Bauhaus-Universität Weimar

Collected Papers On Acoustics
Sabine, Wallace Clement
20 5 
Plaster on an otherwise homogeneous sustaining wall is a first 
step in the direction of a compound wall, but a vastly greater step 
is taken when the plaster instead of being applied directly to the 
sustaining wall is furred to a greater or less distance. In a homo¬ 
geneous wall, the absorption of sound is partially by communication 
of the vibration to the material of the wall, whence it is telephoned 
throughout the structure, and partly by a yielding of the wall as a 
whole, the sound being then communicated to outside space. In 
a compound wall in which the exposed surface is furred from the 
main structure of the wall, the former vibrates between the furring 
strips like a drum. Such a surface obviously yields more than would 
a surface of plaster applied directly to tile or brick. The energy 
which is thus absorbed is partly dissipated by the viscosity of the 
plaster, partly by transmission in the air space behind it, and partly 
through the furring strips to the main wall. The mechanism of 
this process is interesting in that it shows how the free standing 
plaster may absorb a great amount of sound and may present a 
greater possibility of resonance and of selective absorption in the 
different registers of pitch. It is obvious that we are here dealing 
with a problem of more complicated aspect. It is conceivable 
that the absorption coefficient should depend on the nature of the 
supporting construction, whether wood lath, wire lath, or expanded 
metal lath; on the distance apart of the studding, or the depth of 
the air space; or, and even more decidedly, on the nature of the 
plaster employed, whether the old lime plaster or the modern quick 
setting gypsum plaster. A start has been made on a study of this 
problem, but it is not as yet so far advanced as to permit of a system¬ 
atic correlation of the results. It must suffice to present here the 
values for a single construction. The most interesting case is that 
in which lime plaster was applied to wood lath, on wood studding 
at fourteen-inch spacing, forming a two-inch air space. The co¬ 
efficients of absorption before the finishing coat was put on were 
(Curve 3, Fig. 1): 
Ci, .048; C2, .020; C3, .024; C4, .034; C6, .030; C6, .028; C7, .043. 
The values after the finishing coat was put on were as follows 
(Curve 4, dotted, Fig. 1) : 
Ci, .036; C2, .012; C3, .013; C4, .018; C5, .045; C6, .028; C7, .055.


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