National Academy of Sciences (NAS)

Report of the Committee on
Ballistic Acoustics






Washington, DC 1982



A private citizen, Steve Barber of Mansfield, Ohio, voluntarily wrote to the Committee that he was convinced from his own listening that there are clear instances in which phrases recorded on Channel II tape were distinctly audible on the Channel I tape as well. This is quite naturally explained by assuming that the motorcycle with the open microphone (Channel I) was near another police radio receiving a transmission from Channel II, so that transmissions over Channel II would issue from its loud speaker and be picked up by the open microphone and rebroadcast on Channel I. In addition there are simultaneous broadcasts by the dispatcher onto Channels I and II. Both kinds of cross talk are perfectly clear in many cases. The existence of such identical portions of speech on both channels would allow one to establish precise time synchronizations between specific portions of the two recordings. The specific time synchronizations would not apply to the recordings in their entirety, because Channel I ran continuously during the period of interest while Channel II was sound activated and operated intermittently. However, such matching features would enable one to determine the relative timing between many events on Channel I and other events on Channel II.

Barber identified several such matching sections on the two tapes. Four of them are quite clear, but they occur several minutes after the assassination and involve various police communications connected with the follow-up to the shooting; however, they demonstrate clearly that there was cross talk from Channel II to Channel I. As will be seen, they also provide a clear demonstration of Channel I heterodynes suppressing the recording onto Channel I of cross talk from Channel II, which suppression we later also show exists in the interval containing the impulses and shows that the cross talk was recorded through a radio receiver. Two events are especially important for fixing the time of the section of tape analyzed by BRSW and WA relative to the assassination. The first is a 4-second fragment of speech that overlaps the conjectured 3rd and 4th BRSW shots on Channel I. Barber there identifies a phrase, which he says begins with the words "hold everything . . ." as identical to the phrase ". . . hold everything secure until the homicide and other investigators can get there . . ." clearly recorded on Channel II. The significance of this proposed match is that the section on Channel I is concurrent with the last two of the conjectured BRSW shots, whereas on Channel II that communication is part of a clear sequence of emergency communications that followed the shooting and occurred approximately one minute after the assassination. It is, in fact, part of Sheriff Decker's instructions to his men in response to the assassination. This time synchronization, if correct, would prove that the BRSW/WA conjectured shots were unrelated to the sounds of the assassination gunshots. The section of Channel I recording with the BRSW/WA conjectured shots would then correspond to a period of time well after the assassination.

The second crucial event is the transmission "You want me . . . Stemmons", which occurs several minutes after the assassination and is clearly intelligible on both channels. It provides a common reference point for timing events on the two channels. We used it to determine whether the section of the recording containing the conjectured shots occurred before or after Chief Curry instructed the motorcade to "Go to the hospital."


IV-1. Sound Spectrograms

Initially, the poor quality of the "hold everything . . ." portion of the recording made it appear unlikely that a convincing interpretation of the badly garbled speech on Channel I could be made and the Committee was aware of the power of suggestion, or cueing effect, in which a listener to a garbled message will often be convinced he has heard what he has been coached to hear.

For these reasons, arrangements were made through Bruce Koenig and others of the FBI Technical Services Division for members of the Committee to utilize the Division's excellent sound analysis equipment to obtain sound spectrograms ("voiceprints") of the relevant communications on Channels I and II. The spectrograms were prepared under the supervision of Committee members. The sound spectrograms first reproduced were from tape recordings kindly provided by James C. Bowles, Radio Dispatcher Supervisor at the time of the assassination, but a sound spectrogram with a similar pattern for the ". . . hold everything . . ." phrase on Channel I was also made from a tape supplied by James Barger, essentially identical to that used in the analysis of BRSW; later sound spectrograms were also made from new high quality magnetic tape copies of the original Channel I Dictabelt and Channel II Audograph disc. A sound spectrogram is a plot with elapsed time along the horizontal axis, with frequency along the vertical axis and with the darkness of the trace representing the intensity at that frequency. Since the interpretation of sound spectrograms depends on continuous gradations in darkness, copies in a printed report lose clarity. For this reason photographs of the sound spectrograms will be retained in the National Research Council files.

