An experimental fluctuation analysis is usually conducted via thermal anemometry with hot-wires. However, in the vicinity of oscillating shock waves, this kind of sensor can be destroyed due to strong mechanical loads. On the basis of a sufficient modelling, a wedge-shaped hot-film can be suitable for a quantitative fluctuation analysis across a shock wave. Within this study, the modal analysis according to Kovásznay and Morkovin was adapted from hot-wires to the used wedge hot-film. Additionally, 3-dimensional fluctuation diagrams were derived for the separate as well as for mixed modes. The freestream fluctuations were detected across an oblique shock wave with a wedge hot-film in a constant-temperature mode. The shock was caused by a ramp with a 10° ramp angle placed in a flow with a Mach number of M = 2.5 and a unit Reynolds number of Reunit = 4.96·106·m−1. The recorded perturbations were decomposed according to the modal analysis and found to be dominated by the acoustic mode. The fluctuations’ amplification across the shock wave and the subsequent decay could clearly be detected. They are in good agreement with the literature.