The acoustic evidence in the
Kennedy assassination
Michael O'Dell
In 1978 the House Select Committee on Assassinations (HSCA) concluded that there was probably a conspiracy to kill President Kennedy(1). This conclusion was primarily based on acoustic evidence contained in Dallas Police Department radio recordings. An NRC panel later disputed the HSCA conclusions. In 2001 D. B. Thomas(2) published a paper that rehabilitated the original findings. This report demonstrates that the prior reports relied on incorrect timelines, and made unfounded assumptions that when corrected do not support the identification of gunshots on the recording.
History
In 1978 the House Select Committee on Assassinations (HSCA) concluded that there was probably a conspiracy involved in the assassination of President Kennedy(1), a conclusion primarily based on the acoustic evidence contained on the Dallas Police Department radio recordings.On the day of the assassination the Dallas Police Department (DPD) operated two radio channels. Channel I was for normal police radio traffic and channel II was assigned for the use of the presidential motorcade. Each channel was recorded by a different device. Channel I was recorded on a Dictabelt and channel II on a Gray Audograph. Both machines worked by engraving a track into a plastic medium. The Dictabelt used a rotating cylinder and the Audograph used a flat disk, similar to a phonograph record. Both machines were transmission actuated.An unknown motorcycle tuned to channel I had a defective microphone button that caused it to continuously transmit over a five minute period during which the assassination took place (3). If this motorcycle had been part of the motorcade it might have picked up sounds of the gunshots. If true, those sounds could be used to determine how many shots were fired, their timing, and using echo location methods, where the shots came from.Working for the HSCA a team (BRSW) from Bolt, Beranek and Newman Inc. (BBN), headed by James E. Barger, studied the recordings. BRSW performed a series of test shots in Dealey Plaza and used recordings of these shots to compare with signals on the DPD recordings. BRSW concluded that channel I contained impulses probably caused by the gunshots, with a 50% probability that one shot came from the grassy knoll in front of the President (4). Because of the level of uncertainty in this finding the HSCA asked another team try to raise the confidence level of the results. Weiss and Aschkenasy (WA) used an acoustic modeling method and concluded there was a 95% probability of a shot from the grassy knoll (5).Having reached the conclusion of "probable conspiracy", the HSCA asked the Justice Department to pursue the case. The Justice Department requested that the National Research Council (NRC), part of the National Academy of Sciences, review the acoustic evidence. In 1982 the NRC Committee on Ballistic Acoustics, headed by Norman Ramsey, released their report (6) and a version of that report was published in Science (7). The NRC report disputed the statistical significance of the HSCA findings, and following a suggestion from Steve Barber, proved that there is an instance of crosstalk on channel I at the time of the alleged shots (8). Crosstalk occurred when sounds from one channel were picked up by a microphone tuned to the other channel. This instance of crosstalk ("hold everything secure...", referred to as the Decker crosstalk) occurred during the alleged shooting sequence specified by BRSW and WA. Through spectrographic comparison the NRC confirmed that the faint voice that could be heard on channel I was a fragment of speech from channel II that happened about a minute after the assassination. Since this crosstalk happened at the same time as the supposed shot impulse patterns, the impulse patterns could not be of the assassination gunfire. The NRC report also contained a timing chart of events on both channels and included correction factors for tape speed and other effects (9).
Thomas
In March 2001 D. B. Thomas published an article in the peer-reviewed journal of the British Forensic Science Society (2). This article received considerable media attention and revived the debate over the acoustic evidence. Thomas concluded that the NRC made statistical errors of their own and the probability for a grassy knoll shot was actually greater than 96%. He also concluded that by using a different instance of crosstalk to align the channels the shots could be correctly placed in time at the period the assassination actually happened.
When the Thomas paper was published I saw the news reports and was intrigued by the promise that the question of conspiracy in the assassination might be settled scientifically. After reading the article I sought out other informed persons to discuss it with. One of the persons I was fortunate to meet had a multi-generational copy of the DPD recordings and a copy of the original report from the Committee on Ballistic Acoustics. Receiving copies of these from him, and with good audio software (Sonic Foundry's Sound Forge 6), I set out to examine Thomas' findings and the prior work. Later Dr. Ramsey provided me digital CD copies of the recordings from his files. Selections from those CDs are now available on a National Academy of Sciences web site (10).
The paper by Thomas contains several arguments intended to refute the findings of the NRC committee and which helped him conclude that the HSCA acoustic findings were valid. One of the most important of these arguments is an alternative crosstalk synchronization that allowed Thomas to place the shots at the time of the assassination.
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Conclusions
1. The timeline relied on by the NRC report and by Thomas is inaccurate.
2. Both the "hold everything" and the "you want me" crosstalk alignments demonstrate that the suspect impulses happen too late to be the assassination gunshots.
3. There is no evidence that the Audograph machine that recorded channel II ran continuously in the first few minutes after the shooting, and evidence indicates that it did stop. Because the Audograph stopped, later instances of crosstalk cannot be used to align the suspect impulses on channel I.
4. There is no statistical significance of 95% or higher for a shot from the grassy knoll. There is persuasive evidence that BRSW/WA simply found a match to the speech pattern that exists at the same location on the recording.
Acknowledgements
I am grateful to Herman Chernoff, Richard Garwin, Norman Ramsey and Paul Horowitz for their explanations, advice and assistance; Steve Barber for his help and his ear; Don Thomas for his encouragement and openness. Paul Hoch was very helpful with the manuscript. James Barger, Charles Rader, Mitch Todd and Anthony Marsh took time to offer ideas, information or answer questions. Thanks to John McAdams for giving this work a home on the internet.