## Flash, then Bang

Wednesday, January 6th, 2016

If you’ve ever watched a professional fireworks display, you know that the bright flash precedes the loud bang by a few seconds.

If you watch an atomic bomb blast, the bright flash precedes the loud bang by half a minute:

By the way, that bright flash is bright enough “that people can see bones showing through hands covering their closed eyes,” and that loud bang comes from a pressure wave of 0.2 psi, “an immense punch to the whole body” at 10 km.

Also, it is a bang, rather than a boom:

The sound of a nuclear blast is distinctive as more of a bang than a boom because thermal energy is directly proportional to low frequency absorption. or heat soaks up low frequencies. There is a hell of a lot of heat in a nuclear bomb — hence the higher pitched bang sound than you would expect from such a massive explosion.

Over at the Glasstone blog — “a blog all about contradicting the widespread superstition that nuclear wars are unsurvivable and debunking hardened dogma of exaggerated nuclear effects” — our Slovenian guest asked, which comes first, the shock wave, or the bang?

A simple calculation of the arrival time of “sound” corresponds roughly to the arrival time of zero pressure after the shock wave! At very long distances where the shock velocity decays to sound velocity, the blast duration peaks and ceases to increase further. So you can get an idea of the blast wave or sound wave duration by subtracting the calculated sound wave arrival time from the shock wave arrival time!

Some examples. Sound takes 4.7 seconds to travel one mile. So if you are at 10 miles from a 1 megaton low air burst over Trafalgar square, the shock front arrives at 40 seconds after the first flash of the explosion, and zero pressure (sound arrival time) is about 47 seconds, so the total shock duration is around 7 seconds!

There’s some interesting physical mechanisms. Because sound is a longitudinal pressure wave, and is not a transverse wave, it is basically an outward force (pressure = force/area). So you get some simple basic concept physically, like Energy = Force x Distance, or more precisely: Energy = Integral of Force over distance. The distance here is the distance of the outward pressure phase of the shock. Then by Newton’s 3rd law of motion, you get the prediction of the negative (inward directed) pressure phase, which has lower peak (negative) pressure, and so has a somewhat longer duration. Of course, in the very early phase of the blast wave there is no negative pressure, because when the pressure wave is isothermal, Newton 3rd law reaction force consists entirely of the blast going out in the opposite direction, e.g. before the negative phase sets in, the reaction force of the blast force which is going Southwards is simply the blast force that’s going Northwards. After the pressure in the middle drops to ambient due to outward motion of most of the air in the shock front, this reaction force is replaced by the negative phase (reversed blast winds). So it’s possible to get a physical, intuitive feel for the physics of all the details of the blast wave.

The first sound in actual nuclear blast, regardless of the blast duration, is like a pistol shot, according to Jack W. Reed, who saw more atmospheric nuclear tests than anyone else in the West while in the Nevada long-range nuclear blast prediction unit (which had to predict blast reflections from atmospheric temperature inversions to prevent broken windows and injuries in Las Vegas, etc.). Humans can only hear sounds with frequencies of 20 Hz – 20 kHz, so a 1 second blast duration is 1 Hz and is too low to hear. So all you hear in a nuclear explosion is:

1. The crack-like (pistol shot) sound of the abrupt rise in pressure when the shock front arrives (if it takes 1 ms to go from ambient pressure to peak overpressure, that is a frequency of around 1 kHz).

2. The sound of the wind blowing behind the shock front (which is only 40 miles/hour peak wind speed at 10 miles from 1 megaton, but is much higher closer in).

3. Sounds from damage caused like breaking windows, impacts of blown debris.

1. Slovenian Guest says:

Flash then bang? It truly is a deranged weapon!

The embedded video shows a 1953 test of a 16 kiloton nuclear device, filmed from a distance of about 6.6 miles, the detonation is at 0:26, and the bang follows only at 0:57, half a minute later. It was even televised live nationally, the coverage was recorded on a kinescope, making it a rare record with actual sound. A kinescope recorder was basically a special film camera mounted in a large box aimed at a high quality monochrome video CRT. That was the only practical way to preserve live television broadcasts prior to the introduction of videotape in 1956.

People aren’t aware, but most nuclear blast videos have explosion sound effects superimposed, if they have sound at all. Because in reality, from an observers point of view, nuclear detonations happen in eery silence, at zero you only see a blinding flash and feel almighty heat!

Fun fact, the speed of sound is dependent nearly only on temperature, at 0°C it’s 331.3 m/s and at 25°C 346 m/s, slightly faster. While changing atmospheric pressure does not change the speed of sound, for sound pressure without the medium (air) there is no speed, according to this site.

So duck and cover makes sense after all! Who would have thought?
Not Adam Savege from Mythbusters, who said here:

“I came of age when we were still doing nuclear fallout drills, duck and cover, getting on our hands and knees, as if it would help in any way.”

