Part 1: Architect’s Guide 1 - 49 Acoustic insulation provided by a partition lies in its ability to resist the transmission of sound from one side to the other. The sound reduction index is used to measure the performance of the wall. This is expressed in dB. Regulations set minimum values for this index as a function of the building end-use, of the type of facilities being separated and for the airborne sound, impact sound and equipment noise. It should be noted that the insulation provided by a real wall is always less than the index measured in the laboratory, because of lateral transfer and parasitic resonance. The acoustic behaviour of a wall is illustrated by applying the mass-springmass law: The acoustic reduction index increases with the surface density of the wall For a double skin wall (two sandwich panels), this index depends on: - Mass per unit area of each partition - Thickness of the air space between partitions - Thickness of acoustic absorption - Critical frequency of each partition The index of a double skin wall is much greater than a single wall with the same surface density. (The sound emitted from a room and spreading to another room horizontally and vertically passes through first layer of products which causes an initial reduction. It is then ‘trapped’ in the central void of the wall, where it bounces against the second partition and is absorbed by the insulation layer before returning residually through the second partition wall). The acoustic performance of a steel building depends on the composition of the various partitions: external and internal, vertical and horizontal. Construction solutions are available which can achieve the very highest levels of performance. 6.3.2 Partitioning Partitioning usually consists of thin, cold formed steel elements on which plasterboard sheets of varying thicknesses are screwed onto both sides. This creates a central air cavity in which one or more layers of insulating mineral wool are inserted. It can incorporate the main structural building elements. The composition of a partition wall can be adapted to the required level of performance by varying the following parameters: Thickness of the air cavity: the more this increases, the higher the acoustic performance; the size of the main structural elements governs the choice of thickness Composition of each facing (number and type of plates finished in plaster)
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