While a plain blank wall opposite the speaker will throwback a strong reverberation, if this wall be broken up by recesses, or spaced with pilasters, or if a gallery be extended across it, the reverberation will be much less.
The lining of the walls should, if possible, be placed at a considerable distance from the main wall, and supported by it so as to allow as free vibration as possible. Thus when other considerations require that the walls of a building shall be of stone or brick, the acoustic qualities may be recovered by lining up with thin pine, corrugated iron, or a thin coating of plaster on light laths, and suspending this lining as lightly as possible and at a considerable distance from the solid wall. The arrangement of walls in panels is a very advantageous one, as each panel may be so constructed as to be easily set in vibration. Perhaps the ideal arrangement would be an auditorium with brick walls, within which is a shell made up of thin wooden panels, and placed at such a distance from the solid walls that the passage-ways to the entrances on the floor and various galleries maybe placed between.
In this connection it becomes necessary to discuss the proper shape to be given to an auditorium. A good way to arrive at this is to consider first an audience in the open air on a calm day. The open air on a quiet day, when the atmosphere is not disturbed by convection currents, is probably the best possible place for speaking to a large number of people. Wesley is said to have preached successfully to twenty thousand people gathered together in a natural amphitheatre formed by the hills, and on a day when the atmosphere was at rest.
Let us take a small platform arranged for an open-air speaker, and notice how an audience will form itself about it. If the audience is large and each person anxious to hear, we shall find that the outline of the crowd will be that of a section cut through an egg, with the speaker placed at the focus of the smaller end. As in an auditorium we trust to the natural diffusion of sound to absorb the stray rays, we should evidently adopt this same ovate section for the outline of the floor.
But, when we come to consider further that it is desirable to place each member of the audience so that he can see the speaker, and so that the speaker's voice may come directly to him, we see that we conform still further to the egg-shape, for, in order that we may do this, the floor must curve upward as it recedes from the speaker, and the galleries form only a continuation of this curve. To that we may say that the proper shape of an auditorium is in general that of an eggshell, the speaker being at the focus of the smaller end and the audience being seated over the lower half, while the upper half forms the vaulted roof. Like an egg-shell, we have seen that the walls should be thin and capable of absorbing, as fully as possible, all the stray rays of sound. While the egg-shape is the ideal, other considerations fre-