Sound, for example…
The characteristic acoustic impedance (specific acoustic impedance?)…
Z = ρv
is important when calculating the amount of reflection and transmission at an interface. For the reflected portion, notice that when Z2 < Z1, the sign of the reflected portion is negative, which indicates a change of phase.
obstacles and openings
Diffraction is the bending or spreading of light through an opening or around an obstacle.
Goethe "flexibility of light"
Why is it possible to hear around corners, but not see around them?