Timber is a renewable and human friendly construction material and thereby a potential solution to achieve life cycle sustainable buildings. However, it is clear that impact sound and vibrations wit hin the low frequency range still are challenges for wooden joist floors. Another challenge is the, mostly, larger building heights of wooden or hybrid floors compared to the heights of concrete floors. Using timber as the structural joist floor material could imply fewer stories due to maximum allowed building heights, which renders in less income in a building project. Accurate simulations of impact sound may decrease the need for prototypes; thus saving money and time in the timber building industry. Here, a hybrid joist floor consisting of wood, sand and steel is compared to a concrete floor in terms of radiated impact sound into a rectangular cavity. The hybrid floor is designed such that its mass distribution and globa l stiffness are close to the same properties of the concrete floor. Finite element models are used for simulations of the radiated transient sound induced by impact forces having the characteristics of human walking. The simulations indicate that similar surface mass and bending stiffness of a floor intersection give similar impact sound transmission properties around the first bending mode, while it is not necessary so at higher frequencies.