FlipFlopRob

I've been reading up papers about mapping sounds to their auditory representation, and I'm a bit confused about one bit. They've given some diagrams of sound waveforms spectral with the y-axis being spectral frequencies, and the x-axis being temporal modulations What are temporal modulation frequencies in this case? I've been trying to find a simple, down to earth explanation on the net, but there seems to be a few different answers, and I'm not quite sure which one it is, if any. Here is an image of what I'm on about. http://openi.nlm.nih.gov/imgs/rescaled512/2700967_pcbi.1000436.g007.png My guess is its the average frequency of oscillations in a sound? But I'm still really confused! If someone could give me an answer which an everyman can understand, I would be most grateful! "Spectral frequencies between 2500 and 5500 Hz, and temporal modulations in the range 1–16 Hz" - if that helps. Thank you!