Previously, I posted a story about Cats Do Control Humans, Study Finds. Now I have found audios that will give you an idea of the difference between a soliciation purr and one that has no solicition element in it.
click the link to hear the audios: http://www.lifesci.sussex.ac.uk/cmvcr/Domestic%20cats.html
I have noticed the difference when Flip begs for food, his meowing sounds like a combination of purr and cry as well as something else that I can't quite wrap my brain around. I can definitely feel urgency in the purr-cry which makes me want to give him the attention he demands for. Normally when I am not feeding him, his voice is more crisp and softer.
Did you know that: Domestic cats purr in a frequency of 25 to 150 vibrations per second. (Wikipedia)