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[EPFL] [University of Kentucky]
[The International Meteor Organization]
read more: Electrophonic sounds - history
read more: Electrophonic sounds - existing catalogs
Electrophonic sounds:
/Introduction/
*Electrophonic sounds are defined as sounds produced by direct
conversion of electromagnetic radiation into audible sound. In the case
of meteor-related electrophonic sounds, they are heard simultaneously
with the appearance of a bright meteor.*
This fact is in clear contradiction to the physical laws of normal sound
propagation. Electrophonic sounds were for a long time considered to be
a purely psychological side-effect of seeing a bright meteor. Only in
the last twenty years have things changed for the better and the reality
of these sounds is not questioned anymore, although the search for their
actual causes is still far from finished.
The fact that puzzled so many scientists and caused the already
mentioned ignorance is that any sound produced by the meteors would
travel to the observer on the ground within a few minutes after the
meteor disappears, as the meteors usually burn-out at heights above ~30
km. The electrophonic sounds therefore must have a different origin [1
].
A distinction to a normal sonic-boom produced by very large meteors
should be made here. The sonic-boom is generated when a large and solid
meteoroid, (usually of stone or iron type), penetrates into the lower
atmosphere, (lower then about 50 km), while the smaller meteors
disintegrate at heights between 80 and 100 km.
The first plausible mechanism of the origin of electrophonic sounds was
suggested by Colin Keay in 1980 [2 ] and
theoretically modeled by Bronshten [3 ].
According to their theory a bright fireball can, under special
conditions, produce ELF/VLF radio waves [4 ].
This electromagnetic radiation can then be converted into sound by an
ordinary object in the observer's vicinity. The main conclusions of this
theory are that very bright bolides are needed to generate VLF, and they
set the lover limit to -12^ m , (about equal to the brightness of a full
Moon).
Much more important are the results of Keay's laboratory experiments on
the generation of sound by VLF fields on mundane objects that clearly
demonstrate the ability of VLF radiation to produce audible sound [5
].
In 1991, he pointed to the first known detection of a meteor VLF by
Japanese observers [6 ,7
]. Soon after, he refined the theory [8
] and predicted that VLF can be generated
in the moment of the explosive disintegration of a bolide, but also a
little bit earlier. Recently, a new theory of VLF emission from meteors
was proposed [9 ], but still all these
theories cannot explain the variety of electrophonic sound properties
[10 ].
In cooperation with Ceplecha, Keay tried to predict the average number
of electrophonic sounds that should be heard by a single person [11
]. The prediction says that a person which
would spend every night outside has a once in a lifetime chance of
hearing an electrophonic sound. They pointed out that this is a very
optimistic prediction as today many such events would be masked by
man-made sounds and would so pass unnoticed.
/Copyright 1999 Zeljko Andreic, Dejan Vinkovic/
* Read about more recent developments in the ILWCRO press release
regarding the first instrumental detection of electrophonic meteors:
fizika.org/ilwcro/results *
read more: Electrophonic sounds - history
read more: Electrophonic sounds - existing catalogs
*Other sources of information about the electrophonic sound:*
*Colin Keay's Home Page *
*Phil Bagnall's Home Page *
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Dejan Vinkovic (University of Kentucky
)