Several branches in physics have led to the concept of walls which characterize a difference in state of adjacent regions in a medium. Examples are thenaturalBloch walls separating the Weisz domains in ferromagnetics or theartificialporous powder props which are used in the experimental technique of heliumIIfor separating ground state and excited components of the fluid. Another almost trivial example is a mirror (in empty space) intercepting an electromagnetic disturbance. All examples, whether natural or artificial, have the common feature of being impermeable for the state at either side of the wall. The constituent particles of the medium (if any) pass through on condition that they change their state. This suggests, in analogy with the well‐known semipermeable membrane in classical physics, the definition of another semipermeable membrane or wall, which is permeable for matter (if any) but impermeable for its excitations. An analysis of the dynamics of such membranes provides a coherent picture of the classical concepts of acoustic and electromagnetic momentum together with their nonclassical counterparts: phonon and photon momentum. It leads in addition to an interpretation of the photon concept in a material medium, which corresponds to Minkowski's definition of electromagnetic momentum in matter.