ACOUSTIC ARCHITECTURE. 459 



more conveniently handled, which answer the purpose much better. 

 A sheathing of thin pine-wood, lightly suspended, particularly if there 

 be a large and free air-space behind it, will absorb sound very com- 

 pletely. 



There is a very great difference in the absorptive power of different 

 wall materials for sound. Walls of stone and brick absorb hardly any 

 of the sound that falls upon them, but reflect it nearly all ; while walls 

 of thin and dry pine-wood absorb a very much larger proportion and 

 reflect comparatively very little. In order to determine the absorptive 

 power of different wall materials for sound, the author has made bold 

 to extend the general principle in radiant energy, that " bodies which 

 give out rays most readily when excited absorb them most readily 

 when exposed to their action," to such rays as we have in sound. 

 Suspecting from analogy that this principle might be true for sound- 

 waves, he has proved by experiment that this is, at least in a general 

 way, the case, and has then devised the following method of measure- 

 ment : 



If a tuning-fork be set in vibration and held against a wall, it will 

 communicate its vibrations to the wall and the wall will give out a 

 sound, which sound will be feeble or intense just in the same propor- 

 tion that its capacity to absorb sounds falling upon it is feeble or 

 intense. In this way the absorptive power of different wall materials 

 has been measured, and a few of the results are arranged in the order 

 of this power in the following table : 



1. Thin and dry pine sheathing, lightly supported or in panels. 



2. Corrugated iron. 



3. Heavy wood paneling. 



4. Thin, dry, and hard plaster on light laths lightly suspended. 



5. Heavy plastering on laths lightly supported or attached to 

 wooden Avails. 



6. Heavy plaster on laths closely fitted to brick or stone walls. 



7. Thin brick walls. 



8. Thin brick wails covered with plaster directly laid on. 



9. Thick brick walls. 



10. Marble and other stone walls. 



A comparison of this table with results actually obtained in 

 churches in' which these different wall materials are used has amply 

 proved their correctness. 



There is one fact of considerable practical value that seems espe- 

 cially Worthy of attention. Corrugated iron, we see, has a very good 

 absorptive power for sound, while it is durable, safe from fire, and can 

 be easily worked into any ornamental forms desired. It seems, there- 

 fore, peculiarly fitted for the lining up of an auditorium. Bethany 

 Church, on Franklin Square, Baltimore, is built entirely of this mate- 

 rial, and is a decided^ acoustic success. We need, however, not only to 

 attend to the material of the walls, but to their arrangement as well. 



