It really depends on what bird you are after. If your trying to call fall gobblers you want to yelp, deep and raspy and slow. Only 2 maybe 3 yelps in a series.

If your hunting a family flock of hens and poults, you either want to sound like a lost poult or the dominant hen. Lost poults will yelp and kee kee, adult yens will run assembly yelps, yelps and cuts to another dominant hen. The best method of course is to bust a flock and then call them back together but if hunting from a treestand, try the other calling technics I mentioned above.