Some vibration environments are characterized by quasi-periodic excitation from reciprocating or rotating machines (e.g. rotor blades, propellers, or pistons). These can be simulated as one or more high-level narrowband or sinusoidal vibration components superimposed on a low-level broadband random profile. These are called mixed-mode random tests.
There are two types of mixed-mode random tests supported in EDM: Sine on Random (SoR) and Random on Random (RoR). In each of these test types, an additional vibration profile is placed on top of the regular broadband random profile. In Sine on Random, this additional profile consists of one or more sine tones that sweep through a specified frequency range. The random profile may represent a base excitation or background noise level, and the sine tones represent strong single-frequency excitations.
A typical profile of SoR described in MIL-STD-810F is shown below. Note that the random broadband profile is defined in units of Power Spectral Density, or g2/Hz, while the sine components are specified as amplitudes in units of g.