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I I I . 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.
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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
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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 manuscripts on t'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
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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 x 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:
aces going to the dealer on the first deal does not by itself indicate a
95.6% probability chat 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.
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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
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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 judgement 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.
Representative terms from entire chapter:
dealey plaza
