Have you ever walked in an empty house, or listened to yourself singing in the bath? In either case it is hard to recognize common sounds. Usually, much of the energy of sound waves is absorbed by curtains, carpets and other soft furnishings. When these are absent the sound is reflected strongly from the walls, floors and ceilings, and echoes are heard. This can be quite a problem in designing theatres, opera houses and concert halls.

Whether an auditorium is "good" or "bad" for sound, which means whether or not it possesses balanced resonance, is another factor which very much determines, or rather modifies, the tone quality of instruments and voices. This was instinctively felt by many composers and performers of the past, for instance Bach, who used to clap his hands and count until the sound was absorbed in order to get an approximate idea of the acoustics of a place where he had to play. But not till the late nineteenth century was a scientific explanation found for this phenomenon. Now we know that whether or not an auditorium is good for sound depends on the length of its "reverberation period" (which means the length of time a sound takes to die away). Experiments have shown that the most convenient reverberation period for speech and music is between about 1 and 2 1/2 seconds.

Scientists improve the acoustics of an auditorium by using building materials which absorb sound waves at the correct places. They also shape the walls and ceilings to prevent an undesirable reflection being sent to the audience. The acoustics of a hall can also be modified by various devices, such as putting up or taking away hangings which absorb the sound.