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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
1 979 , BRSW concluded ~ 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] was that lathe 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 ~ ~cha~c "`che probability ~cha~c 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...."
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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 benefitted 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~sugggsted by Steve-Barber, a private citizen of
~ ~—,~—, By ~ ~ _~ ~~_~ ~— ~, ~ ~ ~ ~ · ~ ~ i ~ ~ ~ ~ ~ ~ 3 ~ , ·;
Mansfield, Rio, 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
· , -
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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
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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.
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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' manuscripts 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 beleves 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.
Representative terms from entire chapter:
grassy knoll