We began by making sound spectrograms of two of the later proposed matching sections of speech. The match is clear, and establishes unambiguously that identical portions of speech can be identified on both channels. One of these matches is shown in Figure 3, and it demonstrates conclusively that there was cross talk between the two channels. We then made spectrograms of the crucial "hold everything" sections. As discussed in greater detail in Appendix B, Figure 4 is a photograph of composite sound spectrograms for the full four-second message. The beginning of the ". . . hold everything . . ." phrase is approximately at zero on these time scales and the impulses for the BRSW conjectured grassy knoll shot occur beginning approximately at the arrow marked 145.15s (the time of the conjectured grassy knoll shot on the BRSW time scale) and the WA impulses occur 0.2 seconds earlier. As discussed in Appendix B, the black dots mark 27 corresponding features on the two channels.

It is apparent from Figure 4 that there is a marked correlation between parts of the sound spectrograms of the two channels, even though the Channel I recording has much more noise. The correlation becomes much more impressive when the spectrograms of the two Channels are compared in detail. The correlation is particularly striking when one realizes that only the initial second of the ". . . hold everything . . ." phrase can be heard clearly on Channel I, yet the sound spectrograms contain numerous matching features for the entire three and a half second sequence; note for example the impressive match in the final segment from T = 3.2 to 3.6 seconds. In all cases of matching features it is clear from the text of the messages and from the signal intensities that a signal from Channel II was duplicated on Channel I and not the reverse.

The sound spectrograms present much more convincing evidence in the present case than in their application to speaker identification. There, words spoken at different times, supposedly by the same speaker, are compared and a trained interpreter is often required to explain why the subjective match is significant. In the present case, the need is to identify two identical messages extending over a three and a half-second interval. Not only must individual parts of the two sound spectra be alike but they must occur at exactly correct time intervals and with exactly matching frequencies. The existence of these required time and frequency correlations between the two channels imposes rigid constraints on the messages to be matched. Furthermore, all sounds that appear on both Channel I and II are useful in correlating the channels even though some are not spoken words. For example in listening to Channel II it is apparent that there is an intermittent tone that contributes to the flat portions common to Channels I and II. However, this tone varies in both amplitude and frequency and is also useful in correlating the two channels.


IV-2. Analysis of Sound Spectrograms of "Hold Everything . . ."

The Committee used three methods in addition to visual inspection to determine whether these critically important sound spectrograms of Channels I and II contained signals from the same source. The studies are described in greater detail in Appendix B and were made on the sound spectrograms shown in Figures 4 and B-3.

In the first method we identified characteristic features that were present on both spectrograms of Figure B-3 and then determined the relation between the times of occurrence of the two sets of features. Twenty-seven features were selected (indicated by the black dots in Figure B-3); a brief description of each is given in Table B-1 in Appendix B along with its frequency and time coordinates. The existence of correlations between the two spectrograms over a long time interval can be demonstrated by plotting T', the time coordinate of the Channel I spectrogram, as a function of T", the time coordinate of the corresponding characteristic on Channel II. The results are shown in Figure 5. The marked linearity of the plot shows that the similar characteristics of the sound spectrograms of the two channels follow the same time sequence, as they must for one to be cross talk from the other. As described in Appendix B-1, a linear fit to the recorded points gives the equation in Figure 5, and the slope of the line or ratio of recording speeds is 1.059±0.002, which corresponds to a (5.9±0.2)% net difference in the recording speed. The ratio of recording speeds independently inferred from the measured frequency ratios of the same points is 1.064±0.006, a value fully compatible with that obtained from the time sequence. As discussed in Appendix B-1, the probability of obtaining such close agreement by random occurrence of the features at their observed average spacing would be about 2.1 × 10-13, and the probability of randomly obtaining such good agreement on the frequency ratio of the points is about 2 × 10-10.

The second method, which provides further confirmation of the correctness of the identification of the two patterns, is the calculation of the effective relative speeds from the frequency ratios for five sections with particularly well defined frequencies on the two channels, as discussed in Appendix B-1. Such a calculation gives a ratio of recorder speeds of 1.062±0.005 in excellent agreement with the value in the preceding paragraph. Alternative analyses to minimize subjective errors in the pattern recognition are also discussed in Appendixes B-2 and B-3.

To help in the visual recognition of similarities of the two patterns, sound spectrograms have been made with the speed of Channel I effectively changed by 6.7%. The results are given in Figure 4. Both frequencies and times of the two channels now appear to be quite compatible.

