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.
Thomas argued that if a later instance of crosstalk, ("you want me...") were used to synchronize the two channels then the shots did not appear too late. The NRC explained this apparent difference by pointing out that channel II was sound activated and could easily have stopped between the two crosstalk instances. Thomas did not accept this explanation, believing that a regression chart from the BRSW report proved that channel II did not stop during the interval. If the channel II recording had been continuous then one of the crosstalk instances had to have been displaced on the recording by a jumping recording stylus, or some other cause. Because such a displacement appeared to be the only explanation reconciling the timing data, the synchronization of the Decker crosstalk with the "shots" could no longer rule out the possibility that they were the real shots.
Thomas relied on the timing data published in the NRC report (9). Early on, I attempted to reproduce the timing of channel II that appears in that report. The effort was unsuccessful for various reasons. Several different copies, with different histories, were compared and each was found to run at a slightly different speed. None of the copies I had matched the times printed in the NRC report, and no adjustment for tape speed alone corrected the problem. Two of the recordings used by the NRC panel are known as the Bowles version and the FBI version. James Bowles, the Dallas Police Department Communications Officer at the time of the assassination, made tape recordings of both channels from the original media in 1964. In 1981 the FBI made a new tape recording from the original channel II media using a phonograph player as the playback device.
Timing
One of the timing problems with channel II involves the mechanics of the Audograph machine. By comparing the Bowles version of channel II against the FBI version I found a number of forward skips on the Bowles version that weren't accounted for in the NRC report. Herman Chernoff (NRC panel) suggested a mechanical cause (11), and I was then able to acquire some photographs of a machine (12).
The NRC panel made a reasonable, but wrong, assumption about how the Audograph machine worked (13). The tape recording made from the Audograph by Bowles contained numerous repeated sections. On playback the needle would sometimes get stuck in a groove and repeat that groove, sometimes twice. Table C-1 of the NRC report subtracted the time that these repeats took from the elapsed time. This seems to make sense, but due to the mechanical construction of the Audograph machine it was the wrong thing to do.
The needle assembly on the Audograph doesn't move at all. The disc is mounted on a spindle and moves horizontally under the needle as recording progresses. The horizontal motion of the spindle is a result of being mechanically geared to a worm gear, such that, as the spindle rotates, the center of the disc moves away from the needle assembly. There is no freedom of motion in this mechanical system, except for the tip of the needle. The repeats happen because the needle tip has some flexibility and may get stuck for an additional rotation or two, but the horizontal movement of the disc never stops and the needle must eventually catch up to where it should be. When the needle does catch up it will skip over sections of the recording. In the long run any repeats will be roughly matched by forward skips where the needle jumps over grooves. (See Audograph photos.)
It was a simple matter to notice the repeats but forward skips were not noticed. It would have been more accurate if the repeats were not subtracted at all. When the NRC subtracted the time for repeats they reported the elapsed time as shorter than what it really was. There are certain other uncertainties in Table C-1, involving the speed correction factor, the "gap", and the choice of event markers. All of these factors combined made reproducing the prior time studies very difficult. Because of these uncertainties, and with the problem of the overlooked skips, I looked for a reliable way to accurately time events on channel II.
Another copy of channel II was made by the FBI on a high-quality phonograph, instead of using the Audograph machine. The tape of channel II made by the FBI for the NRC panel appears to be a complete recording without skips or repeats in the period in question. The NRC report states, "[the original Gray Audograph was] transcribed, as described in Appendix C, onto tape, with care taken to minimize the 60 Hz hum that was added to the signal and to ensure that no skips or repeats were introduced in the tape recording of either channel. No break interrupted the Channel II recordings as was the case for the Bowles tapes." (15)
Careful examination of the recording during the six minutes after the shooting confirms that there are no obvious repeats in it. A side by side comparison revealed the forward skips in the Bowles version that the NRC didn't account for, and also would have revealed skips in the FBI version had they been there, and had such skips not been precisely mirrored on the Bowles tape.
The FBI tape is a perfect candidate for performing timing studies, except for one problem. As the NRC report says, "The Gray Audograph disk (Channel II) could not be played on an original Gray playback unit without introducing skips and repeats. It was possible to play it successfully without either of these artifacts being introduced by using a phonograph turntable and phonograph arm, cartridge, and stylus. However, phonograph turntables operate at a constant rpm, whereas the Gray equipment's rpm reduced as recording progressed. Moreover, the Gray Audograph records from the inside out, whereas normal records begin at the outside. Thus, when the tapes are played back, there is a speed distortion that causes material at the beginning of the tape (the inside of the record) to be slowed down (time intervals between events are longer and the frequencies are lower than those originally recorded) and material at the end of the tape (end of the record) to be speeded up relative to true speed." (14)
Timing studies on this tape first require a method of correcting for the speed distortion. One such method is described in the NRC report in Appendix C (16). They used the 60Hz hum introduced into the original recording by the Audograph and measured it at multiple points along the tape. When these points were plotted and a linear least-squares fit line drawn the amount of speed distortion at any point on the recording could be determined. The NRC only used this method to time a few events on channel II.
