The principles of Bruel & Kjaer’s Non-Stationary STSF measurement system were developed and applied as a part of a sound field assessment task for the Brite-Euram research project PIANO which ended early in 1996. The PIANO project dealt with new methods to enable faster, cheaper and more accurate ISO 362 pass-by noise reduction measures for heavy road vehicles. This standard specifies a transient outdoor vehicle test and troubleshooting of components must be performed under similar conditions. Cross spectral based near-field acoustical holography implemented in Bruel & Kjaer’s STSF system 1,2 requires stationary signals and therefore did not apply. Non-Stationary STSF (NS-STSF) is an implementation of Time Domain Holography (TDH). 3 TDH processes a recorded acoustic event that consists of simultaneously measured pressure time histories from a planar surface covering the sound source. As a consequence, nonsimultaneous data acquisition (scanning) cannot be used – a full-size microphone array is needed. Compared with STSF, the three most significant advantages of NS-STSF are: 1. Reference signals are not required. 2. There are no requirements for stationary operation of the sound source. 3. Full time resolution is retained throughout the NS-STSF measurement and the Time Domain Holography calculations. Thus, while STSF provides detailed information about where noise is radiated, NS-STSF can show both where and when noise is radiated. While STSF takes into account only that part of the sound field which is coherent with the reference signals, NS-STSF operates with a full representation of the sound field. This article briefly describes the principles and theory of NSSTSF, including the use of envelope intensity to overview the sometimes very complicated time variations encountered. Some typical application examples are given.