Room volume is not dependant on what again?
Are you mad?
Sound is about the movement of air and the molecules that make up air. Sound is very much a volume related concept just as it is distance. Energy is the key word here.
Reverberation plays a large factor as the energy from those air molecules hits and reflects off of walls. The surface material then comes into play before you can make any calculations about how effective a reverberation effect will be in any given room. The preceding assumption on reverberation effect seems to only account for bare walls. Check that assumption at the door.
As one moves further away from a sound source, the SPL decreases. Why?
Molecules lose energy often through heat. They lose energy by colliding with other molecules and transferring a partial part of that energy in the process, the remaining is lost as heat (sometimes light depending on the molecules colliding). In a larger room, the amount of energy lost (and number of molecules involved) by the time a molecule hits a reflective surface will determine the amount of energy remaining by the time that sound wave hits the ear. Larger room, more volume, greater distance to each surface, more molecules available for pushing (in numbers, not density but numbers relate to a linear distance nonetheless), more resistance to an endpoint (again in a linear concept) as well. A small driver can move x # molecules per sq area, less than a larger driver of same design. Sound is about moving air and i'm sorry, but a larger driver in a larger speaker moves more air. It can "fill" a larger room with sound much easier than a small speaker with small drivers simply because it can move more air molecules. This requires more energy to do.
Easy home experiment.
Get in the tub.
Take a spoon and a lid for a 4L ice cream pail.
Push both through the water and tell me how much water moves (wave size) in your tub and how much resistance energy you require to push each of them. Yet, notice, do waves from both of these actions still make it to the wall of the tub? Of course, but at what size? At what equivalent uses of energy?
Air in rooms is finite (when sealed). There is always resistance.
Basic physics.
