45 8 THE POPULAR SCIENCE MONTHLY. 



C (second balcony). 8 to 8.50, good ; 8.50 to 9.20, bad ; 9.20 to 

 10, good. 



D (second balcony). 8 to about 9, good ; 9 to 9.20, bad ; 9.20 to 

 10, good. 



An examination of the foregoing data can leave very little doubt 

 that the condition of the air within an auditorium exercises a very 

 considerable influence upon the facility and accuracy with which sound 

 is conveyed. 



The Academy of Music, in Baltimore, is an example of how a de- 

 sirable condition of the air may be obtained. It must not be supposed, 

 however, that acoustic success depends entirely upon the condition of 

 the air. In fact, the condition of the air is a matter quite secondary 

 to that which next comes up for discussion the material and arrange- 

 ment of the walls. 



It is not uncommon to find churches or halls built nearly square, 

 with a speaker's desk at one end, and a bare wall of stone covered with 

 plaster opposite. If one goes into such a room when it is empty and 

 speaks from the desk, he notices a loud and disagreeable reverberation 

 following each syllable, which, if the room be large enough, comes 

 back to him as a distinct echo. When the room is filled with people, 

 this resonance or echo will considerably decrease ; but, in such a room 

 as we have described, it will not by any means disappear. 



If now we choose a similar room, but with walls sheathed up with 

 thin boarding or a thin layer of plaster laid on to light laths and 

 having a free air-space behind, we shall, in all probability, find that, 

 when this room is filled with people, the echo or reverberation will 

 have almost if not quite disappeared. This is a comparison that the 

 author has often made, and it gives the cue to the whole art of choos- 

 ing the materials of which the walls are to be built. They must be 

 built of such material and arranged in such way that they shall absorb 

 and not reflect sound-waves falling upon them. 



When the speaker utters his first syllable, the sound goes out in 

 straight lines from his mouth to all parts of the house. So much of it 

 as goes directly to the ears of the audience is effective, but all the 

 other rays of sound ought to be as completely as possible absorbed 

 and destroyed, else they will be reflected from the walls and ceiling 

 back to the audience ; and, arriving at their ears somewhat later than 

 the direct sound, will give rise to the confusion or echo whose ap- 

 parent effect is to bridge each syllable of the speaker over into the 

 next, and so cause apparent indistinctness of articulation. 



The presence of an audience in a room causes the absorption of 

 such words as would otherwise be reflected from the floor, and thence 

 to the walls, and so back and forth. But it is out of the question to 

 cover the walls and ceiling with an audience. The absorjrtion of the 

 sound is, however, sometimes effected by draping heavily with cloths ; 

 but it has been found by experiment that there are other materials, 



