Own research.
......but please - let me first introduce myself:
Electronic engineer, now retired. Spent nearly 25 years on a technical university specialising in the development and maintenance of electronic research equipment. Many years of interest in parapsychology lead me to Jurgenson's and Raudive's books and somewhat later to the events in London 1971. Perhaps I should rather say that I became interested in the technical aspect (today I would call it the paraphysical aspect) of the phenomenon. During my employment within a well equipped electronic laboratory, I had the opportunity occasionally to back up my investigations in different ways and also to develop special electronic equipment.This research, which also included several tests in a totally screened room, was spread over more then fifteen years and resulted in some entirely different recording methods - using both analogue and digital techniques. All the old 1971 manipulations using microphone, radio set etc., where an influence from outside sources could only be excluded under laboratory conditions, are left behind a long time ago.
First in the eighties I had the opportunity to read Jurgenson's and Raudive's books and also hear some tape-recordings of their voices. At the same time I also tried to imitate some of Jurgenson's experiments following the descriptions in his book. The results were interesting but I still had the feeling: Maybee - maybee not...
It was the totally insufficient documentation of the genuineness of voices when received by using a microphone or radio-receiver. What was missing was the possibility of a comparison of the original source with the recorded voices. (In the London investigation Beling & Lee.Ltd. had more or less bypassed the problem by using a sensitive radio receiver to receive the hiss (white noise) from a unmodulatted transmitter.)
But it was first after having read a detailed report of the London investigations made by two firms, whose names were familiar to me from my university work that I decided to start my own enlarged check-up.
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Some early analog apparatus - about 1987-1988.
This somewhat confusing accumulation of different apparatus (just one of several) is in principle a radio transmitter with modulator and a screened receiver - surrounded by some mesuring and registrating equipment.
The purpose of the shown apparatus was to simulate the situation during the well known old Jurgenson-Raudive radio recording - but under laboratory controlled circumstances:
1.Both the magnitude of the signal and the modulation was controled.
2.The modulation material was obtained from a tape-cassette and could be repeated. But the main interest was concentrated on the obvious question: What was the original content on just the place where one later heard a "voice"?
And the results of this investigation?
1. The existens of the phenomenon was confirmed - but only when using very low level signals.
2. The arising of a voice seems to occur totally arbitrarily. Repeated experiments when keeping all parameters constant gave different results - with one interesting exeption: The number of "voices" decreased with every repetition so one was forced to a frequently change of the modulation material.
3. A comparison of the "voices" (which I after this investigation would prefer to characterize as a "paranormal influence") with the radiated material showed a variety of degrees of change. The material available at a particular place is sometimes utilized in a very clever way and the result strech from only small deviation from of the original to a totally developed voice. The first are mostly overlooked, the last is easy to understand. But in between is an large area reaching from the " nearly good understandable" to the "several possible interpretations" voices. That explains some of the difficulties and many futile discussions (like Raudive/Ellis) concerning the right interpretation.
! So the main lesson from the investigation: The possibility of a comparation between input and output is unavoidable - both in regard to a reliable interpretation and the necessery dokumentation !