Dark Sonar.

From: Thomas Doniol-Valcroze (tconrad@orbital.fr)
Date: Fri 31 Jan 1997 - 17:01:18 EET

Greetings everyone.

Well, I was very pleased to see that many of you liked my sonar
technical points (and thanks to James Frusetta for having given me
this opportunity!). Here are answers to some of the questions and
comments that have been made about them.
Please note that I am aware this is not a dolphin digest... :)
So I'll try to stay close to any possible Gloranthan use.

Daniel McCluskey:
> I really think that the "carrier" of darksense should be darkness.
I like that, despite the good objections made by James Frusetta (in
#142). It makes sense:
The thicker the darkness, the "denser" the carrier, and so, the faster
the sonar waves, just like in RW water. That would give trolls even
more good reasons to be hunting in the darkest nights.

And I wouldn't hear anymore one of my player (with a troll) saying:
"Well, since it is brigth and blinding noon, I just close my eyes and
I use my Darksense..."

Lewis Jardine:
>By this I mean that ultrasound has to be connected using a similar
>density medium (KY-Jelly!) to get maximum energy transfer.
Not in water. I'm positive about dolphins being able to tell if you
are pregnant without any jelly, and they don't need to be in physical
contact with you either. It is different in air, indeed. But what
about Darkness?

>> -- About techniques:
>That sounds like Dark Scan and Dark Search to me.
Not in my opinion. I see using low and high frequencies like
techniques of Dark Scan.
Dark Search would be finding a coin in the sand, or scanning the
inside of an object using the previously described method.

James Frusetta:
>This is because the return angle is off, refraction at the edge of
>water/air, yes?
Absolutely. And also because of the different speeds of sound in air
and water.

>Is echolocation "specialized" depending on whether or not it's an
>aerial or sea-going critter? Can a bat use sonar if it's swimming?
It is not physically specialized. Thus, there is nothing that prevent
a bat from using it underwater, except that it is not used to it. I
don't think it would be able to interpretate the result of an
underwater scan.
Note the following distinction between dolphins and bats: sonar
ability is innate for a bat, but the bat cannot adapt itself to a new
situation. Sonar ability is not innate to the dolphins: they have to
learn it, to teach it to their youngs, to train using it in different

Since Dark Scan and Search are skills that can be improved by
training, study and experience, I guess that trolls are in the same
situation than the dolphins. The higher the skill, the more adaptable
the troll:
"Grmmbl! I can't sense right through that blasted rain! Go and fetch
Jonakel. He'll be able to sort it out."

>Obviously, the dolphin is not "seeing" the water...
It is very hard to imagine how the dolphin is "seeing" its environment
through the use of its sonar. However, the main idea is that you don't

see things, you see the borders of things. As long as temperature for
example does not change the density of water, you don't have to "tune"
out the water that stands between you and your prey.

A dolphin, or a troll, could easily say if a bone is broken, if there
is internal bleeding, an so on...

>Has there been work done on, for lack of a better term, how "thick"
>or dense an object a dolphin can "look" through?
There has. The main limit is not distance, it's the number of
transitons (interfaces? not sure of that word). The more "layers" of
different materials (ex: water/hull of a ship/air/another wooden
wall/air/skin, and the same amount on the way back), the harder it is.
If you use low freqencies (and thus, have poorer details), distance is
not a problem. Some rumors say that whales can communicate from one
ocean to another! (but I believe this to be only rumor).

About the weapon use of the sonar:
>can aerial critters use it? Are there species of bats that do
>something similar -- stun insect prey?
I don't think bats use their sonar that way. But insects do use sounds
as weapons and defense. Some spiders can produce a strong sound just
by clapping their mandibles, and thus, stun some frail insect prey.
As for the echolocation "glare", you can find an example of this with
the bat. One of its potential preys, a moth, has a great decoil
system: when they feel the bat's sonar waves bouncing on their body,
and thus, know they have been spot, they produce a sound of the exact
same wavelenght, and dodge. The bat thinks this sound is one of its
sonar's echos, and strike where the moth is not anymore...

>Quick question -- what's the level of "background noise" that
>cetaceans can handle?
This is not a quick question! It is a very complex problem involving a
lot of technical stuff (like:"sound waves are additive, but do not
interfere"...), and an even bigger number of highly debated
All I can say quickly is this: Sound pollution is a real threat for
cetaceans. It annoys them a hell of a lot. So, I guess it could be
used quite effectively on trolls.

>I like the pregnant troll ultrasound idea - if they can see its gonna
>be trollkin, would the XU midwives be able to use their sonar to kill
>it, or are Uz Right to Lifers?
Hmm... Sonar cannot be used to kill anything inside the mother! I
don't think it would work, and anyway, it would be too dangerous for
the mother. But I'm sure that some abortive mushrooms can be found.

>Thomas Doniol-Valcroze (a friend of Flipper?):
Don't joke with that! Did I ever tell you that my first love was a
lovely white-sided dolphin called "Whitewings", at the Vancouver
Aquarium? I was thirteen. It was "love at first sight"...
Our romance lasted for three months. But when I got back home, she
never answered any of my letters! sob!

Oh gee. This mail is *very* long. I'm sorry. I'm afraid I can speak
about marine mammals for a very long time...

Yours, Thomas.


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