quently require considerable departures, but it will generally be found that rooms constructed on this plan, if the materials of the walls and the condition of the contained air be secured, are acoustically good.
The Greek and Roman amphitheatres, having the audiences arranged in semicircles, each circle rising above the one in front of it, and having the auditorium either open at the top or covered with awnings so that the waste sound might easily escape at the top or be absorbed by the audience around the sides, were crude approaches to this plan. The modern theatres, in which the floor slopes upward as it recedes from the stage, and in which the balconies are placed one above the other and are of horseshoe form, conform still more closely to it.
Other considerations frequently demand that music-halls and churches shall be square or oblong in shape, and it must not be supposed that acoustic success can not here be obtained. But, in these forms, great care must be taken to avoid large reflecting surfaces, and, by means of paneling or other devices, to absorb fully the waste sound.
We have now examined the two important features on which acoustic success depends. The atmosphere must be in such condition as to best allow the natural diffusion of sound; and, further, the walls must be of such material, and so arranged, as to absorb as fully as possible the waste sound. It now remains for us to look at some of the minor points which contribute to acoustic success.
There is one danger to which buildings having a vaulted roof are peculiarly liable, and that is, that the roof, if constructed of proper curvature and of non-absorbing material, is apt to act as a great concave mirror to gather up waste rays of sound and reflect them back to a focus somewhere in the audience, and so produces a loud and disagreeable echo. The architect can not exercise too great care in selecting absorbing materials, and in so arranging them as to prevent this possibility. A change in curvature, or breaking by transverse arches, will often do this. This focus would be a small area in case of a dome; but, in case of an arched roof running from front to rear, it would be a straight line.
It often happens that churches constructed without regard to acoustic principles are found, when completed, to possess this fault in a striking degree. The only complete remedy in such cases is to entirely replace the ceiling. It may, however, often be largely alleviated by placing the pulpit in a different position, as near one corner or against the wall half-way. down one side. Sometimes the fault may be largely remedied by using a reflector to throw the sound out toward the audience and prevent its going up toward the roof.
Reflectors, or sounding-boards, should be used only with judicious care. Their object is not, like a concave mirror, to gather the rays of sound and throw them out to a focus in the audience. When so constructed they have been found to do more harm than good, especially