The coherence is a dimensionless function that estimates the extent of linear dependence between the input and output at each frequency, and hence indicates the degree of correlation in a Frequency Response Function (FRF).
It reflects the fraction of the power of the measured output towards the power of the measured input, and is bounded between in the interval [0,1]. A coherence value of 1 indicates perfect correlation. If any amount of measured output power is generated by noise, then the coherence value is less than 1 at that frequency. Note that if the value of coherence is low at any frequency, it does not necessarily mean that FRFs are of a poor quality, but it might indicate that more averaging is needed.
Let Gxy(f) be the cross spectral density between signals x(t) and y(t), and Gxx(f) with Gyy(f) be respectively the auto spectral density of x(t) and y(t), where f denotes frequency and t denotes time. The coherenceCxy(f) between x(t) and y(t) is expressed as
Cxy(f) = |Gxy(f)|^2 / ( Gxx(f) Gyy(f) )
Coherence can be viewed in Manage Measurements task under the Spectrum Settings
For more information on Experimental Modal Analysischeck out these online video tutorials describing the Impact Testing Module:
- EMA Impact Testing Module - Part 1
- EMA Impact Testing Module - Part 2
- EMA Impact Testing Module - Part 3
or our online material on EMA module in general:
Otherwise check out an offline tutorial on our EMA module in Step by step: Experimental Modal Analysis.
See Also
