Some thought on trolls' darksense

From: Dag Stålhandske <>
Date: Thu, 6 Sep 2001 18:55:27 +0200

Trolls' darksense


Before writing something about the trolls' darksense, three reservations must be made. First it is based on a short description of the trolls' darksense in a sidebar in Trollpak. I shall quote that text in its' entirety, suffice it for now to state that darksense is described as a form of sonar. So, this paper is based on old, possibly obsolete material that may be contradicted in new material to come. I believe a new book in the trolls, called Feed Uz, is planned. Second, based on the science of sonar, I elaborate on that text. This is an uncertain method given that Glorantha is not a scientific world. Third, it is based on my knowledge of sound and sonar, and while I have a MSc it is not sonar and acoustics that are my specialities, but spectroscopy and optics. I might very well be scientifically wrong.

What has been said previously

The Trollpak Book 1 Uz Lore sidebar page 25 states: "DARKSENSE: This is how a troll gets around. Darksense is to a troll as sight is to a human. It is a form of sonar. The troll generates an ultrasonic noise in his head and receives the echo in his sensitive ears and head bones. This skill is quite refined. A troll can clearly define silhouettes at close range, and differentiate textures with this sense. It can be used to tell the difference between bronze and lead (or other metals), just by the different 'ring' that each gives off when sonared. The troll can tell movement, distance (very precisely), and texture with darksense. It is truly an excellent sense, comparable in many ways to eyesight, though it has its limitations. It is not as good as eyesight at long ranges, and trolls only get silhouettes and general impressions of mountains over a kilometre off, for example. A dark troll would be unable to detect an object as small as a person at a range over a kilometre. In the!  other hand, the troll can 'see' in the dark clearly, and tell by the echo whether a mountain is covered by evergreens or deciduous trees. Trolls are able to focus their darksense and closely scrutinise an area at long range. When doing this, they could see a human two or three kilometres off, but it would take trolls several seconds to receive this information, and they would have to know exactly where to look. In their normal environment, darksense gives trolls a large edge over a human. An interesting fact of darksense is that a troll can use it to sound an animal's body cavities and see whether or not it is hungry (by looking for empty stomachs). This is a peculiar ability, but is only usable very close at hand. The mistress race have sonar as much superior to dark trolls as an eagles sight is superior to a human's. Cave trolls and trollkin have relatively inferior darksense, and supplement it with their eyes."

The basics

From this description, it can be concluded that the troll gives out an ultrasonic signal that travels at speed of sound, normally around 340 m/s and hit objects. Some of the sound bounce back, some is absorbed and some is transmitted and keep travelling inside the object. The troll detects the echo, i.e. the sound that has bounced back to it, and can draw conclusion from where the sound is coming and how much time has elapsed. The sound the troll emits must be of short duration or it will not be able to hear the echo. It is hard to hear your own echo while you are shouting, you will only hear it after you have finished shouting. Therefore you can think of each shout as a snapshot. If looking at something at a distance, the ultrasound must have time to travel to the object and back. For instance to look at an object a kilometre away you would need to wait 6 seconds (2 times 1km divided by 340 meters per second equals about 6 seconds). So in that case you see snapshots with 6 se! conds interval. Normally a troll emits ultrasound about once each second. If rapid information is important, for instance in a melee, a troll will shout ultrasound ten times per second, making himself effectively blind to anything more than 17 metres away. Since the image you get is a snapshot, the situation is equivalent to a human watching something in stroboscopic light. In the strictest sense, a troll can not "see" movement with his darksense. However he is very good at detecting small difference between one picture and the next and perceives that as a movement.

Questions and answers

If a troll can hear a faint echo of the ultrasonic sound it emits, can he hear the ultrasound that other trolls emit? Yes and he can do so several kilometers away if conditions are good. A troll who is using his darksense signals his presence and location to all trolls in a large area. Therefore, in troll versus troll warfare, it is hard to achieve surprise by sneaking up on someone. What you do is to hide in a place, turn your darksense off and wait for the enemy to come. In tunnel warfare, the enemy usually does not have a great many ways of approaching you. When the enemy comes, you will hear his darksense. If he chooses to refrain completely from using the darksense he will have to move slowly and clumsily and probably make enough normal, non-ultrasonic, noise for the ambushers to detect him. Scout trollkin are fairly good at moving stealthily without using darksense. They are therefore often used as point men.

Can this ultrasound be used for communication? A troll's ultrasound generator is not designed to be a speech organ. It cannot manipulate the ultrasound like a tongue and lips can manipulate normal sound. Still, you could make a morse-like signal system with it, but no one has bothered to do so yet.

