Changing parameters of pulseIn() to sense different materials
pulseIn just returns the length of a pulse on a pin. That has nothing to do with materials and can have no concept of materials. It is just time. You cannot change time just to suit different materials.

Instead if you want to calculate the distance depending on different materials you need the actual time of the pulse to then calculate the distance in different ways. 

That is down to your sketch to do that calculation using the time provided by pulseIn and your own adjustment values.