A third approach to the investigation of whether Channel II segments are recorded onto Channel I along with the acoustic impulses was taken by a third member of the Panel and two collaborators. The Channel I and Channel II recordings were digitized and the short-term acoustic spectra were taken and stored in a digital computer. The printouts of these spectra are similar to Figures 3 and 4 and are shown in Figures B-4, B-5, and B-6. These digital spectrograms were computed directly from magnetic tapes and did not involve the use of the FBI sound spectrogram equipment. Many of the features observable in the analog spectrograms of Figure B-3 can be seen in B-6, but no use was actually made of the spectrogram patterns. Instead, the actual data were used to test certain hypotheses, without human intervention. An objective measure of similarity of two spectral matches is obtained from the cross correlation coefficient, defined in Appendix B-4. This cross correlation coefficient would be reduced if one of the recordings were played at the wrong speed, or if the recording at one time were compared with the same or a different recording at a different time.

The first cross correlation coefficients were made from the same Channel I and II recorded copies that were used in preparing Figures 3, 4, B-1, and B-2. It was found that the biggest peak for the cross correlation coefficient occurred for a relative warp (or speed ratio) of 1.06, in agreement with the other two manual approaches for comparing Channels I and II; a 1% deviation of warp from optimum diminished the peak substantially. Unfortunately, that Channel II copy contains many repeats caused by the Gray Audograph machine in playback. Accordingly another tape copy was prepared by members of the Committee directly from the original Audograph plastic disk itself and by the use of a standard turntable and tone arm, thus producing a tape without compensation for the fact that the disk was originally recorded at constant linear track speed. It was this tape that was used in preparing the sound spectrograms shown in Figures B-4, B-5, and B-6. Figure 6 gives the cross correlation coefficient for the "hold everything . . ." segments when the relative speed was selected to give the largest peak and the 750 correlation coefficients were obtained by sliding 2.50 secs of Channel I along 10.00 secs of Channel II, 0.01 secs at a time, using frequencies in the band 600 Hz to 3500 Hz. For comparison the cross correlation coefficients of the unambiguous segment "You want . . . Stemmons" are plotted in Figure 7. The shape of the peak is very similar to that for the "hold everything . . ." segment. The background is somewhat smoother, simply because there is less noise in Channel I at this time. Channel I, however, in neither case gives a perfect reproduction of Channel II. It has lost some of the high and low frequencies, and as one would expect there are tones present on Channel I that are not on Channel II.

The marked narrow peaks of the cross correlation curves clearly show by an objective test that the "hold everything . . ." segment of Channel II is present on Channel I at the same location as the acoustic impulses. There is no doubt that the voice (and other) sounds of Channel II are present on Channel I to an accuracy in location corresponding to a few milliseconds.

We find these three sets of results to be overwhelming evidence that the "hold everything" sections of the two recordings are traceable back to a single acoustic signal from Channel II. If there is no overrecording on Channel I (as we later show to be the case), the correspondence between these two recordings of "hold everything . . ." would be conclusive evidence that the events analyzed by 8RSW/WA were not the assassination shots, since we know from Channel II that the "hold everything" transmission was made at least 50 seconds after the Chief instructed the motorcade to "Go to the hospital." We will discuss in Section IV-4 the possibility of there having been an overrecording on Channel I and our conclusion that there was not. Indeed, the digital analyses in themselves are used in Section IV-4 and Appendix D to demonstrate that the Channel II cross talk on the Channel I recording was already present at the Channel I radio receiver and was not added later in copying or as an overrecording.


IV-3. Timing of Channel I and Channel II Events

In the previous section, a synchronization between events on Channels I and II simultaneous with the conjectured shots was obtained by detailed analysis of sound spectrograms. Other examples of matching features on the two channels, occurring several minutes after the assassination, are so much clearer that no special technical procedures are required to establish synchronizations in these parts of the recordings -- simple listening is sufficient to eliminate all doubt about these synchronizations. By timing both recordings backwards from the time of these matches, it is possible to relate the times of events in the critical portions of both recordings, independent of the correspondence established in the previous section.

The clear match that occurs closest to the assassination is "You want . . . Stemmons," which occurs on Channel II several minutes after the Chief said, "go to the hospital." Figure 3 shows a sound spectrogram of the match. Since Channel II was sound activated and recorded intermittently, we obtain a lower bound on the time between these two transmissions by timing the tape between them. Any halts in the recorder would cause the tape time to be less than the actual clock time between these transmissions.

