Well, Ed, it just so happens that I am an expert at relative loudness! You're in luck! Fortunately for you I like to hang out in the Songwriting Forum because its quieter in here.
My favorite book on realtive loudness is called "Ravens in Winter" You'd be amazed at how subtle changes in volume affect the kinds of communication taking place for ravens! They are really quite complex creatures! Of course, to the untrained human ear they all sound loud- but to the ravens themselves it is all relative.
My other favorite book that highlights the principle of relative loudness is "Tom Brown's Field Guide to Nature Observation and Tracking." You see, when stalking an animal you can often mask the sounds that your feet and body are making by moving with the wind and other natural noises. This allows you to make use of the relative loudness of the natural sounds to mask your own sounds. Viola! The animal cannot hear you because your sounds are wearing a mask and it doesn't recognize them.
Good luck in your studies! Feel free to email me if you need help understanding any of these basic concepts!
Chris