A performance indicator for vibration isolation of building services equipment

There is an increasing concern about the sound emitted into the indoor environment due to the vibration of rooftop building services equipment. Force transmissibility or isolation efficiency is a performance indicator of vibration isolators for building services equipment. In the derivation of the force transmissibility, floor structure is usually assumed to be rigid and immovable. The floor structural dynamic characteristics are therefore neglected. It is used in situations for a single contact point or symmetrical arrangement of symmetrical contact points. In the most common situation where a machine with four symmetrical contact points is placed asymmetrically on a floor, dynamic forces transmitted to the floor through each contact point are different. A force transmissibility vector, which comprises these different dynamic forces transmitted to the floor, is introduced. Measurements of the floor mobility have been conducted in several sites. The effect of floor mobility on force transmissibility vector particularly at disturbing frequencies near the resonant frequencies of the floor is also discussed.