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Page:Popular Science Monthly Volume 21.djvu/473

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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 completely.

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 measurement:

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 proportion 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 especially 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, therefore, peculiarly fitted for the lining up of an auditorium. Bethany Church, on Franklin Square, Baltimore, is built entirely of this material, 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.