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. Measurements of the normalized direction-averaged vibration level difference Dv,ij,n in a 3-room real-size timber frame mock-up have been performed in order to get input data for these new expressions. For cases where bidirectional measurements are not possible, a new expression is proposed to estimate the direction-averaged vibration level difference based on a unidirectional vibration level difference measurement.