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OCR for page 1
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 1 EXECUTIVE SUMMARY At the time of the assassination of President Kennedy the Dallas police recorded sounds from an open microphone; these sounds have been previously analyzed by two research groups at the request of the House Select Committee on Assassinations. Both groups concluded with 95% probability that the recordings contained acoustic impulses which provide evidence for the existence of a shot from the grassy knoll area of Dealey Plaza. On the basis of these results and since shots definitely were fired from the Texas School Book Depository, the House Committee concluded that “scientific acoustical evidence establishes a high probability that two gunmen fired at President John F.Kennedy”. In response to a request from the Department of Justice, the National Research Council Committee on Ballistic Acoustics has over the past year studied these reports and the Dallas Police recording on which they are based. Since the recorded acoustic impulses are similar to static, efforts to attribute them to gunshots have depended on echo analyses; but in these analyses desirable control tests were omitted, some of the analyses depended on subjective selection of data, serious errors were made in some of the statistical calculations, incorrect statistical conclusions were drawn and the analysis methods used were novel in some aspects and were untested at such high levels of background noise. Furthermore, some of the recorded background sounds, such as the delay in the sounds of police sirens, are not what one would expect if the open microphone had been in the motorcade. For these and other reasons discussed in the report, the Committee concluded that the previous acoustic analyses do not demonstrate that there was a grassy knoll shot. The Committee reached this conclusion prior to the availability of conclusive evidence (which we now describe) that the acoustic impulses were recorded on Channel I approximately one minute after the assassination. Following a suggestion volunteered by Steve Barber of Mansfield, Ohio, that the acoustic impulses are overlapped by an almost unintelligible voice transmission on Channel I which might be identified as cross talk from Channel II, the Committee had sound spectrograms made of the appropriate portions of both channels. Copies of these sound spectrograms and analyses of them are included in Section IV of the report. use the print version of this publication as the authoritative version for attribution.
OCR for page 2
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please EXECUTIVE SUMMARY 2 The sound spectrograms show conclusively that the portion of the Channel I recording with the acoustic impulses also contains a weak recording on Channel I of cross talk from Channel II of a message broadcast approximately one minute after the assassination. The Committee has examined the possibilities that the Channel II cross talk might have been overrecorded at a later time on top of the Channel I acoustic impulses or that the Dictabelt examined was a copy with cross talk superposed during copying. The Committee concluded that such scenarios not only are highly contrived and unlikely but also are contrary to physical and acoustic evidence, such as the effect of Channel I heterodyne tones in suppressing cross talk from Channel II. This identification of cross talk between Channels I and II shows conclusively that the previously analyzed sounds were recorded about one minute after the assassination and, therefore, too late to be attributed to assassination shots. A similar conclusion is reached independently by the analysis of the times of the acoustic impulses of intelligible cross talk between the two channels more than three minutes after the assassination. This analysis shows that the previously studied acoustic impulses were recorded after the motorcade was instructed to go to Parkland Hospital. The Committee report lists a number of possible further studies of the Channel I recording and of related matters, but, because of the strength of the demonstration that the acoustical evidence for a grassy knoll shot is invalid, the Committee believes that the results to be expected from such studies would not justify their cost. For these reasons and for others given in detail in the report, the National Research Council Committee on Ballistic Acoustics unanimously concludes that: — The acoustic analyses do not demonstrate that there was a grassy knoll shot, and in particular there is no acoustic basis for the claim of 95% probability of such a shot. — The acoustic impulses attributed to gunshots were recorded about one minute after the President had been shot and the motorcade had been instructed to go to the hospital. — Therefore, reliable acoustic data do not support a conclusion that there was a second gunman. use the print version of this publication as the authoritative version for attribution.