Every surface available I'v herd it really raspy
But some calls just sound like scratching a slate board
And others with a nice sharp break at the end of a rollover
And or a pop not a pop but a pop sound at the end
I have call that sounds really sweet with a double Yelp just before the sharp break at the end
If you already now what your doing just go at it
If you don't know what your doing you might have a good wood pile for the fire