Meteor Perception Sounds

The abilities of video systems like our MOVIE has been discussed already in detail, they provide us with very accurate optical information on what is going on in the sky. Nevertheless, other important types of data have been completely ignored so far. Thus, the aim of this work is to show the chance of getting valuable acoustic information from video recordings.

Normally meteor astronomers observe together in groups. Watching the sky alone with usually almost no meteor activity is boring but if you have a partner you can at least share your annoyance about the situation with him. In addition you have the happy moments when the other saw a nice shooting star whereas you did not concentrate on the sky for a second.
In the last few years I often observed together with DUBKA and NITMI. In those nights I found out that it is possible to guess the brightness of a meteor I have not seen accurately from the sound the other observers make: A little 'Huch!' is not so bad - I only missed an unimpressive +3 mag or even +4 mag meteor whereas an 'Oi!' or 'Wow!' follows a meteors of about 1 mag. If I hear a most excited 'Waaaahnsinn!!!' or any other undefined phonemes that does not correspond to proper German words I can be quite sure that something really bright appeared and I did not see it.

What has that to do with MOVIE? You may know that the recording part of our video system is not a video camera but a Camcorder which, as all devices with this name, has a microphone, too. Thus, many of the human meteor perception sounds are recorded and I can adjust my brightness estimation scale even better by analysing these video tapes. Even more there is valuable information about meteors outside the cameras field of view and about meteor properties the system cannot record.
Unfortunately there is always a lot of background noise (for example from our power generator and the mounting) and the observers are far away from the video system, still many of the sounds are clearly understandable.

In a first approach to the problem I've tried to classify the sounds. For that I have browsed through our tapes starting from the Quadrantids '93 up to the Lyrids from '95. There have been about 200 meteors included in this investigation, in 29 cases I found a distinct sound coming from one or many observers noticing the event.
Five of them had to be be rejected later because their SNR was too bad, so I could not classify them. The others fit neatly into one of the following four classes:

1. faint meteors: Perception sounds following faint meteors are characterised by a great variety of words or phonemes that are used. They are normally very short and not very loud. Whereas our female observer tends to react most often with 'Huch!', we get more variable responses like 'Oi!' and 'Ou!' by male observers.
   It could be shown that different observers say different words after watching the same meteor: In the following example we can hear 'Eij!', 'Ja!' and 'Schön!' at the same time. This makes the acuostic analysis more complicated, but I hope to come up with a list of equivalent words later. Furthermore we should not forget that visual brightness estimations are often not consistent, too.
   Another problem are words and sounds that cannot be written because they just do not exist: Maybe we are dealing here with a mix of two common sounds or the observers has just no time to respond due to high activity.



2. bright meteors: If an observer notices a brighter meteor he has generally more emphasis in his voice. The classification of meteors that are just inbetween faint and bright is sometimes not easy, because the used words are still the same. Anyway, an experienced listener can conclude from the pronounciation of the phrase 'Oi!' alone that the observer just saw a +1,5 mag or +2 mag, hence, a bright meteor. He concentrates completely on the event because he knows that it has been the last bright shooting star for a while.
   Once the meteor becomes even brighter it is easier to classify, because the reactions of the observers change dramatically: They use more and longer words and they tend to 'motivate' the meteor shower with positive attributes. In a typical case you may hear 'Oh! ... Wahnsinn!' and 'Gut!' as a try to stimulate even more events of that type.
   There is a trend that male observers show more emotions at this stadium, they say 'Da! ... Oi! ... Cool!' compared to 'Wahnsinn! .. Oah!'. Even words like 'Boah!' may be heard in such a situation.



3. fireballs: Especially thrilling are, of course, meteors brighter than -3 mag. We call them fireballs from their visual appearance, but what about the acoustic responses? First of all we can classify all sound events that last longer than three seconds to be triggered by fireballs. In the standard case we hear common sounds like 'Oh!' and 'Wahnsinn!' with an intensity and volume that might even wake up sleeping meteor observers from their sweet dreams. If they are lucky enough they might still see the dimishing persistent train of the bolide, if not there is hope for another effect:
   Highly trained observers tend to give away valuable information about the color (blue) and unexpected things like distortions of the train due to winds in the higher atmosphere. Even though this makes fireball perception sounds most significant and important in acoustic meteor observation we also have to consider certain dangers. It could be shown that too bright and slow meteors may result in ecstatic observer states causing an exponential voice volume increase. This is to blame for serious damages of the hearing system of other nearby observers!



4. special events: Perception sounds have to be classified as special events if the observer was not in the standard observing position or mood.
   
   • There are the so called pre-observation meteors that occur during the system setup. Their perception is characterized by ignorance, the observer notices the event but goes on to work without interruption (unless the meteor did belong to the class fireball). This might result in simple expression sentences like 'There was a shooting star!'.
   
   
   • During the observation the camera operator has to change the field of view and control the recording system sometimes. Probability theory and Murphy's law demand a significant increase in meteor activity at those times because both the camera and the observer are not active. Thus, the operator notices regularly meteors at the control monitor and his unhappyness triggers a demotivated sentence.
     It is most interesting that the used words in this situation are always the same which makes it a good indicator for the overall psychological condition of the observer. In addition it is a good test for visual perception abilities of non meteor observers who tend to react to the announced meteor with the question 'Where?'.
   
   
   • After a long night with hundreds of meteors even the hardest observer becomes tired and cannot react appropriately anymore to bright meteors or even fireballs. This is the recommended time to stop acoustic analysis to avoid major errors.
   
   
   • Once the observer has finished the active work he just cannot relax but stands in front of the still working video monitor and realizes that the real show is just starting. It is not uncommon that he or she can count in real time more meteors per minute than in the whole last hour.
   Finally we have to consider different languages and the strange reactions of non meteor observers. Whereas a Dutch observer can be regarded as a good classifier for meteor parameters, too, we have to be careful with unexperienced people:
   
   • Some of them try to do their best to copy the professionals, but they simply do not know standard reactions and use wrong phrases.
   
   
   • Others are completely overwhelmed when they see a meteor of the class 'bright' and respond as if one had to expect a nearby impact soon.
   
   
   • Finally there are human beeings who have never seen a meteor before. Depending on their age and sex they respond completely chaotic and non-predictable, hence, they should be left out in any serious investigation.

I want to conclude that acoustic recordings of meteor observations contain valuable data about meteor brightness, frequency and specific properties. The next step should be the standardization of perception sounds using a describtive sound cataloge to make the reactions even more reliable. Maybe we should generally react in English to make results from all over the world comparable.
The introduction of a normalized AHR (acoustic hourly rate) is an open problem, too.