Saturday, June 13, 2009

Professor LawDog's School of Mayhem and Survival

Good evening, class.

Today's block of instruction concerns the skill-set popularly called "flash-to-bang time" and its use in SHAG* range estimation.

"Flash-to-bang time" requires two things: 1) an event which produces both an effect that you can see -- the "flash" -- and a sound that you can hear -- the "bang" -- and 2) some way to reliably count seconds.

Now, light travels at about 186,000 miles per second. As far as us knuckle-draggers are concerned, this is close enough to instantaneous as to make no nevermind.

Sound, on the other paw, travels about one mile every five seconds, or 375 yards every second. More or less. This is quick-and-dirty ranging -- you want precision, get a laser rangefinder.

For those on the far side of the pond, one mile every five seconds comes out to about 340 metres per second. More or less.

Obviously, we're going to use the difference between the speed of light and the speed of sound, but to do so, we have to be able to reliably count seconds. This is easily done using the "One thousand" or "Mississippi" technique. Since "One thousand" and "Mississippi" both take just under a second to say, they are both fairly reliable -- as long as you remember two things:

First: The number of seconds goes at the end of the word. You say, "One thousand-one, one thousand-two, one thousand-three" or "Mississippi-five, mississippi-six, mississippi-seven" -- NOT, "One, one thousand; two, one thousand, three one-thousand" or "One mississippi, two mississippi, three mississippi".

Second: Don't count past ten. One through ten are nice one-syllable words. Once you get off into those multi-syllable teens, the number takes longer to say and throws your timing off. If you find yourself needing to go to eleven or higher, mark the first ten and start over again at one thousand-one.

So.

Let us say we have witnessed a flash of lightning. We immediately say, "One thousand ONE, one thousand TWO, one thousand THREE, one thousand FOUR, one thousand FIVE", we hold up an index finger, then we start counting again at, "One thousand ONE, one thousand TWO" and we continue, holding up another digit each time we get to one thousand FIVE and starting the count all over again until we hear the bang of thunder.

Remember the speed of sound? About one mile every five seconds? The number of piggies you are holding up is the range to the lightning bolt in miles.

Of course, there are some of us who are simply too cool to hold up piggies. In this case, count your total number of seconds -- remembering to start over if the count goes past ten -- and divide by five to get the range in miles.

"But, Professor LawDog," I hear you say, "What if the range is less than a mile?"

Nae problemo, grasshopper.

As a creative articulation, let us say that we have a boat-load of eco-terrorists hippies in a harbour somewhere over there. Further, let us state that standing next to us is a confused, yet friendly, French tourist who sure would like to know how far away that particular boat is.

Fortunately, the boat has a steam-whistle, and when you see the plume of white stuff (the flash), you start counting seconds. Two seconds later, you hear the shriek of the whistle (the bang).

Sound travels about 375 yards per second ... two seconds ... 375 times two ... carry the little piggy ... about 750 yards away.

Oops, your new friend is French. Okay ... 340 metres per second ... ummm ... call the range 680 metres.

Using this method, any event which you can both see (flash) and hear (bang) can be ranged. In the past I have used a pile driver -- starting my count at the sight of the drivers impact and ending the count at the distinctive "whomp"; I have used the dust raised by the muzzle-flash of a prone rifleman together with the sound of his shot; and I've used the sight and sound of a man splitting cord-wood with an axe.

Thus endeth the lesson.

LawDog

*Scientific Hairy-Arsed Guess

10 comments:

Old NFO said...

And if you only count one thousand one, and hear the big bang, you are WAY too close... :-)

Keith said...

I just got to try it on some lightning of the flash-"one tho.."-BANG variety

aczarnowski said...

I love Mayhem and Survival School. This lesson clarifies some of my less accurate estimations of how far away that storm is.

Cybrludite said...

Nota Bene: The "Bang" of a nuke is supersonic for part of it's trip. You'll need your handy Calculator Set, Radiac And Nuclear Yield: ABC-M28A1 to better gauge your distance relative to the trinitite-lined crater.

Anonymous said...

For a constantly repeating sight/sound combination like a pile driver, you are never sure which sight goes with which sound. You could be counting the sound from an earlier or later sight and be off by once cycle which makes you distance calculation wrong.

Anonymous said...

Conductor on downbeat -> Flash
First sound from the orchestra -> Bang

Mark said...

Good stuff Lawdog!

I've used this method for storms a lot (I generally figure 1,000 feet per second, close enuff for someone counting thousands for time). More importantly, I use it to tell if a storm is coming MY WAY, if the intervals between flash and bang get shorter.

KD5NRH said...

"One through ten are nice one-syllable words."

"Seven" is only a one-syllable word in Arkansas.

The Scribbler said...

Recent studies have actually shown interesting things about the brain, where given a sufficiently small deviation between flash and bang (say, sub 200 yard) the brain will automatically compensate, and you perceive them at the same time. You can actually test it for yourself, if you've got a consistent flash-bang producer (flash bang grenades are generally not advised, though firecrackers are good) and a buddy. Have your buddy walk away and drop a firecracker every few yards. There should be a particular range at which it quite suddenly stops being flashbang and becomes flash -pause- bang. This range is the low end of effectiveness. Strange stuff.

KD5NRH said...

What does it mean when you hear the bang as it lifts you out of bed and cracks a couple of windows, but don't actually see anything but purple streaks for a couple of minutes?

Apparently, it means God felt the pecan tree 40 yards from the house needed explosive pruning at 4AM.