Les,
In a sandwich, is it better to vary densities accross layers or will it not be significally different from a repeat of layers? In other words, would steel/bubblewrap/steel/bubblewrap/steel be equivalent to steel/rubber/lead/bubblewrap/aluminium?
I am sorry, I have been careless. The issue is actually impedance matching. Impedance is the medium's density (ie, mass/volume) x speed of sound in the medium. The speed of sound in a material is the square root of (stiffness divided by density). Broadly, denser materials are stiffer (steel is denser and stiffer than wood, eg), but relatively dense but very un-stiff materials, such as rubber, or sorbothane, can have much lower impedance than somewhat denser materials that are much stiffer.
The principle, however, is the same for electricity and sound, eg: where you want to minimise interface losses, as with power transmission or telephone lines, you match impedances on either side of a junction. Another example of matching is when you have an ultrasound examination and they put jelly on your skin: the jelly has impedance similar to tissue, so it eliminates what would otherwise be a severe mismatch at the probe/air and air/skin interfaces. And you have those little bones in your ear to impedance match the eardrum to the fluid-filled inner ear, because if there were two air-tissue interfaces you would hear poorly.
To maximise losses you want the biggest possible difference between impedances, and you want as many interfaces as possible. Because air is so light its impedance is very low, so a lot of air to something else interfaces is good.
