SOUND INSULATION PARADOX OF MASSIVE CAVITY WALLS IN BUILDINGS

Abstract

In literature, the "box-in-box" system is described as a solution for achieving the highest possible sound insulation in a room. The system involves double walls separating a room from the surroundings in all directions, without any lateral pass for sound. This approach doubles the number of discontinuities in the material through which sound energy travels, thus providing higher sound insulation. Based on such an idea, some dwelling houses were designed with massive cavity walls between apartments with the expectation of better sound insulation. The concept was further motivated by the need to achieve adequate thermal insulation between apartments using insulation material in the cavity. However, in buildings where this was implemented, residents complained about inadequate sound insulation. Measurements showed that the sound reduction index of the double brick wall was lower than expected, even less than that of a wall where the two thinner layers were combined into a single thicker brick wall without an internal cavity. This was surprising, leading to research aimed at finding an explanation. It was concluded that in buildings there is the influence of sound paths through the wall’s lateral junctions, which is more pronounced with two thinner layers. With a cavity wall, the transmission of sound energy through lateral junctions and further by flanking transmission is more pronounced than with a single wall of the same surface mass. Additionally, the high seismic zone in Serbia requires certain adjustments in construction, invisible in architectural drawings, that further diminish the effect of the increased number of discontinuities in the wall. All of this makes the massive cavity wall in the building, contrary to expectations, less effective than a single wall made of the same quantity of material, making it acoustically and financially unreasonable. Research also revealed that material in the cavity has no influence on the sound reduction index value of the wall.