The measurement of the sound reduction index R of building elements is typically performed according to ISO 10140, by measuring the sound pressure level in the sending and receiving room, using standardized loudspeaker-microphone instrumentation. Recent proposals for new building acoustic standards taking into account frequencies down to 50 Hz, provoke new questions. At low frequencies (depending on the dimensions of the rooms), the modal density in the rooms is typically too low, impeding reliable measurements following standard techniques. In this paper, laser Doppler vibrometry is introduced as an alternative approach for the lower frequency region, to determine the intrinsic sound transmission properties of a building element, independently of the particular properties of the test facility. In addition, laser Doppler vibrometry offers the additional advantage in that it can identify the actual boundary conditions of a building element, which can be useful for modeling and prediction of the performance of building acoustics applications. A number of applications will be given.