How much information does a troll get from hearing another troll's ultrasound? He would know that another troll is present and where he is. If several trolls were present, he could count them as easily as a human can count objects within sight. Trolls do not have distinct ultrasound voices, so he cannot recognise individuals. He can however, tell the race of the emitter and whether the emitter is physically mature for his race. He can also tell how the emitter is using his darksense. Few, extra loud shots with great intervals of silence in between means that he is looking at something at a distance, for instance.

Can a troll use another troll's ultrasound emissions to "see" the environment? In theory, yes. However, it is so difficult to decipher the information that the emitting troll must be standing very close to the listening troll and have the same physical orientation if this should work. This method is used if a troll has his ultrasound emitter damaged.

If several troll's are using their darksense simultaneously, do they disturb one another? Yes. You cannot automatically tell if an echo is yours or another troll's. Therefore, they normally take turns emitting ultrasound and listening to the echo. This ultrasound etiquette is so natural for them that they do so without thinking. In melee combat it is necessary to emit sound several times per second. This becomes a problem if there are many trolls in the same melee in the dark. An ultrasonic shouting match is going on during the combat and everyone is having a hard time to "see" very well. If a group of trolls were fighting someone who could see well in darkness with another method than ultrasound, the trolls would be at a great disadvantage. This could happen if Humakti, whose swords give out light visible only to Humakti, were fighting many trolls in the darkness. Still, trolls with darksense are at a great disadvantage to most other beings who are blind in the darkness.

Can a troll deliberately "jam" another troll's darksense? Yes, by using his own ultrasound emitter he can make it very hard for another troll to see things close to the jamming troll. By doing so, he makes his presence and location very clear to the troll being jammed, but the jamming troll makes anything close to him or behind him so blurry it is impossible to detect anything.

Can a troll let the ultrasound bounce to see around corners the way a human could use a mirror for a similar purpose? In theory, yes. In reality it would require an extremely smooth surface, certainly not something you find in Uz caves, and some training to use it properly.

If a troll can see "inside" a living or dead body, would this be useful in medical diagnostics? In our world, it would be quite useful in the emergency room to be able to see broken bones, internal bleeding and sharpnel inside a human body. In Glorantha, healing is done by magic and precise diagnostics are not as useful as in our world. It would be very useful to sort wounded people to make sure that those who needed quick help fast where the ones who got it. It would also be useful in security. With darksense you can do a complete body search, including a cavity search of a person without touching a person.

Is darksense a 360° sense?
Well hearing is a 360° sense, and darksense is a form of sending out sound and hearing the echo. However, assuming that the ultrasound emits from the trolls mouth, it will send a stronger sound in the forward direction than backwards. It is easier to hear someone if he is facing you when he is talking. Therefore, the darksense is significantly better in the direction the troll is facing, but it is fairly good in all directions. You cannot sneak up on a troll from behind if the troll is using his darksense. If you need to surprise a troll, you need to hide nearby and wait for him to pass by close by.

Can a troll move while using darksense?
Ideally, the head should not move from the time ultrasound is emitted until the sound has bounced back. However, for normal troll purposes, i.e. walking or running in tunnels, this is not a problem. The troll should just avoid moving turning the head rapidly. However, when looking at something at a distance, standing still is required.

Do trolls turn off their darksense when they go to sleep? Yes.

Will headgear "blindfold" darksense?
I believe that ultrasound is easier to block out than normal sound. Therefore, a troll is less willing to wear headgear than a human. A troll's head protection is designed to leave the ears free and the troll only puts it on when he is walking into combat. A troll guard, for instance might have a helmet nearby, but until combat is imminent, he will not put it on.

Can a troll's darksense see through mist or smoke? Yes.

How do you hide from a troll's darksense? Try to make your silhouette contrast as little as possible from the environment. In a tunnel, lie on the floor, in the ceiling or stand against a wall. Very complex terrain with a great many complex surfaces like a forest is the best hiding place. Movement makes you easy to detect against an unmoving background. However, movement in the background will conceal your movement very well. So a forest with on a windy day is the right place and time to sneak up on trolls. Camouflage by choosing the right colours is not an issue, since trolls are colour-blind with normal sight and darksense does not reveal colours. Just avoid wearing white or very light colours in the darkness since the trolls can see you with normal sight. As has been stated a troll can "see" what is behind him and smoke or fog will not conceal you.

Can a troll use darksense for prospecting? Yes, but I do not think ultrasound can travel very deep in the mountain, so the usefulness is limited.

End of The Glorantha Digest V8 #519

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