Some buildings such as ancient tombs produce echo of whatever sound you make inside the building. Sometimes you can hear the same sound being echoed many times over. But if a programme of music or a lecture or a play is taking place in such a building, it will be very difficult to hear what is going on if too much echo is being produced. To avoid this cinema halls, lectures halls, auditorium are designed in_a special way so that the sounds produced can
be clearly heard by the audience.
The branch of science that studies and deals with this aspect of sound is called architectural acoustics. The fact that some materials such as plaster, reflect sound well while some others like cloth absorb sound, is taken into consideration while designing these buildings.
Echo and reverberation are the two important properties of sound. A reverberation is defined as a close group of echoes. When a sound produced inside a hall reflects back after hitting the wall we hear an echo,