PUBLIC ADDRESS SYSTEMS 115 



The troubles encountered from echoes usually occur only in large 

 buildings or large open spaces surrounded by buildings, trees, or other 

 obstacles and are generally associated with interferences with the 

 reproduced sound rather than with the original sound. There are 

 cases, however, particularly in auditoriums, where some of the walls 

 or ceiling are large curved surfaces, in which case localized echoes 

 may result. The speaker's voice or extraneous sounds from the 

 audience may be reflected from one or more of these surfaces to focus 

 spots where the volume of sound is consequently abnormally great. 

 It is important, therefore, that the transmitter which is picking up 

 the sound shall not be located at one of these spots. These points of 

 localized echo are particularly troublesome also when they occur 

 in the space occupied by the audience. Under these conditions not 

 only is the sound intensity too great, but the character of the sound 

 is altered and very often badly confused. The avoidance of such 

 difficulties is a matter of test and the proper arrangement of the 

 reproducing mechanism, as will be seen later in some detail. 



The effect of resonance seldom occurs in connection with the am- 

 plified and reproduced sound, inasmuch as the spaces dealt with are 

 large and their natural frequencies are too low to be troublesome. 

 Resonance usually becomes of importance in connection with mount- 

 ing the pick-up apparatus or transmitter. It generally results from 

 attempts to conceal the transmitter by placing it in some form of 

 small enclosure. The best form of housing from an acoustic stand- 

 point consists of a screen cover which protects the instrument from 

 being struck or injured but in no way affects the sound reaching it. 



Resonance produces a distortion which it has been customary to 

 consider as of two varieties. First, there is an unequal amplification 

 of sounds of various frequencies and second, there is the introduction 

 of transients. These transients occur whenever the sound changes 

 but are most easily recognized audibly by their continuation after 

 the source has ceased emitting. They also have frequency character- 

 istics which depend not only on the sound which started them but 

 also upon the character of the resonant portion of the system. 



The troubles introduced by diffraction are seldom of very great 

 importance except where the sound is reflected from regularly spaced 

 reflectors or passed through regularly spaced openings. Quite serious 

 diffraction troubles have been encountered when operating a loud 

 speaker in a large field, surrounded by an open work board fence, the 

 trouble being evidenced by very distinct areas, particularly at the 

 outskirts of the audience, where the sounds were badly distorted. 



