Thanks Marty, I'll give it a try.
Kevin J. Colver
114 N. Clark Lane
Elk Ridge, UT 84651
801-423-1810
-----Original Message-----
From: Marty Michener
Sent: Thursday, February 13, 2003 2:40 PM
To:
Subject: [Nature Recordists] How parabolas do work.
At 10:18 AM 2/13/2003 -0700, it was written:
>I've used a similar system of 2 mono mics mounted in a parabola with a
>barrier between. It works fairly well, however it has less magnifying
>power than a single mic because each mic in the pair only receives the
>amplified sound from 50% of the parabola surface area. Thus it has a
>bit less "reach" than the single mono mic. The stereo effect is more
>pleasing however, and you will need to get a bit closer to your sound
>source.
>Kevin
Hello:
There is absolutely no physical reason why, FOR INCOMING FOCUSED SOUND,
one
needs to put a barrier between two mono mics in a parabola; it just gets
in
the way. For non-focused background sound, the barrier will have about
the
same effect as in non-parabola usage, some slight contralateral damping.
I don't mean to pick on anyone, here. This is a widely stated and used
misconception. I have posted to clarify this point on at least two past
occasions (see attached notes below, from this list Sept 2000).
Parabolas
do NOT "gather" and they do not "amplify". These words have specific
meanings other than focusing, and apply to things and to signals, but
not
to waves. The focused light or sound may be brighter or dimmer than the
intensity of the object itself, it depends on the ratio of object
distance
to image distance. In all our REAL-LIFE nature recording settings, the
image is much brighter - louder - than the object intensity, so we can
use
noisier mics.
Please lets be clear that a parabola is a reflecting lens. Putting a
barrier between the mono mics is like putting a small card inside your
SLR
camera to keep the light from the lens that is supposed to fall on one
spot
from also falling on the spot next to it. The physics and the practice
is
nonsense. The card just cuts down the light you WANT getting to the two
spots, making the lens effectively a larger "f-number" (smaller
numerical
aperture, e.g. f/8 instead of f/5.6.). This is why Kevin finds LESS
sound
than with ONE mic, because of the ill-conceived barrier. In principle,
you
could put an ARRAY of mics, like the array of light sensors in a digital
camera.
I even prepared a diagram of the focusing process, and privately mailed
it
to some folks. Placing two small mono mics two inches apart, with no
barrier, means they gather ALL the sound from ALL the parabola, each
from
two divergent incoming paths. This is just like a camera. It is the
incoming DIRECTION of the sound that determines where it will focus.
Who ever dreamed up the idea of using the "sound from one half of the
parabola" that I keep hearing. I bet it was G*** B*****, at LNS, I
could
never get him to understand this when I talked to him in the 80's or
90's
either. ;^)
As with sounds to your two ears, the waves all cross in the air, they
don't
bounce on each other. You are holding this huge reflector out in front
of
you, so you want ALL the reflected sound getting to ALL your mics for
maximum focused signal per background (non-reflected) sound.
If the focal length is 20 inches, and the spacing between mics is two
inch,
the angle will have a tangent of 1/10, of about 6=B0. With my 48 inch
giant
parabola, I usually used two or three inches, depending on what I was
recording. When you point the parabola at a pond, for instance,
subtending
an arc of six degrees, you point the center to the pond's middle. Then
the
focused sound from the left end of the pond focuses squarely on the
right
mic, and the right end of the pond is focused on the left mic. And the
sound in the earphones is a really clear stereo. I have hundreds of
really
old cassettes of it. Good, live sounding stuff.
Please, no more barriers!
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
Folks:
Let us be really clear about how a parabola works and how it
may be configured to make a stereo recording.
First, a parabola is mathematically not that different from
a (transparent, biconvex refractive) lens - both do focus sound to
a plane, with predictable aberrations from a plane as covered in
any high school physics book under optics. We recordists use
the reflective medium for sound because it is just about impossible*
to make a transparent lens that is linear for sound. To do so
you would need a predictably different sound velocity (inside the
lens) from that in air, with a surface medium also totally
transparent to sound, separating the two media. So we always
use a parabola as a reflector. Under some conditions a helium
balloon seems to act as a very non-linear sound lens.
