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 desirable 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 arrangement 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 choosing 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 apparent 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 absorption of the sound is, however, sometimes effected by draping heavily with cloths; but it has been found by experiment that there are other materials,