This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

The National Research Council was established by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and of advising the federal government. The Council operates in accordance with general policies determined by the Academy under the authority of its congressional charter of 1863, which establishes the Academy as a private, nonprofit, self-governing agency of both the National Academy of Sciences and the National Academy of Engineering in the conduct of their services to the government, the public, and the scientific and engineering communities. It is administered jointly by both Academies and the Institute of Medicine. The National Academy of Engineering and the Institute of Medicine were established in 1964 and 1970, respectively, under the charter of the National Academy of Sciences.

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.

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.

I. INTRODUCTION AND OVERVIEW

At the time of the John F. Kennedy assassination, a microphone, presumably on a police motorcycle, was stuck open and transmitted continuously on Dallas Police Department Channel I during the time of the assassination, making a record of the transmissions on a Dictaphone belt recorder model A2TC. At the request of the House Select Committee on Assassinations, this belt and magnetic tape copies of it were studied by James Barger, Scott Robinson, Edward Schmidt and Jared Wolf (BRSW) of Bolt Beranek and Newman Inc., and later by Mark Weiss and Ernest Aschkenasy (WA) of Queens College. In an initial report to the House Select Committee on Assassinations on September 11, 1978, and in a later report in January 1979, BRSW concluded⁽¹⁾ that the recording contained four sounds, which they attributed to probable gunshots, and that with a probability of 50 per cent, one of them (the third) was due to a shot from the grassy knoll area of Dealey Plaza. Later, WA studied the echo patterns analytically and their conclusion⁽¹⁾ was that "the odds are less than 1 in 20 that the impulses and echoes were not caused by a gunshot from the grassy knoll, and at least 20 to 1 that they were." BRSW subsequently reviewed the results of WA and concluded⁽¹⁾ that "the probability that they obtained their match because the two matched patterns were due to the same source (gunfire from the knoll) is about 95%." This conclusion, together with the known shots from the Texas School Book Depository, was the basis of the finding by the House Select Committee on Assassinations that "scientific acoustical evidence establishes a high probability that two gunmen fired at President John F. Kennedy."

On December 1, 1980, the Federal Bureau of Investigation (FBI) released a report, prepared by its Technical Services Division and dated November 19, 1980, with the findings that the above conclusion of the House Select Committee on Assassinations was not valid and that the acoustical evidence presented "did not scientifically prove that the Dictabelt recording on Channel 1 . . . contains the sounds of gunshots or any other sounds originating in Dealey Plaza . . ."

The Committee on Ballistic Acoustics was established by the National Research Council in the fall of 1980 in response to a request from the Department of Justice for a review of the methodology employed in the evaluations of the recorded acoustic data and of the conclusions about the existence of a shot from the grassy knoll. The Committee was also asked to recommend the kinds of tests, analyses, and evaluations needed to obtain better information from the recording.

Copies of the BRSW, WA, and FBI reports were distributed to members of the Committee before its first meeting, both for study and to make possible the distribution of questions in advance to guests invited to meet with the Committee. On January 31 and February 1, 1981, the Committee met with James Barger and Francis Jackson of Bolt Beranek and Newman Inc., with Mark Weiss and Ernest Aschkenasy of Queens College and with Bruce E. Koenig and others from the FBI Technical Services Division. Subgroups of the Committee have had many separate meetings and have carried out numerous individual studies, the results of which have been distributed to all members. The Committee as a whole met a second time on November 14-15, 1981. The Committee benefited from numerous communications it received from various interested persons. In particular, the manuscript for "The Kennedy Assassination Tapes" by James Bowles, Communications Supervisor of the Dallas Police Department at the time of the assassination, and the cross talk identifications suggested by Steve Barber, a private citizen of Mansfield, Ohio, were of great help to the Committee. Burn Lewis and Ramesh Agarwal of the IBM Corporation assisted the Committee with the digitized studies of the recordings. The Committee would like to express its appreciation to all these people, as well as to Bruce Koenig and others of the FBI who assisted the Committee in producing crucial sound spectrograms.

In the first months of its existence the Committee studied the analytical techniques used by BRSW/WA. These techniques are briefly summarized in Section II of this report. As a result of these studies, Committee members found errors in the previous studies and faults of methodology, described in Section III. These faults were sufficiently serious that, by the end of the first Committee meeting, no member was convinced by previous acoustic analyses that there was a grassy knoll shot.

The Committee agreed to continue its studies to challenge its own conclusion and to search for additional acoustic evidence. The Committee was greatly helped in its studies by the suggestion volunteered by Steve Barber that in the same tape position as the relevant acoustic impulses there was an almost unintelligible voice communication which he thought was cross talk from the Dallas Police Department Channel II, as recorded on a Gray Audograph disk. On the day of the assassination, Channel I was primarily used for normal police activities and Channel II was used for the presidential motorcade. The quality of the recorded cross talk was so poor that the Committee could not conclude by listening to the recordings the two messages were the same. However, the Committee made sound spectrograms, copies of which are included in Section IV-1. As discussed in that section and in Appendix B, a number of different analyses of the sound spectrograms of those portions of the recordings (identified as the "hold everything . . ." segments) show conclusively that a segment of Channel II is recorded on the Channel I Dictabelt at the same location as the relevant acoustic impulses. From the Channel II recording it is clear that the message of concern was broadcast one minute after the assassination. Except for the unlikely possibility of an overrecording with two superposed recordings at different times on the Channel I Dictabelt, which is contrary to the evidence discussed in Section IV-4, this identification between the two channels shows that the sounds analyzed by BRSW/WA occurred one minute after the President had been shot and the motorcade had left Dealey Plaza.