But both a refractive lens and a parabolic reflector focus
parallel waves (in our case of sound) to a single 3D point
in space. Parallel waves correspond to a near flat wavefront,
that is, from a point source that is so far away that the waves
appear to have a single arrival time in the neighborhood of the
parabolic dish. This is like assuming for an optics lesson
that the object is at infinity.
To say that you can't focus incoming sound on two separate
points would be like saying a camera can only take a picture
of a single point of light. Not true. If the waves arrive from
different directions, corresponding, let us say, to two frogs
singing from 100 ft away are 6 ft apart, the two focus
points will be several centimeters apart - one corresponding
in this simple example to the left mic, the other to the right mic.
NO separation device between mics is in fact needed.
The two mics are like two nearby grains of silver halide in
a camera film emulsion. One records the singing of the
left frog (the mic on the right, because the image reverses)
and the other from the other frog.
The mics do not need any separation device because
sound waves, like light waves, freely cross in space, yet
arrive concentrated at but a single point - at least in the ideal
case where the sound wavelength is much less than the
dimensions of the mic and dish.
I have recorded from two omni mics in the focal PLANE
(not point) of my 48 inch (70 pound) dish, many bird and frog
flocks and choruses. The spacing of the stereo separation of
the sources is by the same ratio to the separation of the two
mics as are the relative distances from the reflector. This is
identical to the formula for magnification of an image with
an optical lens Object size/image size =3D Object dist/Image distance.
If the frogs are 6 ft apart at 100 ft distant, and the parabolic focal
plane is 2 ft from the dish, the ratio would simply be 2/100
or 1/50, and the 6 ft would be represented by a mic
placement of 6/50 ft or the mics about 1.44 inches apart.
. . . and he went on and on . . .
and from a private note I wrote on the same subject following up to
somebody else:
I attach a diagram of how they really work. Sound waves
bounce off (and refract around small ) solid objects. This is true,
because the impedance to travel in a solid is so radically
different from that of air. Waves pass by objects small
with respect to their wavelength, like ocean waves
wash past a post or wharf by a dock. They reflect off objects
much larger than their wavelength, in a mirror-like manner.
In between sizes, they bend and curve in a manner
much harder to visualize.
Sound is not "gathered" as you state. It is reflected,
jut like light in a flashlight, sort of in reverse. Angle of
incidence equals angle of reflection. If you simply
place to bulbs in a flashlight, against each other, side by
side, you would have two diverging beams of light - make
one bulb red on the right and one green, on the left, and you
would produce a green beam of light to the right and a red
to the left of center. This is exactly what you do with two
mics side by side in a parabola. No barriers, no membranes.
Correction, that is what **I** have been doing with my parabolas for
over 30 years. The analogy with a refractive lens is much better.
Each lens, whether refractive or reflective focuses waves to a point.
The location of the point only depends on the entry angle of the
sound waves to the lens's diameter and center axis. Just like
a camera lens, sound is focused by every parabola, regardless
of who makes it, to a point. You are absolutely correct that this
point is spread to a globe by the inherent diffusion factor which
depends directly on wavelength. Longer wavelength, larger
the globe. In photographic optics, this is actually called the
"circle of confusion".
To put a barrier in a parabola I would say is worse than useless:
it prevents each half from providing directional sound focusing
to each mic, so makes the sound amplification less by 3 dB
at each mic. Why do it? I have read the explanation of
the PZM effect, and it is all new to me, so I don't pretend to
understand it. If what you say about it is all true, then
you are also correct that the sound at which you aim the
axis would be recorded mono in both mics, and the off
axis sound would be blocked differentially to the two mics.
I am not convinced yet that this would amount to a stereo
recording. It would be nothing like what I am showing in
my diagram and what I can provide you with hours of from
my old recordings.
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