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1
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 record-
ings 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 re-
sults 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 sub-
jectlve 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 approxi-
mately 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.
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2
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 sup-
pressing cross talk from Channel II. This identification of cross talk between
Channels ~ 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:
0 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 re-
corded about one minute after the President had been
shot and the motorcade had been instructed to go to
the hospital.
O Therefore, reliable acoustic data do not support a
conclusion that there was a second gunman.
Representative terms from entire chapter:
acoustic impulses
