I had a DBX unit when I was a baby, it would give a pumping techno music effect to everything. I use the Sonos software now, and that's a whole new set of level issues, because you have soundcard in and out plus software in and out. On top of that, some sound cards have limiters built in, and you have to know whether that's on or off.
When research is done on Wikipedia regarding audio processing, the list goes on and on with the research. I could sit here and post roughly a minimum of 100 links that cover the subject.
I've found some pretty educational sites as well while researching Wikipedia. There are a lot of resources out there. I found an interesting site, or several to be honest that offer a wealth of education.
Let me share a few links if you don't mind.
This one is to a web site that has an Mutimedia Audio Course that you all might find worth reading through. Here's the link to that, you need to scroll down the page a bit: http://www.audiosonica.com/en/course/post/2/Index
The next two sites have resources for audio testing
This has some white papers explaining audio testing. I found it to be educational in learning more about audio. You have a choice of several white papers, download all that cover your interest.
http://www.ap.com/download/whitepapers
Perhaps, when I'm done, I'll write one of my own using simpler terms and offer it here as a new resource.
There is so much to learn and share, the fun part, is sharing what I've learned.
Bruce.