It should be noted that t and r are not restricted to the range of 0 to 1. If Z2 is less than Z1, r will be negative, resulting in a change of the sign of the reflected wave. If Z2 is greater than Z1, t will be greater than 1, resulting in a greater pressure transmitted than incident on the interface. While this may initially be counterintuitive, it does not violate conservation of energy and is a natural consequence of the continuous velocity condition at the interface. A qualitative explanation is that Z2 is the 'harder' material and Z1 is the 'softer' material. The displacements at the interface must be equal due to the boundary conditions, but an equal magnitude displacement will result in a greater acoustic pressure in the harder material.