It certainly would, because it’s all about escaping that shock wave, which will hit like a wrecking ball, remember that the actual fireball doesn’t even touch the ground under a detonation, nobody was ever vaporised, but people were actually decapitated and cut to pieces by flying shards of glass! Again via Glasstone:

“The thermal influence never vaporised anyone at ground zero and has never vaporized more than 1 mm of wood from such a weapon, regardless of distance; while Oughterson and Warren’s Medical Effects of the Atomic Bomb in Japan in 1956 proved that duck and cover prevented burns and the burns-radiation synergism which killed so many. Most of the casualties in both cities were due to blast and thermal radiation, with infected wounds made worse by the synergism of initial radiation exposure, which lowers the white blood cell count.”

People do not melt like that guy in Indiana Jones…

And neither is MAD real, as Isegoria readers already know, it would take 1,241,166 heavy duty nukes to completely wipe out civilization! Mutual assured destruction is propaganda as well.

Another account from the first British atomic device test in 1952:

“Unfortunately the gurus forgot to tell us about the shock wave, so after the flash, me and a couple of other chaps, we jumped on the back of a truck that was close by, to get a better view, then we suddenly saw the trees in front of the thing bend over towards us, and then the next thing we knew we’ve been blown off the truck, because we’ve forgotten about the blast.”

They were 20 miles away and the shock wave took 1.6 min to arrive! Thats enough time to even properly stretch before ducking and covering…

And to come full circle, the need for better broadcast recording brought us reruns and multi-cam sitcoms:

“The true demise of the kinescope, like most things in television, was ultimately driven by an economic concerns and can be attributed to I Love Lucy and its stars and producers Desi Arnaz and Lucille Ball. When beginning their landmark show, the couple insisted on producing in California, their home for many years. Philip Morris, the cigar and cigarette manufacturer, already signed on as the show’s sponsor, wanted the program produced in New York because more potential smokers lived east of the Mississippi: Philip Morris would not settle for inferior kinescopes playing on the East Coast. In response Arnaz and cinematographer Karl Freund devised a method of recording performances on film. Their system used three cameras to record the live action while a director switched among them to obtain the best shot or angle. The show was later edited into the best performance in a manner much like a feature film. The result not only was a superior recording good for repeated airing throughout the country, it also presaged the move of the TV industry from New York to the West Coast, where fully equipped film studios eagerly entered television production and recouped some of the losses they had encountered with the rise of the new medium. Moreover, the new filmed product created, almost accidentally, TV’s most profitable byproduct, the rerun.”

Plus another fun fact:

“The Former Bank of Japan, Hiroshima Branch was representative of Hiroshima’s historical buildings in the early Showa period, with an outstanding classical-style appearance. Despite being exposed to the A-bombing a mere 380 meters (or 0.2 miles) from the hypocenter of the A-bomb, thanks to its sturdy structure, the bank still remains as it appeared when first built. Since the armored shutters on the first and second floors were closed at the time of the A-bombing, the interior was not badly damaged. However, the third floor, where the shutters were open, was completely burned. Only two days later, on August 8, 1945, the Bank of Japan reopened for withdrawals and provided space for temporary branches of other financial institutions in Hiroshima City, which had been rendered unable to conduct business.”

The bank, brand new in 1936, damaged in 1945 and today!

2. Alrenous says:

Mutual assured destruction is propaganda as well.

Narcissism, right? Government don’t care about civilization, but they do care about their own hides. A few nukes can take down a government. It’s hard for them to even realize somebody outside government might see that as a positive.

3. Phil B. says:

Of course nuclear war is survivable. Take a look at this compilation. Several things to notice:

1) A total of 2053 Nuclear weapons were detonated between 1945 and 1998. Two thousand and fifty three!

2) The southwest of the USA in various years looks like it is being hit by an all-out nuclear strike, and the cumulative strikes look like total Armageddon (as in Armageddon out of here).

3) At the end, a summary of the detonation sites is shown. Look at the Russian distribution of detonations. If you did not know where the major cities were located, it looks like the majority have been hit. (They weren’t, but it resembles the usual distribution of cities in a land mass).

It averages over 38 detonations a year — but as many years don’t have any detonations, then some years look like all out nuclear war. I’ll leave you to work out which years these were.

So, where are the mutants with three heads, tentacles for arms, and psychic powers that the Sci-Fi writers predicted? I want my taxpayer money back. I was promised these freaks.

However, to listen to the chattering classes, a single nuclear detonation means the end of all mankind and the planet dying.

