all the
information is happening in empty space, and you either need a translucent=
paper on which it focuses like a ground-glass or film (that is the
parabolic case) or another lens to see it (as with a telescope eyepiece).
A virtual image is like that in a mirror or what we look at when we see (or=
what we listen to when we hear) a real landscape. A virtual image you can=
see. Microphone barriers, and MS setups, and using trees for heads all
work in that latter world. The only way to adequately hear focused sounds=
is to place (or move around) point-receptors, like a single ear or
microphone, IN THE IMAGE.
What is wrong with paragraph three? Of course the stereo pair DO continue=
to do what they do without the parabola, it is a matter of a huge volume
differences.
Try out the single mic case - take two matched omni mics, put one in a
parabola at the focus and point it at interesting sounds. Place the other=
mic anywhere nearby. Make a two channel recording of it, with the mics set=
at the same input volume. How much louder is the one in the parabola? We=
would usually call that setup a recording of whatever the parabola is
focused on, and a background recording in the other channel. Usually not
very interesting, because the parabola provides so much gain, and the other=
channel is out of phase and cannot be used to "subtract" out the background=
or ambient sounds, as we have discussed before.
In a parabola, the barrier-stereo pair IS still recording the stereo sound=
field that would be there without the parabola, but the parabolically
focused sounds are between 15 and 30 dB louder than the ambient field and
so virtually drown out any ambient stereo you could hear.
So, you take your pick:
1.) Either you are NOT getting at least 10 dB gain from your parabola: I
would throw it out if it were this poor.
OR 2.) you ARE, typically getting much more. Now the theory of the need
for the barrier at the very least needs rethinking.
I have recordings made with no barrier in the 80's that sound like the two=
mics were placed, as my example said, at the right and left ends of the
pond. Actually not, because they are in phase, they sound like you had
your head very near the pond, but the left and right birds are louder than=
you could really ever get recorded with two non-focused mics. They are to=
sound I guess what a telephoto image is to real life, much closer, but
somehow a bit artificial.
Hey, if I were wrong, the military would never have spent so much money on=
these parabolic radar reflectors to provide all that tracking station
directionality. And flashlight reflectors wouldn't work any better than=
a bare bulb.
Please try this mind experiment: Put a red bulb (right side) and a green
bulb (left side) next to each other in the middle of a flashlight
reflector, instead of the usual one white bulb, and ask yourself: Where
would you see the two colors shining? Now that is simple: a red beam would=
make a red spot on the wall in front of you, just left of the green spot,
and in the middle where they mix would look all yellow.
Now take OUT the bulbs and put in tiny microphones, and that is your true
picture of sound focusing INTO the reflectors and ONTO your two mics. The=
parabola does the stereo focusing for you; it really is a real image.
my best regards,
Marty Michener, MIST Software Associates PO Box 269, Hollis, NH 03049
EnjoyBirds.com - Software that migrates with you. http://www.EnjoyBirds=
.com
________________________________________________________________________
________________________________________________________________________
|