Hi all!
This is difficult enough to explain in my own language:
----------
In a parabol you need omnis. Everything else get too sensitive to handling
and wind noise.
Stereo depends mainly on 3 things, - amplitude, time and phase differencies=
.
Two omni mics close to one-another make a monomic. Both are pressure sensin=
g
and both will sense "the same" pressure. You will neither get amplitude,
time, nor phase differencies, except at very high frequencies.
To get stereo out of two omnis, close to one-another, you have to create a
difference between them. Our ears have a head in between. It gives both
amplitude, time and phase differencies.
Take a Telinga with a stereo DATmic and remove the dish. What do you have??=
Answer: You have a binaural stereo microphone. You can use it for nature
sounds, but you can't pick out a certain individual with it, unless you get
very close to it.
Now put the parabol on. What do you have?
Answer: You have the same binaural stereo mic, but you have also added a
mono signal, reflected by the dish! Now you can do stereo recordings + pick
out an individual.
The Twin Science (Dual Science) is a microphone holder keeping two mono
mic's inside - one omni, recorded into one channel of the DAT/MD, and one
directional, recorded into the other. This has nothing to do with stereo.
Instead you just record two different mono recordings.
It's rather tricky to use The Twin Science at it's maximum performance.
There is a lot you have to learn, by own experience. You have to make many
recordings and compare, also by changing the focal length.
Klas.
At 23:11 2003-02-13 -0000, you wrote:
>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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Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
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