On mythbusters, they placed a rectangular block of 1 lb of C4 in various locations. They tried setting it off out in the open, and then put their burst disk at the distance from the explosion where it had burst previous. Unfortunately, they did not place additional burst disks at closer distances to determine how much protection a particular method offered.
Anyways, they put the explosive under a mattress and set it off. My rough mental model of an explosion says that shock waves will reflect off of any rigid surface. So had the “mattress” been a steel plate at that height, the shock wave would have been concentrated in the lateral directions away from the plate.
Since a mattress is a material with elasticity, instead it seemed to soak most of the explosion’s energy? Why?
In oder to cause damage off the side of the bed, the explosion has to lift up the mattress somewhat. Directly above the bomb, the force is pushing straight up. So the middle goes up easily. But off to the side, the angle is crappy. It takes a lot more force to lift the mattress the same distance.
Add in the additional time it takes for the explosive force to get to the side of the mattress and you get the nice demo they had on the show. The middle flying up takes the brunt of the force. (The downward force is also wasted pushing the springs and frame down.)
Don’t forget that some of the explosive force also went in to lifting the mattress up off the bed and mattresses are pretty heavy, as anyone who has tried to carry one up a flight of stairs knows, so a fair amount of energy was absorbed there as well.
Another thing to note is that a lot of tests on the Mythbusters, with explosives, aren’t a very good test. They use raw explosives, which are sort of like highly compressed air. When exploded, they create a very suddent release of gas, like a bursting balloon. It creates a pressure wave that can move some stuff and rupture your eardrums (or lungs), but is pretty easily stopped. As they demonstrated in another test, just hiding behind a cheapy table, on its side, will save you, because the pressure wave goes up and over the table.
When someone wants to make a bomb to kill people, they generally surround it with nails, ball bearings, and such. Right next to the blast, they’re accelerated into thousands of tiny bullets that go out in every direction, slicing through everything. That’s where bombs become really dangerous.
I suspect that they’re aware of this and don’t really want to demonstrate how to make real and effective weapons of terrorism on their show, so we’ll probably never see them test how a mattress holds up to a real bomb. I suspect that the mattress would still do a reasonable job, but I’d still rather be in a position where I don’t think anyone has a reason to kill me, than have mattresses strewn all over my house, just in case.
I think there might also be the billiard ball effect. Because a ball is rigid, the kinetic energy in a moving ball is all transferred into the struck Ball B. If there is a Ball C, touching Ball B, all the energy passes through Ball B and is carried by Ball C. If billiard Ball B had been a nerf ball, Ball C would barely move, since Ball B would have absorbed most of the energy. like a mattress…