Alot of it depends on what the builder is trying to achieve or what the customer wants. I know in my trough calls, glass is a bit more raspy, but Cherry produces a better Yalk, There is endless combinations in pot calls, and the pot itself has it's own character sound. No 2 woods sound the same. Then everything you do to it, can change the sound also. In the end, it's not so much about pitch, but the vocabulary you produce on the call that the bird can identify with. Think of it like as if you were suddenly dropped in the middle of China. You still can talk with your voice and so can the local Chinese guy, but your not gonna understand each other. Ya have to talk the language on the call.