For several hundred years, and perhaps longer, witnesses have reported that a few very bright meteors, or fireballs, have produced sounds heard simultaneously with the passage of the meteor. Because of the distances involved, this obviously rules normal acoustic sound waves from a sonic boom (which themselves are quite rare). This anomalous or electrophonic sounds must therefore travel at the speed of light. This would suggest that they are actually electromagnetic waves that are, by some unknown means, transformed into sound waves in the vicinity of the observer.
While these sounds have been observed and reported for at least three hundred years, there still is no satisfactory explanation for this phenomenon. There are a number of theories, but all either fail to explain the variety of properties of the electrophnic sounds, or have other serious problems.
The most widely accepted theory is that the electrophonic sounds are created by ELF/VLF radio waves. While laboratory experiments have indicated that Extremely Low Frequency radio waves can produce sound, there is no currently accepted theory on how the passage of the meteor could create such ELF/VLF waves. And since this mechanism is unknown, the conditions (weather, ionospheric, season, etc.) favoring the generation of such waves is unknown.
Fireballs themselves are very rare. The sonic boom sometimes heard following the passage of a low-altitude meteor is extremely rare. And reports of electrophonic sounds is rarer still. Only a fraction of fireballs will produce this phenomenon. So the chances of an individual experiencing this are very low. And this is why reports are so badly needed.
The Global Electrophonic Fireball Survey (GEFS) by the Center for Computational Sciences at the University of Kentucky is currently collecting eyewitness reports of electrophonic sounds from meteors. Information on what is needed, with other references, can be found at the GEFS homepage. Reports can be submitted via an HTML data submission form, or by e-mail to email@example.com.
It would be well to look at the form now, as the questions about the meteor, the sound, weather conditions, observing conditions, etc. etc. etc. are quite extensive.
The IMO and GEFS are asking that this request for data be spread as much as possible, as the phenomenon is very rare. Also, modern man-made noises probably have masked electrophonic sounds, causing people to dismiss them as something else. And what better group is there to alert than a group of people who are familiar with RF phenomena and also with meteor activity?
One final note, not part of the IMO's request. For high-latitude observers. The aurora has long been said to sometimes produce electrophonic sounds. Even more than with meteors, these have tended to be dismissed as "something else." It would appear that they likely are related to the electrophonic sounds from meteors. So perhaps it is the ionosphere, more than the meteor itself, that is the cause? There remains much in heaven and in earth that is not understood!
For further information, see WGN, the bimonthly journal of the International Meteor Organization, 28-2/3, April-June 2000, pp. 48-53.
Alsok, an early summary of the 1999 Leonids findings (and questions), including VLF signals, on Daniel Fisher's page, Story 2 Item 3. (Good summary of the MAC conference, shows how much we still don't know!)