BadBrain said:
Bobbywhy:
You're old enough to help me with this question.
Survivors of the Hiroshima blast from within the city recall no sound, but the most enormous flash of light, often described as a million photographic flash bulbs going off at once right in your face.
Very interesting to discover this very special acoustical phenomenon. I have not been able to explain the mystery of why survivors near the “hypocenter” heard no sound and those further away (a few kilometers) heard the loud explosion. After a lot of searching I am convinced it has to do with the “Mach stem” (Google it) caused by the atomic bomb detonation. Below are five excerpts and three websites with more information. I would appreciate it if some acoustician could explain this strange mystery.
1.“Those who survived called the A-bomb "pika don". "Pika" referred to the flash of light, and "Don" was an onomatopoeic reference to the tremendous sound.
Survivors close to the hypocenter, the point directly beneath the detonation, heard no sound, and called it merely "pika".
The detonation formed a high-temperature, high-pressure fireball which rapidly expanded to a diameter of about 400 meters in the first second. The fireball emitted intense heat for three seconds, and glowed brightly for about ten seconds. The temperature on the ground near the hypocenter reached thousands of degrees Celsius. On the ground near the hypocenter the overpressure reached tons per square meter. The fireball created a supersonic shockwave, which was followed by winds blowing hundreds of meters per second. The shock wave traveled eleven kilometers in 30 seconds.
The white light acted as a giant flashbulb, burning the dark patterns of clothing onto skin (right) and the shadows of bodies onto walls. Survivors outdoors close to the blast generally describe a literally blinding light combined with a sudden and overwhelming wave of heat. (The effects of radiation are usually not immediately apparent.) The blast wave followed almost instantly for those close-in, often knocking them from their feet.
People farther from the point of detonation experienced first the flash and heat, followed seconds later by a deafening boom and the blast wave. Nearly every structure within one mile of ground zero was destroyed, and almost every building within three miles was damaged.”
http://www.globalsecurity.org/wmd/ops/hiroshima2.htm
2. "Then a tremendous flash of light cut across the sky. Mr. Tanimoto has a distinct recollection that it traveled from east to west, from the city toward the hills. It
seemed a sheet of sun. Both he and Mr. Matsuo reacted in terror and both had time to react (for they were 3,500 yards, or two miles, from the centre of the explosion). Mr. Matsuo dashed up the front steps into the house and dived among the bedrolls and buried himself there. Mr. Tanimoto took four or five steps and threw himself between two big rocks in the garden. He bellied up very hard against one of them. As his face was against the stone he did not see what happened. He felt a sudden pressure, and then splinters and pieces of board and fragments of tile fell on him.
He heard no roar. Almost no one in Hiroshima recalls hearing any noise of the bomb. But a fisherman in his sampan on the Inland Sea near Tsuzu, the man with whom Mr. Tanimoto's mother-in- law and sister-in-law were living, saw the flash and heard a tremendous explosion; he was nearly twenty
miles from Hiroshima, but the thunder was greater than when the B-29s hit Iwakuni, only five miles away.
http://archive.org/stream/hiroshima035082mbp/hiroshima035082mbp_djvu.txt
3. "About 70,000 to 80,000 people were killed immediately, many of whom probably had no idea what had happened. The 560 grams of Uranium 235 took about 10 nanoseconds of fission before a flash of light burst out of the bomb casing, releasing the first wave of gamma rays that traveled nearly at the speed of light. In 1/10,000 of a second, a second burst of gamma rays was released. In 3 milliseconds, a plasma fireball began to form. In 91 milliseconds, the bottom of the plasma fireball began to reach the top of the tallest buildings beneath the detonation. Soon after, a shock wave which traveled at twice the speed of sound came. The human nervous system required 1/30 of a second to register, and 1/10 of a second to flinch, thus for those who were close to the detonation, the blood in the victims' brains were likely evaporating before they could feel anything."
http://ww2db.com/battle_spec.php?battle_id=49
4. "The explosive effect of an ordinary material such as T.N.T. is derived from the rapid conversion of solid T.N.T. to gas, which occupies initially the same volume as the solid; it exerts intense pressures on the surrounding air and expands rapidly to a volume many times larger than the initial volume. A wave of high pressure thus rapidly moves outward from the center of the explosion and is the major cause of damage from ordinary high explosives. An atomic bomb also generates a wave of high pressure which is in fact of, much higher pressure than that from ordinary explosions; and this wave is again the major cause of damage to buildings and other structures. It differs from the pressure wave of a block buster in the size of the area over which high pressures are generated. It also differs in the duration of the pressure pulse at any given point: the pressure from a blockbuster lasts for a few milliseconds (a millisecond is one thousandth of a second) only,
that from the atomic bomb for nearly a second, and was felt by observers both in Japan and in New Mexico as a very strong wind going by."
http://www.cddc.vt.edu/host/atomic/hiroshim/hiro_med.html#EXPLOSION
5. Chronological Development of Air Burst:
http://www.cddc.vt.edu/host/atomic/nukeffct/airburst.html
http://www.fas.org/nuke/intro/nuke/blast.htm
http://blogs.howstuffworks.com/2011...st-wave-is-the-thing-that-causes-destruction/
