On Wed, 26 Dec 2007 23:20:18 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

>In article <3ku4n3lqc9r463rcuoi1k8b6pbhu2avctv@4ax.com>, John Larkin wrote:
>>On Tue, 25 Dec 2007 09:34:20 -0500, "Ethan Winer" <ethanw at
>>ethanwiner dot com> wrote:
>>
>>>Rich,
>>>
>>>> 'I see you, you see me' is pretty much a universal law.
>>
>>Not if I'm in the dark, and you're in the sun. Or if I'm hiding in the
>>bushes and you're not. Or if I have a telescope but you don't.
>
> I thought this refers to a principle that when a ray is traced from
>origin to destination, percentage loss at each lossy point are the same in
>both directions, percentage reflected by partially reflective objects
>in the way is the same for both directions, and percentage making it from
>origin to destination are the same in both directions.
>
> Of course, polarizers and polarized light can complicate this, but we
>can't have a situation where there is a container that light can enter but
>not exit along the same path in a way that allows a thermal radiator to
>heat a target in the container to a higher temperature than the thermal
>radiator is at.
>
> - Don Klipstein (don@misty.com)

There are optical isolators, like electrical isolators, that let light
go from port A to port B, but block light from B to A. They violate no
laws of thermodynamics, because they absorb the light from B, rather
than reflecting it.

http://en.wikipedia.org/wiki/Optical_isolator


John

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:glr8n3pvq5iiua9g6scdcg84vnobs4kuq9@4ax.com...
> On Wed, 26 Dec 2007 23:20:18 +0000 (UTC), don@manx.misty.com (Don
> Klipstein) wrote:
>
>>In article <3ku4n3lqc9r463rcuoi1k8b6pbhu2avctv@4ax.com>, John Larkin
>>wrote:
>>>On Tue, 25 Dec 2007 09:34:20 -0500, "Ethan Winer" <ethanw at
>>>ethanwiner dot com> wrote:
>>>
>>>>Rich,
>>>>
>>>>> 'I see you, you see me' is pretty much a universal law.
>>>
>>>Not if I'm in the dark, and you're in the sun. Or if I'm hiding in the
>>>bushes and you're not. Or if I have a telescope but you don't.
>>
>> I thought this refers to a principle that when a ray is traced from
>>origin to destination, percentage loss at each lossy point are the same in
>>both directions, percentage reflected by partially reflective objects
>>in the way is the same for both directions, and percentage making it from
>>origin to destination are the same in both directions.
>>
>> Of course, polarizers and polarized light can complicate this, but we
>>can't have a situation where there is a container that light can enter but
>>not exit along the same path in a way that allows a thermal radiator to
>>heat a target in the container to a higher temperature than the thermal
>>radiator is at.
>>
>> - Don Klipstein (don@misty.com)
>
> There are optical isolators, like electrical isolators, that let light
> go from port A to port B, but block light from B to A. They violate no
> laws of thermodynamics, because they absorb the light from B, rather
> than reflecting it.
>
> http://en.wikipedia.org/wiki/Optical_isolator

Or just watch them interviewing the suspects on Law & Order

On Thu, 27 Dec 2007 11:51:39 -0500, "Ethan Winer" <ethanw at
ethanwiner dot com> wrote:

>Bob,
>
>> If one reflector was all it took to get 100% reciprocal operation, then
>> they wouldn't use two reflectors.
>
>I don't think that follows. With only one reflector, the gain would be
>insufficient for EITHER person to hear the other. Again, I'm not arguing too
>strongly because I don't really know for sure. But so far I can't see why
>it's not reciprocal.

>> The guy without a reflector can aim his beam at the distant reflector,
>> which might have an area of (say) 10 square feet, and anywhere he aims
>> within that area the beam will be captured and received more-or-less 100%
>> at the focal point.
>
>Yes, but a person talking is not a beam.
>


The reflector provides gain for the listener, by concentrating the
sound from a large area down to a small area at his ear. Let's
say that the reflector is 10 square feet, so all sound impinging
on that area is concentrated down to his ear due to the nature
of the parabolic shape. The gain of this operation could easily
be 100:1 if the ear catches only 0.1 square foot.

