[EPFL] [University of Kentucky]
[The International Meteor Organization]
 read more: Electrophonic sounds - history
 read more: Electrophonic sounds - existing catalogs

Electrophonic sounds:

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

Dejan Vinkovic (University of Kentucky)