Detonation waves are highly unstable and possesses three-dimensional cellular structure. They are believed to correlate with the detonation length, or hydrodynamic thickness, in the direction of propagation. The hydrodynamic thickness is considered an acceptable, or true, length scale for cellular detonations. The hydrodynamic thickness is defined as the distance from the leading shock, to the Chapman-Jouguet (CJ) surface, or sonic surface, behind the detonation front. The sonic surface is the location behind the shock where the flow transitions from supersonic to subsonic. The location of the sonic surface is paramount in characterizing a length scale for detonation-based propulsion and power generation technology. A better understanding of this length scale will greatly influence the ability to characterize and maintain sustained detonations. It is of importance to note that there is a lack of experimental data supporting current hydrodynamic theories. The current study plans to produce such experimental data by determining the location of the sonic surface by detonating hydrogen-air mixtures in a Pulsed Detonation Engine (PDE) and Rotating Detonation Engine (RDE) facility. Velocity and temperature profiles are constructed, for both cases, in order to create a spatial evolution of the Mach number profile for the identification of the sonic surface. The hydrodynamic thickness for both cases is revealed and compared to current detonation theories.