Questions about sound and echo

[Tog_]

It's story time again.

In this one, I have two groups. One is a large group that has a legitimate purpose for setting off a large explosion at the north end of a particular area. I'm thinking maybe 5 depth charge equivalent blasts set off in unison, but spread out quite a bit. The goal is is to dislodge something on the bottom of a shallow (20 m?) bay.

The second group has a covert task and will be using the sound of the first explosion to mask the sound of their, much smaller, ones (Small shaped charges to blow a hole in something like an oil tank)

As for the setting, it's a long (4500 m) bay ringed almost entirely with a narrow strip of land. To the outside of that land are very thick, tall walls. Basically, it's a bathtub.

The plan, as it stands now, is to have the sound of the first explosion set off the second at just the right time to have the sound of the second explosion mix in with the echo of the first as it gets to the far wall.

I've looked up the speed of sound in air, and salt water, and found values of 334 m/s and 1475 m/s respectively. I'd like the sound of the first explosion to activate the timer to set off the second.

Is it feasible that this would mask the sound of the second explosion?

[Sticks]

Why?

[Tog_]

Why?
Why am I asking?
It's for a story I'm working on.

[Jens]

I'm not 100% sure about this, but I think the echo goes out in different directions, like other sounds, so your idea would work if you time it correctly for the person who is to be hearing it. But I imagine you do it more simply by just placing your explosion in between the big explosion and the listener, and timing it correctly, no?

[Neverfly]

Why am I asking?
It's for a story I'm working on.

I think he meant: Why Time it with the Echo?

Why not just blow the charges at the same time as the main blast?

[Michael Noonan]

The sound may get masked or may not if the frequency of the two blasts are too dissimilar. Just as the human ear can pick up a variety of notes in a piece of music the pitch of the two blasts has to be carefully planned as well.

[Michael Noonan]

The sound may get masked or may not if the frequency of the two blasts are too dissimilar. Just as the human ear can pick up a variety of notes in a piece of music the pitch of the two blasts may need to be carefully planned as well.

Do you have experts with specialised listening equipment monitoring the exercise? If so are their detectors at a known point if absolute accuracy is required.

But the biggie I think is that to an explosives expert it does need to sound right. A deep bottom blast for instance may do very little to mask a high ping of metal rupturing. How devious do we need to be heh heh heh ...

[Tog_]

Hmm. Okay. I was trying to ask with out going into the specifics, but it looks like I need to.

Team one is the one with the big boom. They have absolutely no idea that team two even exists.

Team two is using the explosion as a cover to set off their own charge which will blow a hole in a storage tank and disable the automatic clean up system so that the EPA can come out and see just what the tanks had in them. The company that owns the tanks is known to a few as being very evil overall, but has a good public face, and a wall of lawyers that would make both MS and Disney quake in terror. The only way to see what they are up to is an illegal search. The case would never go to trial, but the goal is actually to get the EPA in there to test the chemicals that have spilled to create bad press, and get public opinion to shift. There are a total of 5 people that know about this part of the plan.

As for using the explosion for the timing, They don't know exactly when the first set will go off, so they have a sensor in the water to start the timer after a delay of X seconds to match the interval between the sound in the water reaching them and the sound of the blast itself. They can't use a radio to trigger the timer because it might be intercepted. I'm not sure how long the delay between actually seeing the first blast happen, and the time the actual sound is generated might be, or if it could be determined exactly.

[Acolyte]

If it's in a circular bay, I'd imagine there'd be rolling echoes going on. There'd be echoes from the beach or wall behind the initial explosion following along behind the original crest of sound & wave. From experience, the underwater explosion sound doesn't really transfer out to atmosphere very well so the main detection risk would be underwater sensors.

Something else to think about - underwater pressure waves are a LOT more lethal than air bursts. Water doesn't compress worth a damn - big explosion at a couple of thousand metres is going to have pretty severe effects on those waiting to blow the tank.

You could maybe have them on the far side of the tank from the larger explosions - that would also offer better concealment of their smaller blast.

A question - why are they using explosives? Why not cut into the tank with a thermic lance or similar?

[Tog_]

The tank is above ground, and the area is a walled off rectangle, about 5 by 1 km.

All information about what might be in the tank says that it is some sort of chemical, but they have no idea what its properties might be. There is a very real concern that getting on you would be "bad". The explosive on the tank is a small shaped charge that is intended to split the side just enough to create a persistent leak. It's not enough to blow a huge hole in it or anything.

The rupture only has to look like an accident for the first few hours. They are aware that it will be found to be intentional very quickly, but as long as there is reason enough to get a real investigator on the scene without having to go through the lawyers, it will be a success.