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How long does an explosion's boom last?

  1. Mar 28, 2012 #1
    How long does the boom from an explosion last? As in, how long does it make the sound for?
     
  2. jcsd
  3. Mar 28, 2012 #2
    the sound is produced the reaction taking place. energy released from breaking bonds in given out into the surroundings, some in the form of heat/light and the rest in the form of things like sound.

    for an explosion to occur, you want the reaction to take place incredibly quickly, so it doesn't 'produce' sound for very long.

    however the sound that it does produce is likely to echo around for a number of seconds.

    to put an actual value on it... doesn't really mean much. depends on a lot of variables
     
  4. Mar 28, 2012 #3
    The energy that comes out of the explosion makes a shockwave. The shockwave passing you is the boom that you hear.... same as a jet breaking the sound barrier. That is why you can not stand to close to an explosion besides the heat, the impact of the shockwave will kill you too. The shockwave is the expansion and compression of the air molecules. As for how long does it make the sound for. It makes the sound for an instant. It just takes some time for you to be able to hear it because of the speed of sound. That is why you can see the flash and then hear the boom if you are standing far enough away. Does this answer you question?
     
  5. Mar 28, 2012 #4
    Not really, I wanted to know how long the boom itself lasts, rather than how long it gets to you. It must take a finite amount of time. How long is it?
     
  6. Mar 28, 2012 #5
    "Explosion" doesn't really give you enough parameters to be able to tell.

    Think about it - what's causing the sound? Is it due to something rapidly oxidizing/burning and vibrating against the air, like a firework? If so, how long does it burn? That's how long your sound is. Fast explosion? Quick, short sound. Slow burn? Longer sound.

    It's possible for there to be multiple phases of an explosion, like a volcano. The "boom" will start making sound by the lava pushing its way through the ground (and then that sound travelling through the air to you), and then by its explosion through the air, and then even by impacting the surface when it lands.

    Then you have to take into account your surroundings. If you're in a large room, or in a valley, the sound could continue to bounce around (echo) for a longer time. Also you have to take the medium into account. An explosion will make a boom of 0 seconds in a vacuum.

    Like other folks have said here - sound is just caused by expansion and compression of the medium, in this case air. How long is it expanding and compressing? That's how long the sound is. It's like asking "how long is the sound when someone yells?" The answer is "as long as they're yelling for".
     
    Last edited: Mar 28, 2012
  7. Mar 28, 2012 #6

    davenn

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    That was a really good response to the OP, Vladimirr.
    Particularly the comment about the zero boom in a vacuum.

    I was also thinking of examples of lightning and thunder and the factors that determine if the rumble of the thunder goes on for many many seconds or if it just that sharp crack/ bang common with a closer strike

    Dave
     
  8. Mar 30, 2012 #7
    OK, how long does a nuke's explosion's boom last?
    How long does a C4 explosion's boom last?
     
  9. Mar 30, 2012 #8
    LIke everything else, it all depends.

    The "BOOM!" (i.e., acoustical disturbance resulting from an explosion) is due to the release of energy in the form of heat into the ambience, which heat, when received by the surrounding molecules which make up the ambient air, causes massive acceleration of those molecules by means of increasing their temperature (temperature being, after all, only the average kinetic energy of the atoms and/or molecules making up a substance). This sudden acceleration of the ambient air molecules generates a shock wave in the air due to the increased volume of space required to accommodate the increased motion of the air molecules, which increase in volume needed to confine a given amount of matter is called an acoustical overpressure wave, as it involves actual motion of matter for propagation, and thus stimulates the organ of the human body which senses such shockwaves (the ears), which then transmits a signal to the auditory center of the brain to be perceived as sound. This is then followed by an acoustical underpressure wave, as the thermal energy imparted into the particles constituting matter is expended as kinetic energy, and the mass returns to its original volume, which volume contraction also stimulates the ear, and, via the ear, the auditory center of the brain. Sound is a function of the brain of the auditor, and not of the acoustical disturbance alone.