Time intervals were measured using two different sets of tape recordings. First, we used the tapes obtained from Bowles to time events in critical portions of the recordings. Since relative time between Channels I and II is all that is of significance in this comparison of events, time in this set of measurements was made in somewhat arbitrary Channel I elapsed time units. The timing was difficult to do because there were "repeats" (see Appendix C) on the Channel II magnetic tape and speed differences between segments of it and the Channel I tape. Appendix C describes how these timings were made and how compensations for repeats and speed differences were accomplished. The results of the spectrogram analyses just discussed were used to obtain the speed correction (a factor of about 1.06). The durations of repeats were determined from strip charts of the signal level as a function of time.

The result of these timings, also given in Appendix C is that:

a) On Channel II, "Go to the hospital" occurs at least 189 seconds before "You want me . . . Stemmons.

b) On Channel I the portion of the tape on which BRSW/WA found "shots" occurs 171 seconds before "You want me . . . Stemmons."

c) Since Channel II operated intermittently, any time that elapsed while the recorder was stopped would increase the 189-second interval between "Go to hospital" and "You want me . . . Stemmons." There were five places where the recorder could have stopped.

By this analysis, the last of the conjectured shots occurred at least 20.9 seconds after Chief Curry issued his instructions "Go to the hospital"; therefore, they could not have been the shots of the assassination.

After the preceding analysis of the tapes obtained from Bowles was completed, the Committee gained access to the original Gray Audograph and Dictaphone recordings. These were 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. These recordings, of course, did not eliminate the effects of the intermittent operation of Channel II, and time interval measurements are still lower bounds. The 60 Hz hum from the original recordings was used to determine the relationship between playback speed and original recording speed and to convert the measured-elapsed time intervals to real elapsed time units. (Recall that arbitrary Channel I elapsed-time units were used for the first set of measurements made on the Bowles tapes.) It was easy to make this correction on Channel 11, but difficult on Channel I, because the Dictabelt was in poor condition. The conversion method is described in Appendix C. Except for this speed-time correction, obtaining comparable measurements of the time intervals between critical events on Channels I and II and the common "You want . . . Stemmons" transmission was straightforward.

The result of these timings made on tapes obtained directly from the original recordings, also given in Appendix C, is that:

a) On Channel II, "Go to the hospital" occurs at least 206 seconds (real time) before "You want me . . . Stemmons."

b) On Channel I, the portion of the Dictabelt on which BRSW/WA found "shots" occurs 178 seconds (real time) before "You want me . . . Stemmons."

By this analysis, the last of the conjectured shots occurred at least 30.9 seconds (real time) after the instructions "Go to the hospital." This measurement is believed to be more accurate than the one obtained from the Bowles tapes, since the tapes obtained from the original recordings showed no evidence of skips, repeats, or breaks.

Both of these results confirm the previous finding from the sound spectrograms that the section of tape in which BRSW/WA found "shots" recorded events that occurred after the assassination. Note that the results from timing events do not require a match between the two recordings of "hold everything," but they do not preclude such a match. Halts in the recorder would increase the time between the conjectured shots and Chief Curry's instructions. Furthermore, any delay between the assassination and the instructions, "Go to the hospital," would increase the discrepancy between the timing of the conjectured shots and the actual assassination.


IV-4. Possibility of Superposed Recordings

The Committee has considered seriously the possibility that the impulses analyzed by BRSW/WA might have been overlaid at a later time by the "hold everything . . ." message. Conceivably such an overrecording could have occurred by an accidental knocking backwards of the Dictabelt or the recording head by about one minute in the first minute following the assassination or by the substitution (either accidentally or deliberately) of a new Dictabelt copy for the original, with the copy being made by audio coupling while a Channel II recording was playing in the background. The Committee found conclusive evidence that this was not the case. The evidence is of four kinds: (1) physical examination of the Dictabelt for indications of overrecording or of substitution of a copy for the original; (2) the unlikely nature of any of the highly contrived scenarios required to provide such an undetectable overrecording either accidentally or deliberately, (3) the compatibility of the timing implied by the "hold everything . . ." identification with other firmly established evidence, and (4) the conclusive acoustic evidence on the Dictabelt itself that the cross talk recordings were made through a radio receiver with automatic gain control. These different forms of evidence are discussed in Appendix D, where all are shown to be compatible with the recordings being made at the same time and some are incompatible with the hypothesis of later superposed recordings by audio or direct electrical coupling. Only the evidence of category (4) will be reviewed in this section.

