L. MECHANICS AND ENGINEERING. 519 



inserted in the top will show the effect of contraction of the 

 top of a chimney. 16 (7, yi., 66. 



ACOUSTICS OF PUBLIC BUILDINGS. 



The acoustics of public buildings has been lately studied 

 very systematically by Orth, who, however, in general but 

 confirms the results of the studies of Langhaus, w^hose w^ork, 

 " Ueber Theater," etc., published in 1810, seems as yet to 

 have been but little improved upon. The investigations that 

 these architects have made into the acoustic properties of 

 rooms have led to the following conclusions : 



1. The simple physical laws controlling the reflection and 

 intensity of sound are alone necessary to be considered. The 

 interference of sound-waves is a phenomenon of minor im- 

 portance. 



2. The concentration of the sound of the speaker's voice, 

 within a small space, is of less importance than the suppres- 

 sion and diffusion of disturbing noises and reflections. 



3. The chief cause of the poor acoustic properties of a large 

 room is a reflection of sounds, such that they reach the hear- 

 er's ear, by a certain small interval of time, later than the di- 

 rect voice of the speaker, and effect the ear as does an echo. 



4. The acoustic properties of a room can be represented 

 graphically, on a diagram, by tracing out the reflections, and 

 entering for each point numbers showing the strength of the 

 sounds, both direct and reflected, and lines showing the in- 

 terval of time between them. For the time-scale may be 

 substituted a scale of feet showing the diflerence in the 

 leno'ths of the routes of the various sound-waves. 



5. A difference of routes of fifteen to twenty-two feet not 

 only exerts no disturbing influence, but may even be favor- 

 able, by strengthening the direct by the reflected sound ; in 

 feebler sounds the difference may be even somewhat greater ; 

 but a difference of thirty feet is, by all means, to be avoided. 

 On the other hand, if the difference is 185 to 215 feet, the in- 

 tensity of the reflected sound becomes so weak that it may 

 be disregarded. 



6. The ceiling which, in theatres, may be acoustically ad- 

 vantageous, is in lofty church edifices of much less impor- 

 tance, and may even be disadvantageous by producing audi- 

 ble echoes. It is, therefore, important to destroy the reflec- 



