A timber floor was constructed on a 100mm thick Interspan concrete slab within a high-end residential apartment. The finished timber was nailed using concealed fixings to a plywood substrate. The substrate was atop a 6 mm thick rubber underlay mat which was affixed to the concrete floor system using a polyurethane adhesive. Impact testing on the completed timber floor / concrete slab was completed prior to the construction of the ceiling in the apartment below. Testing showed that the impact isolation class of the timber floor was much lower than the adjacent tiled areas. The measured impact isolation class (IIC) of the flooring system suggested a shortfall in performance of around ?IIC 10 in comparison to that calculated for the timber floor system based on available data and theory. Vibration measurements on building elements in the apartment below showed that the floor transmission was the primary, though not only, path of noise transfer. To determine the reason for the performance shortfall, a hole was cut in the completed floor and testing carried out using a tapping machine. Impact testing on the bare concrete slab showed that the slab was underperforming at high frequencies by around ?IIC 6 when compared to a theoretical prediction model. An initial comparison of IIC values could have lead to the erroneous conclusion that the poorer than expected performance of the slab was the reason for the overall shortfall in performance of the system, however analysis of the measured impact sound pressure levels showed that the timber, substrate and underlay were resulting in a higher level of impact noise at 1 kHz and 1.25 kHz than compared to the bare concrete slab alone. An inspection of the substrate and underlay showed that the adhesive used was significantly harder than expected. The adhesive had also penetrated the rubber matting which had likely resulted in an overall stiffening of the floor system. It was also noted that the use of the adhesive had formed small interstitial voids on both sides of the underlay. It was determined that one of these mechanisms was the primary cause of the shortfall in performance. In order to compensate for the shortfall in performance and to achieve the Building Code minimum level of performance (FIIC 50), a heavy, resilient ceiling and isolated wall linings were specified in the apartment below.