155. Introduction. -Throughout the previous chapters it has been pointed out that the aim of sound
reproduction is to produce sound similar to the original music or speech as near as possible, that is, to
obtain an exact likeness of the original sound. The conditions necessary to obtain good sound
reproduction in an auditorium are:that the sound should be of sufficient loudness in all parts of the
auditorium, that the original quality of sound should be maintained, that the successive sounds in rapid
speech and music should be clear and distinct, and that extraneous noises should be absent.
Obviously, to fulfill the conditions necessary for ideal sound reproduction requires that all forms of
distortion introduced by the studio acoustics, recording equipment, sound film, reproducing equipment,
and house acoustics be negligible. The importance of good auditorium acoustics has become fully
recognized since the advent of talking motion pictures.
Our everyday dependence on sound has made the observation of the more usual acoustic defects in
auditoriums the common experience of everyone. Each of us at some time or other has noticed an echo,
a distinct repetition of sound, from a hillside or the wall of a distant building in the open, or, more often,
from a high curved ceiling or a rear wall in a theatre. Likewise, everyone has experienced reverberation, a
blurred prolongation of "after" sound, when talking or shouting in a hard tile walled swimming pool or in
an empty hard plastered corridor or in a completely marbled lobby. Again, each has noticed the
phenomena of resonance, the tendency of certain notes to be over-emphasized, in a powerfully resonant
room, such as a small tiled bathroom or small marbled alcove.
It is the purpose of this chapter to discuss the more general acoustic phenomena in theatres, and give a
brief picture of the general requirements of a theatre with ideal acoustics.
156. Review of the Nature of Sound. -Before discussing the action of sound in a room let us review the
fundamentals of sound studied in the first chapter.
Sound is a form of wave motion, and as such has definite characteristics. Upon the amplitude of the
wave motion or vibration depends the loudness of sound; upon the frequency of vibration (cycles)
depends the pitch and quality of sound; and upon the relation existing between the direct and reflected
waves depends the intelligibility of speech sounds, and the clearness of musical sounds.