Term	Definition
E[]	The expected value.
g	The acceleration of gravity.
[H]	A matrix of frequency response functions (FRF’s) relating the control system response to the drive signals. Typically the elements will have units of g/V. Each element is a frequency response function. A third dimension typically is the amplitude as a function of a set of frequencies relating to the DFT of the input and response signals.
Ns	The number of drive signals, the number of shakers.
Nc	The number of control signals.
Na	The number of acceleration return signals.
Nd	The number of output control variables.
R	The reference control spectral density matrix; the desired spectral density matrix.
{s}	The vector of shaker drive voltages.
S	The spectral density matrix of the drives in shaker space.
SCD	The spectral density matrix between the control signal and the drives to the shakers.
Ta	The acceleration to control space transformation matrix.
Ts	The drive in the control space to voltages {s} to the shakers transformation matrix.
Z= H†	The system impedance matrix, typically units of volts/g.