On the reflexion of sound from a porous surface

When sound-waves impinge on a plane surface forming the boundary between two media of different mechanical properties, part of the incident energy is reflected and part is transmitted as a refracted wave, the sum of the energies of the reflected and refracted waves being equal to the energy of the incident wave. The proportion of the incident energy which is reflected depends not only on the mechanical properties of the two media, but also on the angle of incidence of the primary wave. If the first medium is air and the second a liquid or continuous solid, by far the greater part of the incident sound-energy is reflected. For example, if the second medium is water, 99·9 per cent. of the energy of the primary wave is reflected at normal incidence, while if the angle of incidence exceeds 13 degrees the refracted wave becomes evanescent, and total reflexion occurs. If the second medium is not continuous, but of a porous nature, a somewhat different state of affairs ensues. The boundary between the two media is no longer well defined, for the first medium (air) can penetrate into the pores of the second medium and reflexion is then accompanied by a motion of the air in and out of the pores. Examples of porous substances of the kind con­templated are the various “acoustic plasters” produced in recent years for the rapid absorption of sound in auditoria, when the reverberation is found to be in excess of that demanded by good taste. Other examples, of a rather different sort, are hairfelt, carpets and similar materials.