ACOUSTIC ARCHITECTURE. 461 



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 ar- 

 ranged 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 sup- 

 posed 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 acous- 

 tic 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 con- 

 cave 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 dis- 

 agreeable echo. The architect can not exercise too great care in se- 

 lecting 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 acous- 

 tic principles are found, when completed, to possess this fault in a strik- 

 ing 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 audi- 

 ence 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 con- 

 structed they have been found to do more harm than good, especially 



