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OCR for page 8
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please II. DESCRIPTION OF STUDIES BY BRSW AND WA 8 II. DESCRIPTION OF STUDIES BY BRSW AND WA The acoustic studies by BRSW and the further acoustic studies by WA were reported to the House Select Committee on Assassinations, and are printed1 in volume VIII (Appendix to Hearings) Investigation of the Assassination of President John F.Kennedy, March 1979. This section is a simplified presentation of the procedures described in those two reports, and as later discussed when the committee met with Barger, Weiss and Aschkenasy on January 31 and February 1, 1981. For details one should refer to the original reports 1. The BRSW team began by listening to tape copies of the recordings of both police radio channels for general orientation. Because the recorders were sound-activated, they could have stopped frequently for varying amounts of time, except that the Channel I recorder ran continuously for 5.5 minutes when a transmitter, presumably on a police motorcycle, became stuck in the transmit mode (the Channel I recorder appears to have run continuously during the period of interest). Using the frequent annotations of time by dispatchers on both channels, BRSW estimated the approximate time of the beginning and end of this 5 1/2 minute stuck-button transmission. Because of the severe noise and distortion on the recording, the BRSW team could neither confirm that this segment contained gunshot sounds, nor eliminate the possibility that they were present, by simple listening or by examining the waveforms of sounds on the tape. Therefore, they went to Dealey Plaza in Dallas August 27, 1978, and made recordings of test shots with various kinds of guns and ammunition, two shooter locations, and many microphone locations along the approximate route of the motorcade. For each combination of shooter location and microphone location, there is a characteristic and complex pattern of echoes in the recording of the test shot, because after the first sounds travel by a direct path to the microphone (usually the bullet shock wave and the muzzle blast sound), subsequent sounds arrive (due to echoes from buildings and other large objects) with varying delays, depending on the length of the path they have taken. A typical test shot waveform, made with a shooter use the print version of this publication as the authoritative version for attribution.

OCR for page 8
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please II. DESCRIPTION OF STUDIES BY BRSW AND WA 9 and 3 microphone locations close to those later claimed for an assassination shot from the knoll, is shown in Figure 1. While the Channel I recording is noisy (as can be seen in Figure 2), distorted and contains many confusing sounds which are not gunshots, the test shot recordings are clean, faithful records of the acoustical response of Dealey Plaza—except for certain noted changes that took place between 1963 and 1978. The BRSW team then compared, manually, each of 432 test shot waveforms with all the parts of the 5 1/2 minute record that could reasonably have included assassination gunshot sounds. This comparison was done using a binary correlation metric, with a ±6 msec window, applied to strip chart recordings of the relevant waveforms. This means that for each suspected assassination shot and each test shot, the strip chart recordings were aligned for a best match and a score was obtained by calculating the correlation coefficient. The correlation coefficient was defined as the number of impulses (large peaks) in the waveform of the suspected shot that came within 6 msec of an impulse in the test shot, divided by the geometric mean of the numbers of impulses available in the suspected shot and in the test shot. This binary correlation procedure does not make use of all the information available—thus impulses that barely resemble each other affect the score the same as impulses which match each other quite well— but it is a compromise that permits a relatively easy computation of similarity. The BRSW correlation was meant as a selective screen. Only candidate shots that gave a binary correlation greater than 0.6 were studied further. In all, fifteen pairs, involving only six sets of impulses on the Channel I recording, survived this screening. For each surviving pair, the location of the microphone that recorded the test shot should approximate the location of the motorcycle at the time the suspected assassination shot was fired. This time, in a relative sense, could be determined by the location of the suspected impulses along the length of the tape. A plot of microphone position versus suspected shot time was made. In such a plot, a sequence of actual assassination use the print version of this publication as the authoritative version for attribution.

OCR for page 8
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please II. DESCRIPTION OF STUDIES BY BRSW AND WA 10 shots should lie along a line that describes the movement of the motorcycle during the interval of the assassination shots. Unrelated impulses (false alarms) plotted in this manner could lie on this trajectory or elsewhere, depending to some degree on chance factors. Most of the pairs could be identified as false alarms in this way. The remaining pairs were judged, by the closeness of the trajectory fit, to contain at least two assassination shots and at least some false alarms. The hypothesized motorcycle trajectory was subsequently claimed to be consistent with that of the motorcycle driven by Officer H.B.McLain in the Kennedy motorcade based upon an interpretation of photographic evidence. Of the six sets of impulses that give high binary correlation coefficients with test shots, BRSW selected four as likely assassination shots by eliminating those whose echoes were inconsistent with a reasonable trajectory. One of these four, the third, was judged to have been fired from the grassy knoll, BRSW stated the probability that this set of impulses represents a shot from the grassy knoll is 50%. The impulses associated with this identification are shown in Figure 2, with the muzzle blast as identified by BRSW being at A on that figure. Judging that the principal limitation on their ability to make a more definitive claim was the microphone spacings for the test shots, which led to the ±6 msec window, BRSW suggested that WA be asked to try a theoretical acoustical modeling technique. This procedure was applied only to the hypothetical third (knoll) shot. Several test shots were examined, with a shooter location on the knoll, and prominent echoes were related to the Dealey Plaza objects that might have reflected or diffracted sound with the proper timing. After a set of echo- producing objects had been identified, a theoretical model of sound propagation in Dealey Plaza, incorporating possible variations in shooter position, microphone position and velocity, and air temperature, was used to predict the relative timings of various echoes that would be expected in the actual Channel I recording if the segment in question contained the sounds of a gunshot fired from the grassy knoll. In effect, for every choice of shooter position, microphone position, microphone velocity and air temperature, WA could theoretically determine the time of impulses of a hypothetical test shot, which they use the print version of this publication as the authoritative version for attribution.

OCR for page 8
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please II. DESCRIPTION OF STUDIES BY BRSW AND WA 11 could correlate, using the binary correlation measure previously described, with the Channel I segment identified by BRSW as a possible knoll shot— with the significant difference that matching pulses were required to be within 1 msec rather than within 6 msec. WA did not try all possibilities, but rather used the results of each trial to guide a search until they were able to find a shooter position, etc., for which most predicted impulses in the theoretically reconstructed test shot agreed with actual impulses on the Channel I tape. The best agreement was found for a part of Channel I which is 0.2 seconds earlier than that suggested by the BRSW investigation. On seeing the WA results, the BRSW investigators agreed that their earlier screening procedure had missed the WA identified segment and that the WA identification should be used. The waveform now identified by both BRSW and WA as the sound of a gunshot fired from the knoll is shown in Figure 2 with the WA identification of the muzzle blast being at B. The time scales given in Figure 2 are only approximate and are slightly different from those used by BRSW, by WA and by this Committee elsewhere in the report. use the print version of this publication as the authoritative version for attribution.