MASS LAW

DEFINITION The Mass Law states that the sound insulation of a single-leaf partition wall has a linear relationship with the surface density (mass per unit area) of the partition, and increases with the frequency of sound.

“Sound insulation increases by 5dB whenever the mass is doubled” Single leaf construction includes composite construction such as plastered brickwork, as long as the layers are bonded together. Theory predicts an insulation increase of 6dB for each doubling of mass, but for practical constructions the following working rule is more suitable. “Sound insulation increases by 5dB whenever the mass is doubled”

For example, the average SRI (Sound Reduction Index) of a brick wall increases from 45dB to 50 dB when the thickness is increased from 102.5mm to 215mm. This doubling of mass does not have to be achieved by a doubling of thickness as the mass of a wall for sound insulation purposes is specified by its surface density measured in kg/m2 (rather than cubic metre). Concrete blocks of different densities can produce the same surface density by varying the thicknesses of the blocks.

EXAMPLE The acoustic transmission loss (TL) of a simple partition is given by the equation: TL = 15 log σ + 13dB Where σ = is the area density (kg/m2) of the partition. Estimate the TL for a concrete wall of 750mm thickness assuming a bulk density of 2400kg/m3 for the concrete used in the wall.

If we can find σ this calculation should not pose a problem TL = 15 log σ + 13dB σ = 2400x0.750 So TL = 15 x log 1800 + 13dB TL = 61.8 dB

EXAMPLE The acoustic transmission loss (TL) of a simple partition is given by the equation: TL = 15 log σ + 13dB Where σ = is the area density (kg/m2) of the partition. Estimate the TL for a concrete wall of 500mm thickness assuming a bulk density of 2500kg/m3 for the concrete used in the wall.