Kennedy Assassination

Police Radio Recordings

Dealey Plaza in Dallas

November 22,1963

grassy knoll

Oswald

How many shots were fired?

According to the Warren Commission, Lee Harvey Oswald fired three shots. The first shot hit both Kennedy and Governor Connolly and wounded them. The second shot missed. Thethird shot killed the president.

The time between the first and third shots is known from the Zapruder film. There was time for Oswald to fire three times, but not more.

A fourth shot would mean that there was a second gunman.

Two Police Radio Channels• Channel M was devoted to the motorcade, recorded on a

waxed disk (audiograph)• Channel R was devoted to routine police business,

recorded on a waxed belt (dictabelt)

On both recorders, recording stopped when the carrier signal went away, so the recordings are not continuous.

But, somewhere in Dallas a police motorcycle’s radio transmitter button was stuck in the transmit position. So there was continuous recording on channel R for the entire time relevant to the assassination.

stylus

Dictabelt Geometry (channel R)

stylus

Gray Audiograph (channel M)

How could assassination gunshots get onto channel R?

Maybe the motorcycle with the stuck transmit button was one of the motorcycles in the motorcade.

It would also have to have had its transmitter/receiver switched to the wrong channel.

Two unlikely coincidences – but why not investigate?

Naïve approachListen to channel R. Do you hear gunshots?

Count the gunshots and note when they occurredrelative to one another.

Sadly, the recording quality is so bad that one can’t recognize gunshots!

The waveform on Channel R

Work by Dr. James Barger and his team, (Bolt Beranek and Newman)

• 1978 House Select Committee on Assassinations asked BBN to examine Channel R.

• Gunshots have distinctive echo patterns, like fingerprints.

direct path

reflection

refraction

bulletmuzzle blast reflection

supersonic shock wave

refraction

supersonic shock wave

BBN Measurement Approach

test shot locationsmicrophone locations(36 used)

Microphone locations

Some BB&N Recorded Impulse Patterns(templates)

Sliding Templates

Barger’s team slid each template past a long strip recording from channel R.When enough pulses on a template matched up with peaks in the recording,a match was suspected.

Peaks were considered to match pulses if they were within ±6 ms.

correlation coefficient =

N = number of matchesP = number of pulses in the templateQ = number of peaks in the waveform

A match was retained when its correlation coefficient was more than 0.5.

Fifteen matches were found with correlation coefficients > 0.6.

N PQ

Were there 15 shots?

No. Some matches were from the same part of the channel R recording matching templates from nearby microphones. Barger’s team found four timeswhen the record matched a template.

We’ll refer to these as four putative gunshots.

But are they gunshots? What else can the data tell us?

For each match, we have a location of the microphone,a location of the shooter, and a time from the beginning of the recording.

Trust a theory when it makesa prediction of an unlikely event.

Barger’s screening procedure identified the only four times along the record whichcould have been gunshots, but it also, independently told where the motorcycle would have to have been. For that, there was independent photographic evidence.Barger’s team thought it was unlikely that there was a motorcycle where one neededto be.

But the staff of the HSCA told them that there was a motorcycle right there,and Barger’s team became believers.

(The claim that the motorcycle, driven by Officer B. J. McLain, was photographedthere is actually quite suspect.)

Confidence?

Barger’s calculations of probability suggested that he was95% sure that there were shots detected. But there’sless confidence in each individual shot. For the crucial third shot his estimate of the confidence was 50%.

That third putative shot was a match to a template recorded from a test shot fired from the grassy knoll!

50%?

For a politician, 50% is the worst possible result - How, they askedcould you give us a more definitive result?

Barger suggested that instead of deriving the templates from microphones, their impulse patterns could be calculated, using an accurate survey map of Dealey Plaza. The tolerance for matching each peak to each pulse could be reduced from 6ms to 1 ms.

He recommended contracting that study to Mark Weiss and Ernest Aschkenasy, of Queens College.

Weiss & Aschkenasy’s workParameters:•shooter position x,y,z•microphone position x,y•microphone velocity Δx,Δy•speed of sound•speed of the dictabelt

Weiss and Ashkenasy identified a combination of these parameters that gave them a template with 13 impulses, and 11 of these were within ±1 ms of some of the 15 peaks in part of the channel R record. They claimed that they had a 95% confidence in their identification.

