The best advice I received was to try and make tree yelps, the softer the better. My tendency was to have way to much tension in my jaw, mouth, and lips. Making the softer sounds helps you to realize how little effort, air, and lip pressure it takes to get good results. I can't explain the acoustics, but a quiet mellow cluck and yelp on a trumpet in the house or truck will translate into a commanding yelp or cluck in the woods that reverberates and echos like no other call I have used. Del's method is also very helpful in getting the front end on a two note yelp.