When we are listening to a speaker or to music in an accommodation space, the sound passes directly to our ears. However, sound also passes to the room surfaces and is reflected, eventually reaching our ears from many directions. The distances travelled by the sound by these routes varies, so that we receive the sound from several directions at slightly different times, the reflections being called reverberant sound, and some of the routes involving reflections from several successive surfaces. Sound involves atmospheric pressure oscillations, the frequency of the oscillations giving the pitch of the sound and the amplitude of the oscillations giving the intensity or loudness. If sound from a source arrives by two different routes of different length, the oscillations will be out of phase; if they are half a wave out they will precisely cancel out to destroy the sound, but if they are one wave out they will reinforce and double the amplitude of the sound. This failure of coincidence of sound vibrations by different routes is known as interference and often causes serious distortion to speech and music. Interference can be used to judge the nature of a room, the degree of interference being expressed usually in terms of the reverberation time, which may vary from 0.5 seconds in a living room to more than 12 seconds in a large cathedral. If the time is less than about a second, reverberation will decay quickly with little interference with sound, but a long time will make it very difficult to understand speech, although it may account for the ‘atmosphere’; in a cathedral we can only clearly hear very slow speech because of the long reverberation time, but the ‘echoes’ also account for the ‘atmosphere’ which is characteristic of cathedral music. Reverberation time depends directly on the volume of the accommodation unit, but indirectly on the sound absorption of the surfaces or the area of each different type of surface multiplied by its absorption coefficient; this represents the amount of sound energy that it can absorb at particular frequencies. Obviously, sound absorption is not limited to the structural surfaces but applies equally to introduced surfaces, so that the reverberation characteristics of an empty room are drastically magnified by furnishing, whether it is curtains and carpets or seats and tables, all of which increase absorption, reducing echoes and reducing reverberation time.