Sounds like a simple enough question but I slogged through several search references that in the end didn’t actually answer the question. Anyone?
This paper (PDF) tests sound propagating though granular solids including sand (starting on third page); they give a value of 110 meters per second for sand, sound also propagates through particles that are touching each other, which happens to be a relatively small number of particles (in continuous chains); louder sounds and different frequencies will give different results since the grains can shift.
The reason I ask is this: concrete and brick can be surprisingly poor protection against high-velocity bullet rounds, to the extent that high-powered rounds like the Barrett .50 cal have been used to shoot targets who thought they were safe behind a solid wall. Whereas the ordinary sandbag is extraordinarily effective at shielding against high-velocity rounds but can sometimes be defeated by slower but heavier bullets like handgun rounds. The Box O’ Truth (http://www.theboxotruth.com/) website covers this in detail.
I was wondering if the determining factor is that sand has a slow enough speed of sound that high-velocity bullets are effectively hypersonic through sand- causing their energy to rapidly dissipate. This hypothesis fits a few other observations: another material with a low speed of sound is rubber, with values quoted between 40 and 150 meters per seconds; shooting ranges sometimes use old airplane tires as a backstop. Concrete and brick have sound velocities between 3000 and 4000 m/s, well above most bullet speeds. While handgun rounds are already fairly slow and once slowed to the speed of sound through sand their penetration is based on their mass, which is often greater than some rifle rounds (5.56 NATO = 63 gr vs. .45 ACP = 185 gr)
The fact is also critical in many border-alert ground devices, I would imagine.
‘Sand’ describes a lot of granular substances that may or may not be mixed with other substances including water, and packed to different densities. Pretty hard to establish a precise speed of sound that way.
I was thinking that it has more to do with the fact that sand is made up of many small particles and disperses energy pretty well, same for rubber which is elastic, as compared to concrete or brick which will shatter. Another comparison is a ball pit (also a bunch of particles); you wouldn’t want to do this without something to absorb the impact.
Ok, so I was looking up what factors determine the speed of sound through various materials. Density is one- the more dense, the slower the speed. Two others are elasticity and several somethings called modulus (sheer, bulk, etc.). I gather that in layman’s terms, how “springy” the material is, the faster the speed of sound. My new question is this: does this mean that lead putty would have an extraordinarily slow speed of sound?
How thick are sandbags versus concrete and brick walls? Especially walls ones that may have air holes in them. Since sand and concrete and brick are of a relatively similar density, maybe this is the determining factor, and the heavier bullets can get through from sheer momentum.
Perhaps the explanation for rubber backdrops is that they may be more forgiving and less likely to shatter versus concrete and brick rather than having stopping power per se.
Exactly, the speed of sound is determined by density and springiness, as measured by bulk modulus, specifically the square root of the ratio of bulk modulus to density.
As mentioned above, it will depend on the composition of the sand and what fills the space between sand particles, for example air, water, clay, etc.
Slight :nitpick:
Whenever old tires of any type are used as a ‘backstop’ at shooting ranges, all of the ones that I’ve seen were filled with sand/soil.
I’ve seen videos of ‘shoot houses’ that were partially constructed with sand filled tires.