CEN/TC 126/WG 2 is currently revising the EN 12354 series on prediction models for sound transmission in buildings based on the performance of elements. One major goal of this revision is the extension of the current models towards lightweight building constructions. In a first step, new expressions are being proposed to predict the flanking sound transmission due to airborne excitation. The goal of this paper is to provide input data for these prediction models by measuring the newly introduced normalized direction-averaged vibration level difference [...] in a 3-room real-size timber frame mock-up with basic walls and floors. Further, measurements of the flanking sound reduction index ijR using the sound intensity technique have been performed in order to investigate the validity and applicability of these expressions down to 50 Hz.