The thing here that is the constant and only rule is physics. Conservation of energy. Converting one form of energy into another. This is why there is no need to test products to predict basic behaviour. You can do the prediction simply. You need to test to determine how exactly the conversion performs, but you know something works or won't work - the variable is how much?
Polystyrene has very specific features. Its thermal performance is well know - to the touch it's always warm, but it's very hard - especially when cut via heated wires that cut and seal by melting the surface. Formed building foam being rougher on the outside than packing foam. Physics lets you determine that one will reflect similar to a hard panel, and the other will difuse because of the number of slightly different angles. The hard surfaces absorb very little and the polystyrene takes up very little of the sound energy as heat. On soft foam, the proportion of sound converted to heat is higher, but we know this is frequency dependent - bass goes right through largely unaffected, HF getting the heat conversion. A membrane absorber is quite hard, which should reflect, but it moves, and the sound is converted to kinetic energy - the vital bit being there's less sound left! Polystyrene because if the lack of mass/weight won't allow this conversion.
As an audio inexpert, my level of physics lets me predict the basic functions but I cannot quantify it, but I can understand what is happening. Plasterboard vs MDF is a good one. Plasterboard seems a good audio product because of how it responds to sound - some reflection, some absorbtion, plenty of mass, but MDF has a hard sealed surface, so has mass and reflection properties. How does painting plasterboard impact on the figures and performance? Don't know from doing it. Acousticians can read and really understand the specs. Less able people like me can only understand the lower tier of data. Good to learn though!