A second demonstration of the same conclusion involves a more obvious and later instance of cross talk between the two channels; no sound spectrograms are needed to identify the "Do you want . . . Stemmons" sections on both channels. This analysis is discussed in Section IV-3 and Appendix C. By tracing time backward from that match on both channels, the Committee found that the President received his mortal shot at least 30.9 seconds before the impulses analyzed by BRSW/WA. The uncertainty in the exact timing discrepancy arises from the fact that the Channel I recorder operated continuously, whereas the Channel II recorder operated intermittently and was supposed to stop after 4 seconds of silence. So in all cases when the Channel II recorder was inoperative, the "missing time" must be added to the "at least 30.9 seconds" noted previously. The Committee's two quite different techniques for determining the length of time between the real assassination and the one deduced from the study of sounds on the Channel II tape can, of course, be brought into agreement, at one minute, by the reasonable assumption that the Channel II recorder was not operating for a total of 44 seconds in a section of the recording in which the recorder operated for 206 seconds and in which there are many places where there are 3 to 6 seconds of recording silence. The recorder may have stopped during some of these times and it definitely did stop for 2.9 seconds during one of them.

It is important to note that this second timing method cannot be brought into agreement with the timing demanded by the BRSW/WA analysis unless one assumes that there are backward skips totaling 30.9 seconds on the analyzed playbacks of Channel II or that there is overrecording; the tapes show no evidence of the backward skips required by the BRSW/WA analyses. For the timing method based on the "hold everything" analysis, the recorded impulses could come from assassination shots only if there were accidental or intentional overrecording. The Committee investigated possibility of overrecording by microscopic examination of the grooves on the Dictabelt, by examining the effects of heterodynes on the intensities of the sound spectrograms and by examining the possibilities by which overrecording could have occurred and then have been hidden, either accidentally or on purpose. These investigations are reported in Section IV-4 and Appendix D. For the reasons discussed there, the Committee concluded that the BRSW/WA timing could not be made compatible with the observed Channel I and Channel II cross talk.

Features of the recorded sounds, especially the siren sounds, strongly suggest that the open microphone was not in Dealey Plaza at the time of the assassination, even though the BRSW/WA analysis required it to be there and, in fact, identifies the open microphone explicitly as on the motorcycle of Officer McLain.

A feature of the Channel I tape that has puzzled most persons who have listened to it is the complete absence of siren sounds for about two minutes after the time BRSW/WA pinpointed as that of the assassination. Witnesses have testified that soon after the assassination many sirens were activated in the motorcade, as anyone would have predicted. Although the sirens could be partially suppressed by the microphone directivity and the high level of background noise, their total absence for two minutes is strange for a microphone in the motorcade, especially when the siren sounds are later so clear. Furthermore, when finally the siren sounds do appear they have the characteristics one would expect for sirens that were moving at some reasonable speed relative to, or passing, a microphone, and not the sounds of sirens moving along with the microphone. An important signature is the gradual rise and fall in the loudness of the siren sounds, over a period of several seconds. The sounds go from inaudible to loud, and then back to inaudible, and one recognizes that there is more than one siren that passes the microphone. It is difficult to devise a credible scenario that takes Officer McLain's motorcycle from its observed position behind the President's car to Parkland Hospital (where Officer McLain was photographed with Mrs. Kennedy as they entered the hospital) and which permits the long absence of siren sounds, followed by the very distinctive sounds mentioned above. The siren sounds are much more compatible with the suggestion in James C. Bowles' manuscript⁽²⁾ that the open microphone was in the command post near the Trade Mart than that it was in the motorcade. The siren sounds are discussed further in Section II of the report and in Appendix E.

An evaluation of the November 19, 1981 FBI report is given in Section V. In Section VI and Appendix F, the Committee, in response to its charge, lists some of the tests, analyses and evaluations that could be made to obtain better information from the Dallas Police Department recordings, but the evidence against the BRSW/WA conjectured grassy knoll assassination shot is already so strong that the Committee believes the results to be expected from such studies would not justify their cost. The Committee's conclusions are given in Section VII and the Executive Summary.

REFERENCES

1. Appendix to Hearings Before the Select Committee on Assassinations of the House of Representatives Ninety-Fifth Congress, Volume VIII, US Government Printing Office, Washington, DC, 1979.

2. James C. Bowles, The Kennedy Assassination Tapes: A Rebuttal to the Acoustical Evidence Theory (copyrighted and unpublished).

3. 3. Hearings Before the President's Commission on the Assassination of President Kennedy, US Government Printing Office, Washington, DC, 1964.

4. Report released December 1, 1980, by the Federal Bureau of Investigation and prepared by the FBI Technical Services Division, Washington, DC, and dated November 19, 1980.

5. Minitab Manual, by Thomas A. Ryan, Jr., Brian J. Jainer, and Barbara F. Ryan, published by Minitab Project, Statistics Department, 215 Pond Laboratory, Pennsylvania State University, University Park, PA 16802.