Thank you Marty and I will give it a go with my 32" without barrier.=20
In my inquiry to Lang your additions are why I inquired if he used
one. I suspect that a barrier would have more of an effect in widely
off axis sound but I have noted poor stereo out of using omni's with
a 5.25 barrier and no dish.
The telinga seems to get better stereo without the dish than I obtain
with two omni's and a 5.25 thin barrier and no dish. The Telinga I
have not commented on for other reasons as well. It is difficult and
complex to understand and a one of a kind product. It does not
appear to be a parabolic for its entire diameter and its mics I can
not understand without destructive testing which I can not do with
loaned equipment. I will leave it for what it is as many like what
it accomplishes.
Rich Peet
--- In Marty Michener <>
wrote:
> 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).=20
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.=20
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.=20=20=20
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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