The number and strength of loud speakers, their relative location, and the radiated sound spectrum must be examined relative to acoustical principles. It follows that uniform distribution throughout a room is not only uneconomical but does not lead to best audibility. In view of interfering sounds, formulae are developed allowing computation of the required number and strength of loud speakers and their most favorable location, and the intensity for each observer. It is further shown how the absorption in air is to be considered. Using different sound spectra in loud speakers, there arises a greater total loudness than would be computed on a straight energy basis. Very large auditoriums do not follow the generally accepted acoustical laws. Short examples show the practical use of the formulae.