Because a sonogram does not provide a highly precise measurement I chose a direct method of measuring wavelength. The audio software allows time markers to be placed on the waveform. The marker resolution is equal to the sample rate, 44,100 samples per second. A band-pass filter between 40Hz and 80Hz was applied and the hum measured where it was prominent. Cycles were measured where the waveform intersects the central zero line. Time markers were placed at the proper points to measure multiple cycles. The total duration of the measured cycles divided by the number of cycles is the wavelength at the midpoint of the markers.
From a series of such measurements along the length of the recording, I plotted the power hum on a graph, and drew a least squares fit line (Fig. 1). The measured frequency is plotted as a ratio to the correct 60 Hz. This results in a correction factor for a given point equal to the amount the apparent playback speed is faster or slower than real time.
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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.
Appendix
Figures A-1 thru A-3. Photographs of an Audograph machine.
Table A-1. Background Power Hum Measurements of DPD Channel II
Table A-2. Events on DPD Channel II
Table A-3. Events on DPD Channel I
References and Notes
1. US Congress, House of Representatives, House Report Wo. 95-1828, Select Committee on Assassinations. US Government Printing Office, Washington DC. 1979 (hereinafter referenced as HSCA). Final Report, pp. 76, 94. [http://www.history-matters.com/archive/jfk/hsca/report/contents.htm]
2. Thomas DB. Echo Correlation Analysis and the Acoustic Evidence in the Kennedy Assassination Revisited. Science & Justice 2001; 41: 21-32. [http://www.forensic-science-society.org.uk/Thomas.pdf]
3. Barger, JE, Robinson, SP, Schmidt EC & Wolf, JJ. Analysis of Recorded Sounds Relating to the Assassination of President John F. Kennedy. Bolt, Baranek & Newman, Inc., 1979, (hereinafter referenced as BRSW Report). HSCA Proceedings Vol. 8, p. 70. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0019a.htm]
4. BRSW Report. HSCA Proceedings Vol. 8, p. 107.
5. Weiss, MR & Aschkenasy, A. An Analysis of Recorded Sounds Relating to the Assassination of President John F. Kennedy. Dept. Computer Sciences, Queens College, City University New York, 1979 (hereinafter referenced as WA Report). HSCA Proceedings Vol. 8, p. 32. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/contents.htm]
6. National Research Council, Report of the Committee on Ballistic Acoustics. Prepared for Department of Justice, Washington DC. Report No. PB83-218461, 1982 (hereinafter referenced as NRC Report). [http://www.nap.edu/books/NI000372/html/]
7. Committee on Ballistic Acoustics, National Research Council. Reexamination of Acoustic Evidence in the Kennedy Assassination. Science, Oct. 8, 1982.
8. NRC Report. p. 25. [http://books.nap.edu/books/NI000372/html/25.html]
9. NRC Report. Table C-1. p. 62. [http://books.nap.edu/books/NI000372/html/62.html#pagetop]
10. The National Academies Press, Audio files of JFK Assassination Recordings, http://stills.nap.edu/html/JFK_audio
11. Herman Chernoff, E-mail correspondence. 10/4/2001.
12. Gray Audograph, Gray Manufacturing Company, Hartford, CN. Model BIC-4 Master. Serial number M160134.
13. NRC Report. p. 61. [http://books.nap.edu/books/NI000372/html/61.html]
14. NRC Report. p. 67. [http://books.nap.edu/books/NI000372/html/67.html]
15. NRC Report. p. 28. [http://books.nap.edu/books/NI000372/html/28.html]
16. NRC Report. p. 68. [http://books.nap.edu/books/NI000372/html/68.html]
17. The National Academies Press, Audio files of JFK Assassination Recordings, http://stills.nap.edu/html/JFK_audio/tr5_128.mp3
18. NRC Report. p. 34. [http://books.nap.edu/books/NI000372/html/34.html]
19. NRC Report. p. 23. [http://books.nap.edu/books/NI000372/html/23.html]
20. BRSW Report. HSCA Proceedings Vol. 8, p. 101.
21. Thomas DB. Hear No Evil. The Acoustical Evidence in the Kennedy Assassination. http://pages.prodigy.net/whiskey99/hearnoevil.htm
22. Thomas article. p. 29.
23. BRSW Report. HSCA Proceedings Vol. 8, p. 72. Fig. 8.
24. BRSW Report. HSCA Proceedings Vol. 8, p. 70.
25. Herman Chernoff, E-mail correspondence. Mar 7, 2002.
26. Bowles, JC. The Kennedy Assassination Tapes, A Rebuttal to the Acoustical Evidence Theory, 1979. [http://www.jfk-online.com/bowles1.html#ref]
27. WA Report. HSCA Proceedings Vol. 8, p. 32. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0018b.htm]
28. Thomas article. p. 26.
29. BRSW Report. HSCA Proceedings Vol. 8, p. 66. Fig. 5. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0035b.htm]
30. BRSW Report. HSCA Proceedings Vol. 8, p. 65. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0035a.htm]
31. BRSW Report. HSCA Proceedings Vol. 8, p. 101. Table II. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0053a.htm]
32. BRSW Report. HSCA Proceedings Vol. 8, p. 50. [http://www.history-matters.com/archive/jfk/hsca/reportvols/vol8/html/HSCA_Vol8_0027b.htm]
Diego Verdegiglio