W&A’s good match was to a different part of the record,

one not identified by BB&N’s team

W&A BB&N

Committee on Ballistic Acoustics

• Norman F. Ramsey. Harvard University, chairman; • Luis W. Alvarez • Herman Chernoff• Robert H. Dicke• Jerome I. Elkind• John C. Feggeler• Richard L. Garwin• Paul Horowitz • Alfred Johnson • Robert A. Phinney• Charles Rader • F. Williams Sarles

The Barber HypothesisWe began by trying to find flaws in the work of the previous teams. There were such flaws. They had ignored the amplitudes of peaks in the record. There was inconsistent use of prior statistics in the probability calculations, Weiss and Aschkenasy’s distance measurements were susceptible to bias.

But this quickly became irrelevant. Steve Barber, a rock musician and assassination buff, brought to the committee’s attention a speech sound on channel M which could also be heard on channel R at the exact same time as the putative grassy knoll shot!

Dallas Sheriff Decker is saying “Hold everything secure until the homicide and other investigators can get there.”

Hold Everything Secure

The phrase is unmistakable on channel M and it self-evidently comes after the assassination .

The phrase on channel R barely sounds like speech at all. But once you are told that it might be “Hold everything secure’’, you can hear it.It is not only coincident with the putative gunshot-- it is the sound identified as the gunshot!

But is it there, or is it psycho-suggestion? We needed to be quantatitive.

Spectrograms compared with points of similarity

Recorders ran at slightly different speeds. This changes both the time and frequency of each feature – consistently.

Cross correlation between "hold everything" segments of channel M and channel R sound spectrograms with time scale slightly adjusted to produce the best correlation peak. The curve is produced by sliding 2.50 seconds of channel R along 10.00 seconds of channel M, 0.01 second at a time, using frequencies in the band 600 to 3500 hertz.

Stemmons Phrase

Cross correlation of the two spectrograms for the “Stemmons” phrase using the samespeed correction as the “Hold everything secure” phrase.

How did the speech from Channel M get onto Channel R?

The Stemmons phrase (and some others) prove that speechoriginally transmitted on channel M can get onto channel R. But how?

Hypothesis. At the Dallas Trade Mart there was a police staging area. A loudspeaker broadcasting the motorcade radio channel could have been near the motorcycle with the stuck transmit button.

Evidence in favor of this hypothesis comes fromsiren sounds recorded on the dictabelt. These are not right after the putative shots, but several minutes later.And they have doppler shifts, first high, then low, whichindicate that the sirens were passing the stuck-button motorcycle’s location.

From Dealey Plaza to Dallas Trade Mart, along Stemmons Freeway

The Press Conference We Didn’t Have

We wrote a draft report. The committee’s government facilitator wanted to release it at a press conference.

I’m very proud to say that I objected to this. I said that we should first show our work to Barger and to Weiss and get their comments. The committee agreed with me.

Weiss made no objections but neither did he concur. Barger agreed with our work and made several useful suggestions about closing up possible loopholes, which we did.

Then we released the report and had the press conference.

Kennedy Assassination Evidence Faked

One of the loose ends was the possibility that the recording of channel M could have been nefariously altered to include the sounds that eventually found their way onto channel R and were used to discredit the gunshot theory. Was the wax disk of channel M an original or a copy? I thought of a way to tell.

The audiograph disc is like a phonograph record and each track, as it is recorded, can distort the adjacent track. On playback, a loud soundcan be heard one full revolution after it was actually recorded. But the audiograph is recorded at constant speed, not constant radial velocity.

So the time between the loud sound and its faint bleed-through should vary in a known way and if it were measured and was wrong, it would prove that the disc was a copy.

A moment of doubt

Several of us used stopwatches to time the delay between a loud sound andits faint bleed-through. We started the stopwach on the first sound and stopped it on the second sound. We all measured a delay that was a few milliseconds too long!

After a few moments of absolute horror, I realized that we were the victimsof a psychoacoustic factor. Our nervous systems reacted less quickly to afaint sound than to a loud sound.

We re-measured the delay using a computer, and found exactly the predicted delay.

The Thomas Hypothesis

Maybe the dictabelt stylus recorded gunshots, then continued recording for another minute and suddenly jumped back to record “Hold everything secure ..” over the gunshots.

Coincidences

Barger, Weiss Aschkenasy needs• Stuck button (which happened)• stuck button in motorcade• switched to wrong channelThomas needs additional coincidences• stylus jumped backward• stylus landed on recorded gunshot• speech did not perturb gunshot peaks

Summary• The recorded police radio does not contain

gunshots.• The time of the suspected gunshots is about a

minute too late.• The sound suspected to be a gunshot is actually

speech.• The motorcycle with the stuck transmitter button

was almost certainly not in the motorcade.• This neither proves nor disproves any theory

about a second gunman.