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ANef_is_Enuf's avatar

How can an explosion feel more intense to someone that is farther away?

Asked by ANef_is_Enuf (25249points) August 25th, 2010

A few years ago a house blew up nearby.
At the time my sister was living in an apartment building not far from the explosion site. She said that they heard the blast, but it was faint, they couldn’t feel anything, yet they could see the smoke and fire.
I happened to be just 1–2 miles further up the street when the house exploded. On the contrary, the whole house shook. It was so loud we were shocked… in fact, we immediately thought a car had run into the house. It was that intense.

So if my sister was much closer why is it that they hardly felt/heard anything, while a mile or two away we were being rocked out of our seats?

Is this a stupid question? I can’t figure it out for the life of me.

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13 Answers

FireMadeFlesh's avatar

The only reason I can think of is that the apartment block is much more stable than your house. Houses generally don’t require the same level of engineering that larger buildings do.

ANef_is_Enuf's avatar

@FireMadeFlesh ah, it wasn’t my house, actually. It was my grandmother’s. That would make sense, and it may not be off base, but all of the neighbors felt what we did. We all thought that it happened to us. It wasn’t until about an hour later that we heard about the explosion.

FireMadeFlesh's avatar

@TheOnlyNeffie Are the houses in the area old?

ANef_is_Enuf's avatar

@FireMadeFlesh how old is old? Most of the houses in that particular development were built between the mid 70’s and now.

LuckyGuy's avatar

In free space the pressure amplitude of an explosion s drops off with the distance ^3. That means for example if you are 2x the distance, the pressure is ½^3 or 1/8. So, the apartment house got hit with much more pressure.
The apartment however is built with walls and beams strong enough to support a hold the entire building during heavy wind and even an earthquake or two. That means the walls are probably poured reinforced concrete. a foot thick (not sure what code requires in your area) Then the concrete is covered with different layers of waterproof material, Stud walls, insulation drywall. . In the inside. On the outside it might have brick, insulation, siding etc. All of this makes for something very rigid in not apt to move or pass low frequency vibrations easily.

Your house walls only have to support one relatively light attic. You might have a concrete block basement but the walls are most likely 2×4 or 2×6 studs with plywood on the outside, covered with 1” of sheet insulation and siding. On the inside you have insulation and dry wall. It is very flexible and allows low frequency to pass through. That is why those damn motorcycles are so loud when they pass by.

Also there is a chance the pressure wave was channeled by neighboring houses. That has an effect too.

ANef_is_Enuf's avatar

I love this site.

Thank you both, I feel so much better now that I have an explanation.

FireMadeFlesh's avatar

@TheOnlyNeffie I have no idea when laws for these things started to change, but people only really got interested in safety (as far as I can tell) in the 70s. My guess is that houses built before 1980 would be worse than those after, but that is only a vague guess.

@worriedguy also has some great points. I would imagine that a city street would channel the blast to some extent so the force dissipation would be less than the theoretical r^3.

stranger_in_a_strange_land's avatar

@worriedguy has it. Light frame construction of single unit houses and the “wind tunnel” channeling of the blast effect.

CyanoticWasp's avatar

@worriedguy pretty much nailed it.

Another thing to consider is that not all explosions radiate equally. For example, an explosion that happens in the front of a building (think Oklahoma City, Murrah Federal Building) will be shielded by the rest of the building, so that people and structures in back will experience less… and structures and people across the street may be leveled, as the building reflects the force of the explosion out toward the front.

So depending on the physics of “the house that blew up”, it’s not surprising that the effects are felt differently in different locations and distances.

ANef_is_Enuf's avatar

This has been so informative. :)

I have been baffled for a long time, I even doubted what I felt for a while there.

Austinlad's avatar

Now that I sort of understand the answer to the question, my question is—why did the house explode ???

ANef_is_Enuf's avatar

@Austinlad it was vacant and (like most vacant houses in this area) had been looted. All of the pipes were stolen, which somehow lead to a gas leak.. and something ignited. Kaboom. It was very destructive, there were significant damages to neighboring houses within a block radius.
The details are and always have been sketchy. Rumors of a methamphetamine lab run rampant.

Austinlad's avatar

Thanks, @TheOnlyNeffie. I was thinking you might have been in the IRA or something. ;-)

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