4. You guys are a breath of fresh air :)

To answer a question posed by a friend this morning, I just finished calculating that Little Boy’s blast effects (assuming yield = 15 kt) are equal to the full conventional bomb-loads of ~54 B-52Hs (assuming 18 X Mk 84 2000 lb bombs each). So the two main US B-52 wings (5th and 2nd Bomb Wings) could largely equal the destruction done by Little Boy.

Remember that the destruction from blast effects doesn’t scale linearly with yield but with the 2/3 power of yield, because the blast wave from the bomb is spherical but the target is spread out on a plane.

5. Isegoria says:

I found the following fact (from Table C, “Per-Cent Mortality at Various Distances”) morbidly fascinating: from 0 feet to 1000 feet from ground zero, percent mortality was 93.0. Not 100.0 percent. Not 99.9 percent. But 93.0 percent. Certain earthquake-proof concrete buildings survived the blast intact; I guess that accounts for the 7.0 percent.

6. Slovenian Guest says:

Another gem: Ground zero, population 5!

On July 19, 1957, five men stood at Ground Zero of an atomic test that was being conducted at the Nevada Test Site. This was the test of a 2KT (kiloton) MB-1 nuclear air-to-air rocket launched from an F-89 Scorpion interceptor. The nuclear missile detonated 10,000 ft above their heads.

Correct sound, and one of them is even commenting!

This was a test of the Genie air-to-air rocket and its W25 warhead. The rocket, fired by an F89J, traveled 4,240 meters in 4.5 seconds before detonating.

For what it’s worth, we manufactured 10,171 Genies, which were operational from 1957-84. Total cost was about \$1 billion in today’s dollars (excluding the cost of the nuclear warhead). Approximately half (5,000) were nuclear-armed. The Genie was in large measure a response to the 1955 bomber gap (as was almost everything else we poured into continental air defense from the mid-1950s to the mid-1960s). All told, 11,000 warheads, nearly sixteen percent of all nuclear warheads we ever produced (and thirty-four percent of all operational weapons in 1965) were intended to counter and defeat what was at best a marginal Soviet airborne threat.

via Nuclear Secrecy.

7. Alrenous says:

Includes mortality by distance in concrete buildings.

More here.

8. Slovenian Guest says:

Luckily there is no free nuclear lunch:

The blast wave cannot cause destruction without using energy, and this use of energy depletes the blast wave. The American manuals neglect the fact that energy used is lost from the blast. Visiting Hiroshima and Nagasaki, Penney recorded accurate measurements of damage effects on large objects that had been simply crushed or bent by the blast overpressure or by the blast wind pressure, respectively. At Hiroshima, a collapsed oil drum at 198 m and bent I-beams at 396 m from ground zero both implied a yield of 12-kt. But at 1,396 m data from the crushing of a blue print container indicated that the peak overpressure was down by 30%, due to damage caused, as compared to desert test data. At 1,737 m, damage to empty petrol cans showed a reduction in peak overpressure to 50%: ‘clear evidence that the blast was less that it would have been from an explosion over an open site.’

A similar pattern emerged at Nagasaki, with close-in effects indicating a yield of 22-kt and a 50% reduction in peak overpressure at 1,951 m as shown by empty petrol can damage: ‘clear evidence of reduction of blast by the damage caused…’ If each house destroyed in a radial line uses 1 % of the blast energy, then after an average of 200 houses in any radial line from ground zero outwards are destroyed, 87 % of the blast energy will have been lost in addition to the normal fall in blast pressure due to divergence in an unobstructed desert or Pacific ocean test. You can’t ‘have your cake and eat it’: either you get vast blast areas affected with no damage, or you get the energy being used to cause damage over a relatively limited area. The major effects at Hiroshima in the horizontal blast (Mach wave) zone from the air bursts were fires set off when the blast overturned paper screens, bamboo furniture, and such like on to charcoal cooking braziers being used in thousands of wooden houses to cook breakfast at 8.01 am. The heat flash can’t set wood alight directly, as proved in Nevada tests: it just scorches wood unless it is painted white. You need to have intermediaries like paper litter and trash in a line-of-sight from the fireball before you can get direct ignition, as proved by the clarity of ‘shadowing’ remaining afterwards (such as scorch protection of tarmac and dark paint by people who were flash burned). In general, each building will absorb a constant amount of energy from the blast wave (ranging from about 1 % for wood frame houses to about 5 % for brick or masonry buildings) despite varying overpressure, because more work is done on the building in causing destruction at higher pressures. At low pressures, the building just vibrates slightly. So the percentage of the blast energy incident on the building which is absorbed irreversibly in heating up the building is approximately constant, regardless of peak pressure. Hence, the energy loss in a city of uniform housing density is exponential with distance, and does not scale with weapon yield. Therefore, the reduction in damage distances is most pronounced at high yields.

Again via Nigel Cooks Glasstone

Who also has a quantum field theory blog!