In nearly all inhabited buildings it is desirable to prevent the conduction of sounds from one room to another through the floors or walls, and in school houses, office buildings and apartment houses sound-proof construction should be considered as necessary. The usual method of attempting to prevent the passage of sound through the floors is by lining the floors with some material that is expected to absorb and dissipate the sound waves. As commonly practiced, however, this method is usually only partially successful, owing partly to the failure of the lining to fully accomplish its object, but more to the solid connection maintained between the floor and ceiling by means of the nails and joists.
The author is of the opinion that perfect insulation from sound waves can be obtained only when the upper floor and the ceiling beneath have no direct connection by rigid bodies, either by contact or by nailing.
The most effective sound-proof construction is undoubtedly that described in Section 63, where two sets of joists are used. Owing to the great danger of such construction in case of fire, however, it is not to be recommended except in rare instances, and then only when the ceiling is fireproofed, and all communication with vertical air spaces cut off.
The next most effective wooden construction, in the opinion of the author, is that in which the upper and under flooring is separated by an efficient deafening material, and in which no nails connect the two. Such a construction is shown in Fig. 178, the cleats to which the upper flooring is laid being merely laid on top of the deafening and not nailed. This could be further improved by filling the space between the cleats with mineral wool, and by a layer of mineral wool 1 or 2 inches thick on top of the lathing.
As already stated, however, the common custom is to merely lay a thickness of deafening material between the upper and under flooring, and nail the upper flooring through it and the under floor into the joists, and while this method is not perfect, if a good deafener is used it partially accomplishes its object.