Many people on the sidewalk in front of the TSBD heard the shots as "firecrackers". By the time Altgens snapped his photo at z255, the onlookers had still not reacted to two rifle shots, despite the facts they were very close to the rifle AND the muzzle of the rifle was pointing toward them.
No.
The people under the sniper?s nest, 60 feet below, near the southeast corner of the building, were 60 degrees away from where the rifle was pointing. The bullet might pass right over their heads, about 60 feet above their heads, but in 3-D space the rifle was off of their position by 60 degrees. The rifle was not pointing in their general direction, almost straight down, but only 30 degrees below the horizon.
The people along the north side of the street near where the limousine was at z152 (see the map below):
http://content.invisioncic.com/r16296/monthly_2017_11/5a1653a2b6d50_DealeyPlazaDetailedMap112217.gif.368da7d606dfb2a91ee3af598949031f.gifwould have the shot muffled by the tree, which blocks the line of sight from the sniper?s nest to the sidewalk more than it does the center of the street.
And even if we ignore the effect of the tree, ignore that the rifle was not pointed within a few feet of them, the sound of the rifle from 45 yards away would be no louder than a firecracker 10 yards away. And for all they knew, someone had set off a firecracker.
Also, we should not forget that the Carcano is not one of the louder rifles. I understand it is not nearly as loud as some. As to be expected from a rifle that fires bullets that travel around 2100 feet per second as opposed to 2800 feet per second. So, while 163 db may be a good estimate for an average rifle, the Carcano was probably less loud than this.
Are you aware how much louder a rifle is if you are ahead of the muzzle, as opposed to standing behind the shooter?
Yes. 163 db. When the muzzle is pointed in the general direction of the observer.
Which is why a much larger firecracker is only 150 db, but a much smaller bullet can produce a much louder noise.
Provided the rifle is pointed in the general direction of the observer. And it is important to remember that in 1963, much more powerful firecrackers were legal then the ones you can legally buy today.
Even in an ?ideal? situation (ideal for hearing a loud noise) with the rifle pointed nearly at the observer, a rifle 90 yards away will only be as loud as a firecracker that is 20 yards away. And a rifle that is 45 yards away can only be as loud as a firecracker that is 10 yards away.
So, it won?t be obvious to an observer, at least one not experienced at being fired at with rifles, from the loudness alone, if they are hearing a firecracker or a rifle from four times further away.
Why did the onlookers in front of the TSBD not display instantaneous startle reactions, as would be expected of people exposed to 163 dB? Why did many of them describe a deafening muzzle blast as a firecracker?
The people who were close, within 60 feet, did not have the rifle pointed in their general direction.
The people who did have the rifle pointed in their general direction, were not that close, but 45 yards away (at z153). And, unless standing in the middle of the street, had a tree between them and the rifle, which would muffle the sound to some degree.