dear all

some vibration analyzers have coherence parameter . i want to know , can h use this measurement when i have some unknown frequency on machine or other related part to find relation of those frequency with rotational frequency . how can help me this parameter for vibration diagnosis?

pls share your experience

best regars

Original Post

Coherence is an averaged function looking at the relationship between an input and an output and is used to verify how much the output is related to the input. generally used in frequency transfer functions, impact tests and similar and gives you an idea of the validity of your data.



thanks sir

for example , i see high vibration on slid of pump rather than body of pump.

can i put out put channel on skid and input channel on body of pump to realize how much of vibration on skid related to body vibration?


best regards

Sure, you can do a transfer function of slid (did you mean skid?) compared to pump housing. Coherence is a measure of causality between input and output. Let's say you measure the COH = ~1 between the ?skid? and pump.  In this case, the relationship between the two are the same but not sure what useful information that gives me or whether it helps me figure out the problem.

As Gary stated, it is used primarily in impact/modal testing to provide confidence in the cause/effect (e.g. impact to response) relationship.

For the case you mentioned (large difference between skid and pump), might imply looseness in bolting.

I might be wrong in my reasoning, others can elaborate.


Jim P



Last edited by ivibr8

Top of page 11

Think of it this way, you are comparing two signals and you want to know if the frequency on sensor A is the same as sensor B. IF the frequency is the same coming into both sensors then there should be a coherence value of .95-1.0.....IF the frequency is not the same then coherence could be from .6 to 0.....

An indication of a natural frequency in an FRF plot would be a peak present at a given frequency, and at that frequency there is a 180 degree phase shift and a coherence close to 1.0


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