Now when we turn around and speak into the reflector
it *spreads* the sound from the small source (mouth) over
that same 10 square feet. Is there gain here? Yes, but
not the same as the prior case: Here the only gain (as far
as the guy at the other end without a reflector is concerned) is
due to the fact that without the reflector the sound would be
a spherical source from the mouth, falling off with the square
of distance. With the reflector, assuming everything was just
right, the best you could hope for would be sound that was
collimated into a plane wave that did not fall off with distance.
But the sound would still be spread over 10 square feet in
the ideal case, and much more in reality. The ear of the listener on
the other end only catches a tiny percent of that without a
reflector. If his ear only catches 0.1 square foot, there is a *loss*
of 100:1 compared to having a reflector.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
http://www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

Bob,

> Let's say that the reflector is 10 square feet, so all sound impinging on
> that area is concentrated down to his ear due to the nature of the
> parabolic shape. The gain of this operation could easily be 100:1 if the
> ear catches only 0.1 square foot.

Yes, but my point is that the source / person is not 10 square feet!

--Ethan

On Dec 28, 10:46 am, "Ethan Winer" <ethanw at ethanwiner dot com>
wrote:
> Bob,
>
> > Let's say that the reflector is 10 square feet, so all sound impinging on
> > that area is concentrated down to his ear due to the nature of the
> > parabolic shape. The gain of this operation could easily be 100:1 if the
> > ear catches only 0.1 square foot.
>
> Yes, but my point is that the source / person is not 10 square feet!

So far, everyone is making a bunch of assumptions,
many of which simply don't hold, with the result being
that the conclusions people are reaching are HIGHLY
suspect.

For example:

1. A parbolic reflector, has TWO focal points. If the
source or receiver is placed at the principle focal
point, the other focal point is an infinite distance
away and the system then will focus an incoming
set of parallel rays to the focal point and vice versa.
Move the focal point farther from the principle point,
and the other focal point now moves closer to the
system and the incoming/outgoing rays are no
longer parallel, but convergent.

Asserting that a parabolic disk will focus a set of
parallel rays to a small point ASSUME that the source
or receiver is placed at the infinity focus point of the
parabola.

A reciprocal transmitter-receiver system would
consist of two parabolic reflectors, with the
transmitter and receiver placed at the principle
focus of each reflector. The transmitter would
emit a parallel bundle of rays which the receiver
could refocus back to the focal point again.

BUT, a system does not REQUIRE a reciprocal
physical arrangement. You could have, say, a
transmitter with no reflector placed 100 feet away
and a receiver with its reflectors aimed at it and the
receiver moved out somewhat to compensate.

2. The ability of any such system to focus to a small
point is a function of the size of the reflector and
the wavelengths involved. With a reflector a few
FEET in diameter and wavelengths ranging from
several inches to many feet, the efficacy of a parabolic
reflector is limited. At low frequencies where the
wavelength is large compared to the dish, it simply
doesn't focus anything. At higher frequencies and
thus shrter wavelengths, the region of focus is,
essentially, defined by the wavelength. I.e. at say
1 kHz, you are simply NOT going to have any precision
in focusing smaller than a couple of feet at best.

On Dec 25, 3:25 pm, Uncle Al <Uncle...@hate.spam.net> wrote:

> Whispering galleries and inside elliptic reflectors, yes. This is the
> preferred way to build confessionals in church - at one focus - with
> rentable scholarship rooms at the other focus.

Really? I"m going to have to look into this. I"m thinking I'd really
like to line up a few "scholarship" hours at peak confession times!

> Parabolic reflector (satellite dish), not so much. Incoming will be
> focused but outgoing will be collimated.

Obviously the "whispering gallery" setup would use TWO parabolic
reflectors which is the equivalent of the elliptic chamber. Both are
conic sections and work nearly the same. A single parabolic reflector
is the usual microphone setup. But a single reflector is the "I see
you, you see me" equivalent of only ONE person with a telescope as
opposed to each person having one. If both persons have similar
telescopes then that is the optical equivalent of a "whispering
chamber". And I might mention that acoustic lenses exist as well so a
reflector isn't essential to create the effects.