Bauhaus-Universität Weimar

Collected Papers On Acoustics
Sabine, Wallace Clement
hewn stone. On the general principle of investigating any proposal 
so long as it contained even a possibility of merit, these suggestions 
were put to test. The concrete floor of a room was covered with a 
gravel so sifted that each pebble was about one-eighth of an inch 
in diameter. This was spread over the floor so that pebble touched 
pebble, making a layer of but a single pebble in thickness. It 
showed not the slightest absorbing power, and there was no per¬ 
ceptible decrease in reverberation. The room was again tried with 
sand. Of course, it was not possible in this case to insure the thick¬ 
ness of a single grain only, but as far as possible this was accom¬ 
plished. The result was the same. The scarred, the sanded, the 
pebbly plaster, and the rough hewn stone are only infinitesimally 
more efficient as absorbents than the same walls smooth or even 
polished. The failure of such roughening of the wall-surfaces to 
increase either the absorption or the dispersion of sound reflected 
from it is due to the fact that the sound-waves, even of the highest 
notes, are long in comparison with the dimensions of the irregu¬ 
larities thus introduced. 
The absorption of sound by a wall is therefore a structural 
phenomenon. It is almost infinitely varied in the details of its 
mechanism, but capable of classification in a few simple modes. 
The fundamental process common to all is an actual yielding of the 
wall-surface to the vibrating pressure of the sound. How much the 
wall yields and what becomes of the motion thus taken up, depends 
on the nature of the structure. The simplest type of wall is obvi¬ 
ously illustrated by concrete without steel re enforcement, for in 
this there is the nearest approach to perfect homogeneity. The 
amount that this wall would yield would depend upon its dimen¬ 
sions, particularly its thickness, and upon the density, the elasticity, 
and the viscosity of the material. It is possible to calculate this 
directly from the elements involved, but the process would be 
neither interesting nor convincing to an architect. It is in every 
way more satisfactory to determine the absorbing power by direct 
experiment. A concrete wall was not available. In its stead, the 
next more homogeneous wall was investigated, an eighteen-inch 
wall of brick set in cement. This wall was a very powerful re¬ 
flector and its absorbing power exceedingly slight. Without going


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