Report of the Committee on
REPORT OF THE
COMMITTEE ON BALLISTIC ACOUSTICS
COMMISION ON PHYSICAL SCIENCES,
MATHEMATICS, AND RESOURCES
NATIONAL RESEARCH COUNCIL
NATIONAL ACADEMY PRESS
Washington, DC 1982
III. EVALUATION OF THE BRSW AND WA METHODOLOGIES AND CONCLUSIONS
A reliable analysis of the Dallas Police Department Channel I recording presents serious difficulties. The noise level is high, there is conflicting evidence as to the location of the open microphone, some of the background sounds are difficult to interpret, the absence of certain expected background sounds is difficult to understand, and the transmitting and recording systems distorted the acoustic signals. As pointed out by WA, to the ear the sounds resemble static much more than they do gunshots and it is only the poor fidelity of the radio dispatching system that might permit the latter interpretation. But such static-like sounds could be generated by a number of other acoustic, electrical or mechanical sources in the environment and in the radio transmission, receiving and recording equipment. Tests and analyses more discerning than the human ear are required to determine the probable cause of the sound impulses. The WA analysis is ingenious but it is novel in some aspects and both the BRSW and the WA echo techniques for gunshot location had not been applied previously by either group to a situation with as high a level of noise and distortion as this one.
Furthermore, the BRSW/WA studies were seriously limited by funds and by the time schedules with fixed deadlines. A number of essential tests to confirm both the analysis procedure and the interpretations were omitted. Some of these are listed in Section VI and Appendix F. The WA studies, for example, were limited to the single conjectured grassy knoll shot from Dealey Plaza. The results of such an analysis should not be considered reliable until the method has been adequately tested on some other cases. In particular, the impulses conjectured to be sounds of gunshots from the Texas School Book Depository should have been analyzed by the same method. Not only would this have provided a control on the method, but it would also have provided much stronger evidence as to whether the open microphone was or was not in Dealey Plaza at the correct time. Similarly, more of the test shots should have been analyzed to compare the observed echo patterns with those predicted from structures identified in the echo patterns with a different neighboring microphone location.
The original BRSW report claimed a 50% probability of there being an additional shot from the grassy knoll. Even this seemingly modest claim is based on both questionable assumptions and on incorrect computations (see Appendix A). This claim was used as a justification for the later more detailed studies of WA. The result of WA's analytic echo-prediction technique in the subsequent analysis of this BRSW conjectured shot would appear to improve the credibility of the grassy knoll hypothesis. However, the Committee noted that the identification of shots and impulses by BRSW was completely different from that by WA as demonstrated by the more than 200 millisecond (or more than 200 ft.) displacement between the two identifications (this is the displacement between A and B in Figure 2). Dr. Barger has pointed out that, if the acceptance window in matching impulses is increased to 14 ms and with the particular locations of most of the assumed reflecting objects and the short 87 msec total time span of the relevant impulses, the two different identifications may be reconciled by assuming that the BRSW echo pattern had been subject to one additional wall reflection. Even with this interpretation there remains a serious flaw; namely, that the BRSW analysis missed the identification that WA considers to be the primary one.
The impulses selected for the BRSW study were not always the largest impulses. Frequently, large impulses were omitted and some impulses close to the noise level were retained. There are far more impulses that do not fall into the BRSW classification of "probable sounds of gunfire" than do. Since the results of correlation coefficient calculations are highly dependent on the impulse and echo selection process, it is especially critical that the scheme used to distinguish these sounds stand up to close scrutiny, with the process used being spelled out in detail so others can duplicate the analysis. From the published reports, it is impossible to do so. Furthermore, weak spikes on the Dictabelt often are selected to correspond to strong patterns, in the test patterns and vice versa.
Although the conclusions of the BRSW analysis were supported by some later interpretations of photographic evidence as being consistent with a motorcycle in the procession at approximately the position indicated by their analysis, it is by no means certain that this was the motorcycle with the open microphone, that its radio was improperly tuned to Channel I, that the open microphone was even in Dealey Plaza, or that the relative times of the four sets of impulses studied by BRSW and WA were consistent with the three known actual shots. There is important evidence to the contrary on all four of these points that should not be ignored.
In his manuscript(2) on "The Kennedy Assassination Tapes," Captain James Bowles proposes the hypothesis that the motorcycle carrying the open microphone was not part of the motorcade that passed through Dealey Plaza but was near the police command post at the Trade Mart at the time of the assassination. He supports this hypothesis by a subjective assessment of the motorcycle engine sounds (both during the motorcade and subsequent to the assassination shots), the absence of crowd noises on Channel I (which are clear on Channel II), the puzzling long-delayed timing of the siren sounds, the voice broadcasts, interviews with police officers, and the fact that all motorcycles in the motorcade were supposed to be tuned to Channel II, not Channel I. The questions raised by Bowles and by others pose serious doubts about the location of the open microphone in Dealey Plaza, a necessary requirement for the BRSW conclusion.
