Now, it is possible to interpret a complex number as a vector, which has real and imaginary components for each frequency. In SIM II, many transfer functions are measured and stored so an average may be then computed. This test or coherence threshold is explained below. Coherence Blanking Coherence blanking avoids frequency response data being displayed when the coherence between the input and the output signals is low. The spectral power density can be seen as a measurement of the RMS power of a random signal for each frequency, and the cross power spectral density can be seen as the power ratio, for each frequency, between two different random signals.
Coherence is defined only for measurement series over time.
When coherence does not exceed the threshold, the trace will not be displayed on screen for that frequency, showing there is noise contamination. Signal to noise ratio is the relationship between coherent output power and non-coherent output power. It can be seen that coherence and signal to noise ratio are equivalent functions, but signal to noise ratio shows more familiar values to technicians and engineers. Signal to noise decreases with noise, hum, reverb, echoes, distortion or dynamic compression, that are contamination forms uncorrelated to the input. When coherence is 0.1, signal to noise ratio is -20dB, showing that the output noise is larger than the input signal.