Fibers in fibrous structures have some level of orientation in machine — or cross­direction and parallel to material surface. Hence, fibrous materials are inherently anisotropic and the propagation constant, k, and surface impedance, z1, changes ac­cording to the direction the sound waves propagate. Sound waves must be permitted to enter the material in order for the material to be able to absorb the sound rather than reflect it.17 Fibrous mats with fibers arranged vertically to the surface, such as needle-punched products, allow sound to enter the material.69 Accordingly, fibers ar­ranged perpendicular to the material surface may be placed to face the sound source.