No siren sounds are heard on Channel I at a time when they should have been heard by an open microphone in the motorcade; sirens are not heard for approximately two minutes after the impulses attributed by BRSW/WA to assassination shots, following which clear and unambiguous sounds from a group of sirens occur on Channel I. The sirens seem to come from a group of at least 3 vehicles with the intensity of the sound first increasing and then decreasing. This is consistent with sirens heard at a stationary point if the presidential motorcade had passed close by. It is not the siren sound expected if a motorcycle with a stuck button had been part of the presidential motorcade. In the first quarter mile of the trip to the hospital, the presidential motorcade encountered a complex pattern of underpasses, roads and ramps as it approached the entrance to Stemmons Freeway. But there is no trace of a siren sound in Channel I during this interval of time. This initial long absence of any indication of siren sounds, followed by the pattern of loud and clear sounds of several sirens passing by, suggests that the radio transmitter with the stuck button was not part of the presidential motorcade. This radio transmitter may have been on a motorcycle parked somewhere, perhaps, as suggested by James Bowles, at the Police Command Post near the Trade Mart, where it would be natural for there to be adjacent police radios tuned to different channels, thus accounting for the instances of cross talk described in Section IV. The problems associated with both the presence and absence of siren sounds are discussed in further detail in Appendix E.
The concluding two sentences of the BRSW report state: "The probability of obtaining just one match by chance in any of 180 independent tries is equal to 5.3 × 10-2, or about 5%. Therefore, 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%." The WA report concludes with a similar statement. Such statements do not allow for the existence of hypotheses alternative to the two primarily considered (the hypothesis of gunshots or the hypothesis of impulses randomly located according to a Poisson distribution in relevant sections of the Dictabelt). Various reasonable alternative hypotheses include non-white (non-Poisson) noise or other typical noise and static distributions which are ordinarily lumped together in time and which thereby may give a higher correlation with the non-random distributions of test shot echoes. Furthermore, even if the only alternative to impulses from a gunshot were the hypothesis of randomly located impulses, a single observed result whose P value under the random location hypothesis is 5% does not imply a 95% probability that there was gunfire from the knoll (the P value or significance level in current statistical theory is the probability, assuming the hypothesis to be true, of observing data as or more extreme than what actually is observed). The situation is analogous to that in a card game where the significance level for the dealer to receive three aces is P = 0.044 but 3 aces going to the dealer on the first deal does not by itself indicate a 95.6% probability that the dealer is dishonest if there were no prior reason for suspecting him of cheating. The issue of the probability of gunshots is one of posterior probability and is discussed further in Appendix A-3.
In addition to the above misinterpretations, the BRSW/WA calculation of the P value for the hypothesis of random pulse location is incorrect. There are several errors of which the most serious is the failure to allow in the probability calculations for the fact that the location of the shooter in the WA analysis was adjusted to maximize the number of coincidences. These errors are discussed in Appendix A, where it is shown that, with these corrections and a conservative adjustment, a significant level as high as P = 0.223 can be obtained for the hypothesis of random location; this value is much less impressive than the BRSW/WA value of 0.05. Furthermore, as discussed previously, even if it were granted that the hypothesis of randomly located impulses on relevant portions of the tape were in serious doubt, it would not follow that the alternative of gunfire from the grassy knoll was convincing. All plausible alternatives to both of these hypotheses would have to be eliminated, and no convincing effort has been made in this direction.
The analyses reviewed above and in Appendix A lead the Committee to the following conclusions about the probability analyses of BRSW/WA.
(1) The conclusion of a probability of 0.5 of a shot from the grassy knoll on the basis of the BRSW analysis is invalid as is also the conclusion of a probability of 0.95 for such a shot on the basis of the WA analysis.
(2) There are several inaccuracies.
(3) Except for a rather conservative alternative analysis given in Appendix A, the data do tend to cast doubt on the hypothesis of random impulse locations according to a Poisson process.
(4) Alternative hypotheses to a random Poisson process and a shot should have been examined as possible explanations of the coincidences. These might invoke the nature of the bursts of noise prevalent during the period under study and a consideration of other possible non-Poisson distributions.
There are some valid arguments in support of the BRSW/WA conjecture that the impulses may be due to a gunshot from the grassy knoll. The selected impulses fit better than randomly the echo patterns of the test shots, the trajectory of the microphone inferred from the BRSW analysis is reasonable for a microphone attached to a motorcycle, and some interpretations of photographic evidence are consistent with a motorcycle being in approximately the correct location. However these points are not strong since there are many ways in which static-like impulses can be nonrandom, unreasonable microphone trajectories were rejected, there were many motorcycles in the area and the impulse and echo selection procedures used by BRSW could affect the results.
For the reasons given above, in Section VI and in Appendixes A and F, no member of the Committee on Ballistic Acoustics was convinced last Spring by the arguments given that there was a grassy knoll shot. The members of the Committee reached their initial negative conclusion prior to the availability of the sound spectrograms and event timings discussed in Section IV and Appendixes B and C, so this negative judgment was in no way a result of the subsequent evidence that the portion of the tape containing the relevant acoustic impulses was recorded about one minute after the assassination. With the added evidence in Section IV, there is now a conclusive case against the impulses studied by BRSW/WA being associated with a shot that contributed to the assassination of President Kennedy.
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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.
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