ABSTRACT

The ability of a partition to resist the flow of sound energy through it is termed its sound insulation and this is largely determined by its mass.

Mass Law Most of the principles involved in direct sound transmission are revealed by considering a single leaf wall. Chapter 1 on the fundamentals of acoustics showed that acoustic wave motion is transmitted through air in the form of successive compressions and rarefactions travelling at a speed of approximately 340m/sec (1120ft/ sec) in the direction of propagation of the sound waves. In other words, air is able to transmit longitudinal wave motion by virtue of its compressibility. A wall, however, is constructed from an incompressible material and, as such, clearly cannot transmit acoustic wave motions in the manner of the surrounding air. Fig 1 illustrates how the incident sound wave acts on the partition-through its pressure fluctuationsand sets it into flexible or bending movements. These movements are usually extremely small but they radiate sound from the opposite side of the partition and also set supporting structural members into vibration. The flexible motions can be likened to ripples travelling across the surface of the partition with velocities very much higher than the speed of sound in air.