    The sharpness of the sound of the explosion is related to the speed of the oxidation reaction which sets the shockwave in motion. The first explosion I ever heard was when I was living in Lower Manhattan in the early 1980's, and a petroleum tank farm across the river in New Jersey went up. As the speed of the reaction was relatively slow (poorly oxygenated, separate tanks required time to be ruptured and their contents ignited), all I heard was a low roar, followed by a strong wind blowing away from the explosion (the explosive overpressure wave, as the shock wave propagated away from the seat of the explosion), followed by a slightly less strong wind blowing in the opposite direction (the underpressure wave, during which the displaced air reoccupied the partially evacuated space).

    Then again, not every explosion requires an oxidation reaction, nor any chemical reaction whatsoever. My biggest explosion came during a soaking rain storm in Rhode Island, when a large, waterlogged tree located more than twenty miles away from me was struck by lightning, the heat of which lightning strike caused the water in the tree to rapidly evaporate. It being the fact that steam wants to expand to 1600 times the volume it occupied as liquid water, and that the water was trapped within the cells and in the intercellular space within the tissues of the tree, a massive rupture of the cellular and histological structure of the tree resulted, creating an acoustical shock wave in the air, as well as a seismic wave through the ground within which the tree's root structure was embedded. I perceived this explosion as a low, rolling sound which rapidly built up to a crescendo, finally terminating in a loud CRACK just as the floorboards beneath my feet suddenly sprang up and tossed my feet a major fraction of an inch off the floor.

    The loudness (I've already used the term "volume" with reference to space, so I'll not confuse matters here by using it to refer to perceived sonic intensity) of the explosion would be related to the amount of energy released in the explosion, whereas the sharpness would be related to the duration of the energy release as a function of time.

    An interesting point: when the nuclear weapon released upon Hiroshima exploded, people hearing the blast from far away heard an enormous boom, but those located within the city recall no sound whatsoever. So far as I know, this phenomenon has never satisfactorily been explained.
     
    Last edited: Mar 30, 2012
  10. Mar 30, 2012 #9

    Bobbywhy

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    jetwaterluffy, from reading the responses so far I imagine that you have received a satisfactory answer to your original question. Here is my two cents worth from a passage taken from my memoirs as a civilian advisor in Vietnam during the war:

    “The United States used B-52 bombers to carpet bomb in Laos, Cambodia, and Vietnam, each bomber laying down hundreds of 500-pound exploding death bombs in wide swaths to kill everything in its path. The sound of “carpet bombing” is similar to rolling thunder, except thunder eventually stops. The sound of all those bombs exploding continues on and on, like a continuous explosion, deafening when you, the skinny civilian tech-rep, happen to be only a few thousand meters away from the target area.”
     
  11. Mar 30, 2012 #10
    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.

    I'm old enough to understand this analogy quite well, but young people these days have never seen a photographic flash bulb igniting.

    How might one express this analogy in terms today's young people will understand?
     
  12. Apr 1, 2012 #11
    Sort of. I was hoping for a number, though. And why has everyone got an avatar now?
    EDIT: oh, I see, april fools day.
     
  13. Apr 1, 2012 #12
    A lightning strike heats the air and produces a shock wave in a similar way to what some explosives might. We all have experienced thunder. Depending on how close the lightning is and the surrounding topography the boom might be a sharp crack that lasts only a split second or it might roll on for several seconds. So you see, the answer to your question is already in your personal experience.
     
  14. Apr 1, 2012 #13

    sophiecentaur

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    I just found this thread and I don't think anyone has introduced the word dispersion. The initial impulse from an explosion can be very short (high explosive "crack") or relatively long ("boom"). People have already discussed this. The initial impulse will consist of a large range of frequency components and these are all timed to produce a peak. The profile of this peak will be modified over distance.
    As the pulse travels through the air and over the ground, some frequencies will be attenuated, some will travel faster than others and get multiple reflections. The result is that the original single peak will be spread out in time and turn form a crack into a rumble which can last up to a second (hence the "clouds bumping together" story that they tell kids).
     
  15. Apr 1, 2012 #14

    Bobbywhy

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    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 travelled 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 [Broken]

    5. Chronological Development of Air Burst:
    http://www.cddc.vt.edu/host/atomic/nukeffct/airburst.html [Broken]

    http://www.fas.org/nuke/intro/nuke/blast.htm
    http://blogs.howstuffworks.com/2011...st-wave-is-the-thing-that-causes-destruction/
     
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