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ARCHITECTURAL ACOUSTICS 333
mass from one pound per square foot to one hundred pounds per square foot, are given in the following table:
Mass per Square Insulation
Foot of Wall Area Value in db
1 pound 24
5 pounds 33 '
10 20 40 60 100
38 43 48 51 54
The values given in this table are for a frequency of 512 d. v. In general, the insulation is slightly less at 128 d. v. and slightly more at 2048 d. v. than the values given in this table.
For porous, flexible materials the insulation is almost proportional to the thickness of the material, and therefore for a given material the insulation value would be proportional to the weight of the material used per square foot of wall section. Often a combination of the dense, rigid partition and the porous, flexible material will provide the required amount of sound insulation. The use of multiple layers attains its highest insulation value when the separate layers are free from all connections or ties. That is, a stage inside of a stage with no connection between the two except through the earth will provide an insulation which is nearly equal to the sum of the insulations provided by the two structures. If the walls or ceilings of these two structures are connected in any way the over-all insulation becomes less than the sum of the insulations of the separate structures. In case the two structures are rigidly connected together they become essentially a single wall and the total insulation is almost proportional to the logarithm of the mass per square foot of wall area — the same as for a dense, rigid wall.
5. The insulation of Vibration. It is often necessary to provide large mountings in the recording room which will be free from mechanical vibrations. For example, the wax shaving machine is particularly sensitive to solid borne vibrations. Further, these solid borne vibrations are often communicated