THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

12Walls, Enclosures, and Barriers12.1 IntroductionAcoustics constitutes an important factor in building design and in layouts ofresidences, plants, offices, and institutional facilities. A building not only protectsagainst inclement weather; it must also provide adequate insulation against outsidenoises from transportation and other sources. Interior walls and partitions need tobe designed to prevent the intrusion of sound from one room into another. Exposureof workers to excessive occupational noises can be decreased by construction ofappropriate barriers or enclosures around noisy machinery. A shell constructed ofthe densest materials may be the most effective barrier against noise transmission,but such an enclosure can lose a major portion of its effectiveness if there are weaklinks that tend to promote sound transmission rather than hinder it. For example,a large proportion of sound energy enters a building through its windows, evenclosed ones, and many cracks and crevices inevitable in real structures permitsound to enter the structure’s interior.12.2 Transmission Loss and Transmission CoefficientsSound absorption materials tend to be light and porous. Sound isolation materials,however, generally are massive and airtight, thereby forming effective soundinsulation structures between the noise source and the receiver. When airbornesound impinges on a wall, some of the sound energy is reflected, some energy isabsorbed within the wall structure, and some energy is transmitted through thewall. Sound pressure against one side of the wall may cause the wall to vibrateand transmit sound to the other side. The amount of incident energy transmittedto the wall depends on the impedance of the wall relative to the air. The amountof the sound transmitted through the wall that is finally transmitted to the air onthe receiver side also depends on the impedance of the wall relative to air. Adouble wall construction that incorporates airspace between panels repeats theprocess between the two panels, which results in even more insulation againstsound.281