This has been well established. There was an "echo" on the head shot for some reason. And obviously Oswald could not have fired two shots in such rapid succession.
Its never been explained why there was an "echo" on the head shot but not on any of the other shots.
There is at least one rather good explanation. “Crack-Thump”.
A supersonic bullet does not make one noise. It makes two. “Crack-Thump”.
Interruption: Yes, but why not “Crack-Thump . . . Crack-Thump . . . Crack-Thump”.
Answer: I’ll get to that.
The “Crack” is from the supersonic bullet. The “Thump” is from the muzzle blast. The “Crack” travel’s most of the way, if an observer is near the trajectory of the bullet, like along the street, as most witnesses were, at the speed of the bullet, an average speed of 2,000 feet per second. The “Thump” travels at a slower speed, the speed of sound, 1,125 feet per second.
For the first shot, at a distance of 130 feet, the bullet will arrive after (velocity = distance / time, and time = distance / velocity) 130 / 2000 or 65 milliseconds. An observer may hear it shortly after that.
The muzzle blast travels 130 feet in 113 milliseconds. The difference is 48 milliseconds.
If one does the math, one finds that the theoretical time difference between the “Crack” and the “Thump” is:
First shot at z153: 48 milliseconds.
Second shot at z222: 70 milliseconds.
Third shot at z313: 98 milliseconds.
The delay between the “Crack” and the “Thump” is twice as long for the third shot than it is for the first shot. And a good deal longer for the third shot compared to the second.
Now, the exact delay will depend on the location of each witness, and will vary from witness to witness. But in general, the interval between the “Crack and the “Thump” will be greatest for the third shot. So, it may be that for the first and second shots, the “Crack” and the “Thump” arrived too close to each other to distinguish them as two separate sounds. But the third could be distinguished as two separate sounds.
Another factor is the alertness of the witnesses. After the second shot, when both the President and Governor were wounded, most witnesses did not realize that shots had been fired. Most continued clapping their hands, as can be seen in the Zapruder film. So, they might ignore the detail of what might be a backfire, like whether it was a “bang” or “bang-bang” as they concentrated on their few seconds of seeing the President and First Lady close at hand. But the third shot, with the head explosion, may cause them to become instantly alert and remember with more detail the sounds that had just occurred.
Finally, while there was an extra sound with the first shot, striking the street, there was no extra sound with the second bullet, which passed through mostly the soft tissue of the President and the Governor. In contrast, the third shot had extra sounds caused by:
• The shot hitting at almost 1900 feet per second the skull.
• The sound of a fragment hitting the windshield.
• The sound of another fragment hitting the chrome windshield frame.
So, there are some extra possibilities why a witness might be more likely to mistake the third shot as two shots occurring almost together.
