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II. DESCRIPTION OF STUDIES BY BRSW AND WA
The acoustic studies by BRSW and the further acoustic studies by WA
were reported to the House Select Committee on Assassinations, and are
printed1 in volume VIII (Appendix to Hearings) Investigation of the
Assassination of President John F. Kennedy, March 1979. This section is a
simplified presentation of the procedures described in those two reports,
and as later discussed when the committee met with Larger, Weiss and
Aschkenasy on January 31 and February 1, 1981. For details one should
refer to the original reports1.
The BRSW team began by listening to tape copies of the recordings of
both police radio channels for general orientation. Because the recorders
were sound-activated, they could have stopped frequently for varying
amounts of time, except that the Channel I recorder ran continuously for
5.5 minutes when a transmitter, presumably on a police motorcycle, became
stuck in the transmit mode (the Channel I recorder appears to have run
continuously during the period of interest). Using the frequent
annotations of time by dispatchers on both channels, BRSW estimated the
approximate time of the beginning and end of this 5 1/2 minute stuck-button
transmission. Because of the severe noise and distortion on the recording,
the BRSW team could neither confirm that this segment contained gunshot
sounds, nor eliminate the possibility that they were present, by simple
listening or by examining the waveforms of sounds on the tape. Therefore,
they went to Dealey Plaza in Dallas August 27, 1978, and made recordings of
test shots with various kinds of guns and ammunition, two shooter
locations, and many microphone locations along the approximate route of the
motorcade. For each combination of shooter location and microphone
location, there is a characteristic and complex pattern of echoes in the
recording of the test shot, because after the first sounds travel by a
direct path to the microphone (usually the bullet shock wave and the muzzle
blast sound), subsequent sounds arrive (due to echoes from buildings and
other large objects) with varying delays, depending on the length of the
path they have taken. A typical test shot waveform, made with a shooter
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and 3 microphone locations close to those later claimed for an
assassination shot from the knoll, is shown in Figure 1.
While the Channel I recording is noisy (as can be seen in Figure 2),
distorted and contains many confusing sounds which are not gunshots, the
test shot recordings are clean, faithful records of the acoustical response
of Dealey Plaza -- except for certain noted changes that took place between
1963 and 1978.
The BRSW team then compared, manually, each of 432 test shot waveforms
with all the parts of the 5 1/2 minute record that could reasonably have
included assassination gunshot sounds. This comparison was done using a
binary correlation metric, with a + 6 msec window, applied to strip
chart recordings of the relevant waveforms. This means that for each
suspected assassination shot and each test shot, the strip chart recordings
were aligned for a best match and a score was obtained by calculating the
correlation coefficient. The correlation coefficient was defined as the
number of impulses (large peaks) in the waveform of the suspected shot that
came within 6 msec of an impulse in the test shot, divided by the geometric
mean of the numbers of impulses available in the suspected shot and in the
test shot. This binary correlation procedure does not make use of all the
information available -- thus impulses that barely resemble each other
affect the score the same as impulses which match each other quite well --
but it is a compromise that permits a relatively easy computation of
similarity. The BRSW correlation was meant as a selective screen. Only
candidate shots that gave a binary correlation greater than 0.6 were
studied further. In all, fifteen pairs, involving only six sets of
impulses on the Channel I recording, survived this screening.
For each surviving pair, the location of the microphone that recorded
the test shot should approximate the location of the motorcycle at the time
the suspected assassination shot was fired. This time, in a relative
sense, could be determined by the location of the suspected impulses along
the length of the tape. A plot of microphone position versus suspected
shot time was made. In such a plot, a sequence of actual assassination
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shots should lie along a line that describes the movement of the motorcycle
during the interval of the assassination shots. Unrelated impulses (false
alarms) plotted in this manner could lie on this trajectory or elsewhere,
depending to some degree on chance factors. Most of the pairs could be
identified as false alarms in this way. The remaining pairs were judged,
by the closeness of the trajectory fit, to contain at least two
assassination shots and at least some false alarms. The hypothesized
motorcycle trajectory was subsequently claimed to be consistent with that
of the motorcycle driven by Officer H. B. McLain in the Kennedy motorcade
based upon an interpretation of photographic evidence.
Of the six sets of impulses that give high binary correlation
coefficients with test shots, BRSW selected four as likely assassination
shots by eliminating those whose echoes were inconsistent with a reasonable
trajectory. One of these four, the third, was judged to have been fired
from the grassy knoll, BRSW stated the probability that this set of
impulses represents a shot from the grassy knoll is 50%. The impulses
associated with this identification are shown in Figure 2, with the muzzle
blast as identified by BRSW being at A on that figure.
Judging that the principal limitation on their ability to make a more
definitive claim was the microphone spacings for the test shots, which led
to the + 6 msec window, BRSW suggested that WA be asked to try a
theoretical acoustical modeling technique. This procedure was applied only
to the hypothetical third (knoll) shot. Several test shots were examined,
with a shooter location on the knoll, and prominent echoes were related to
the Dealey Plaza objects that might have reflected or diffracted sound with
the proper timing. After a set of echo-producing objects had been
identified, a theoretical model of sound propagation in Dealey Plaza,
incorporating possible variations in shooter position, microphone position
and velocity, and air temperature, was used to predict the relative timings
of various echoes that would be expected in the actual Channel I recording
if the segment in question contained the sounds of a gunshot fired from the
grassy knoll. In effect, for every choice of shooter position, microphone
position, microphone velocity and air temperature, WA could theoretically
determine the time of impulses of a hypothetical test shot' which they
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could correlate, using the binary correlation measure previously described,
with the Channel I segment identified by BRSW as a possible knoll shot --
with the significant difference that matching pulses were required to be
within 1 msec rather than within 6 msec. WA did not try all possibilities,
but rather used the results of each trial to guide a search until they were
able to find a shooter position, etc., for which most predicted impulses in
the theoretically reconstructed test shot agreed with actual impulses on
the Channel I tape. The best agreement was found for a part of Channel I
which is 0.2 seconds earlier than that suggested by the BRSW
investigation. On seeing the WA results, the BRSW investigators agreed
that their earlier screening procedure had missed the WA identified segment
and that the WA identification should be used. The waveform now identified
by both BRSW and WA as the sound of a gunshot fired from the knoll is shown
in Figure 2 with the WA identification of the muzzle blast being at B. The
time scales given in Figure 2 are only approximate and are slightly
different from those used by BRSW, by WA and by this Committee elsewhere in
the report.
Representative terms from entire chapter:
dealey plaza