The digital analyses of the sound spectra can be used to demonstrate that the Channel II imprint on the Channel I recording was already present at the Channel I receiver and was not added later in the recorder or as an overrecording. The by radio nature of Channel II cross talk is demonstrated by its detailed behavior in the presence of Channel I heterodynes when another Channel I transmitter is keyed on with a more powerful carrier signal. The frequency offset between the two carriers gives rise to a heterodyne tone in the Channel I recording. However, the Channel I receiver was fitted with automatic gain control (AGC) to hold the output level approximately constant; as a result, the cross talk signals decreases in intensity in a few tens of milliseconds (as does any residual transmission from the original stuck-mike transmitter). At the end of the Channel I heterodyne, the AGC gradually increases the receiver gain, and signals on the stuck-mike transmission increase in intensity in the recording. An excellent probing signal for the Channel I gain would be a Channel II steady tone acoustically coupled from the field loudspeaker to the stuck-mike transmitter. This would come in at constant level, and the variation in level on the Channel I recorder should mimic the AGC action If the Channel II signals were present in this way. Inspection of the digital spectrogram of Figure B-4 (and digital tabulations of the data) show that numerous Channel II brief tones have constant level from beginning to end. However, a crucial demonstration is provided by the Channel I heterodyne beginning in Figure B-6 at time 32.02 seconds. The underlying Channel II brief tone is clearly substantially reduced in intensity at the beginning of the Channel I heterodyne, and gradually grows back when the Channel II brief tone results after the Channel I heterodyne ceases. More detail is available in the two digital plots of Figures B-7 and B-8. This behavior is validated by similar Channel II brief tones underlying Channel I heterodyne signals in the "You want me . . . Stemmons" phrase and in a phrase, "I'll check . . ." likewise present on both channel. This is discussed in further detail in Appendix D along with other evidence that has led the Committee to conclude that the acoustic impulses attributed to gunshots were recorded about one minute after the President was shot.



Although the Committee agrees with the "Findings" in the November 19, 1980, FBI report, (4) it disagrees with one of the arguments used to justify the Findings. It considers invalid the criticism of the WA report on the basis of the high value of the binary correlation coefficient found by the FBI for a match between the supposed grassy knoll shot and one of the recorded gunshots in the unrelated later shootings at Greensboro, North Carolina. Although the FBI obtained a high value of the correlation coefficient, that value was not nearly so significant as the one obtained by WA, which involved many more "time windows" (90 windows were used although this number was erroneously reported as 45 on page 76 of the BRSW report) and 39 Greensboro shots were available from which the most favorable could be selected. Although the Committee considers this particular FBI argument against the BRSW/WA report to be invalid, the Committee, for the reasons discussed in this report, agrees with the FBI conclusions.



Our charter asks us to recommend the tests, analyses and evaluations needed to obtain better information from the recordings. If there were to be further studies of the Dallas police Department Channel I recording in the hope of demonstrating the validity of the conjectured shot from the grassy knoll, the information listed in Appendix F should be sought. However, as discussed in Sections II and III, the evidence against the BRSW/WA conjectured grassy knoll assassination shot is already so strong that the Committee believes that the results to be expected from such studies would not justify their cost.



For the reasons discussed above and in the appendixes, the Committee on Ballistic Acoustics has reached the following unanimous conclusions:

(a) 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 a 95% probability of such a shot.

(b) The acoustic impulses attributed to gunshots were recorded about one minute after the President had been shot and the motorcade had been instructed to go to the hospital.

(c) Therefore, reliable acoustic data do not support a conclusion that there was a second gunman.


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1. Appendix to Hearings Before the Select Committee on Assassinations of the House of Representatives Ninety-Fifth Congress, Volume VIII, US Government Printing Office, Washington, DC, 1979.

2. James C. Bowles, The Kennedy Assassination Tapes: A Rebuttal to the Acoustical Evidence Theory (copyrighted and unpublished).

3. 3. Hearings Before the President's Commission on the Assassination of President Kennedy, US Government Printing Office, Washington, DC, 1964.

4. Report released December 1, 1980, by the Federal Bureau of Investigation and prepared by the FBI Technical Services Division, Washington, DC, and dated November 19, 1980.

5. Minitab Manual, by Thomas A. Ryan, Jr., Brian J. Jainer, and Barbara F. Ryan, published by Minitab Project, Statistics Department, 215 Pond Laboratory, Pennsylvania State University, University Park, PA 16802.


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