Sound insulation between different parts of a building depends partly on their acoustic isolation from each other. This, in turn, depends on the dynamic stiffness of the connections between the different parts. Complete physical isolation between different parts of a building is impossible but the more rigid the connectors between the layers of a separating element, the more the sound is transferred across from one compartment to another. Knowledge of the dynamic stiffness of connecting elements can be of considerable assistance when assessing and specifying sound insulation.
Floating floors and cavity walls can be modelled as simple systems comprising springs and masses. A floating screed floor on a heavy supporting floor, for example, can be treated as a 'mass-spring' system and a cavity wall as a 'mass-spring-mass' system (see box).At its resonance frequency, the system's vibration amplitude is greatest and this can mean reduced sound insulation.
Above the resonance frequency of the floating floor or the cavity wall system, sound insulation increases significantly, so calculating the resonance frequency identifies the frequency at which sound insulation will be harmed and above which it can be expected to improve.
The sound insulation rating of floors and walls is determined by sound pressure level measurements between 100Hz and 3,150Hz. However, sound insulation at frequencies between 50Hz and 100Hz can be important subjectively so, in line with European standards, the resonance frequency of sound insulation between dwellings should be below 50Hz. This is so that the steep rise in sound reduction improvements are maximised as soon after 50Hz as possible.