What are the audio differences you hear with turntable Mats.

None

I selected my current mat by amplifying the weak spot of turntables and listening for which mat did the best job of reducing the audible effects of that weakness. 

When you know what to listen for and how to do it, selecting a mat becomes easy if you buy a few different ones to test.

the differences in sound are quite significant.

Thanks. Useful info

I concur with avanti, based on personal experience. I settled on Hexmat Yellow Bird on acrylic platter of Clearaudio, brings out more details yet not harsh and fatiguing.