The following was written by Adrian Lincoln in response to a customer asking about averaging Frequency Response Functions (FRFs)
Further to your reply it should be noted that there is no such mathematical or physical quantity corresponding to an averaged structural FRF except in those special cases where you are testing a symmetric structure comprised of nominally identical components (such as turbine blades). For symmetric structures, where you can impact at a similar position on each component and measure the corresponding acceleration at a similar response position, then the individual FRF’s should have the same characteristics and therefore can be averaged.
For structures that don’t have multiple symmetric components then you should not perform any sort of averaging because the FRF’s will not have the same characteristics. For example, if you have beam-like structure and are measuring the FRF’s between different locations along the beam, then these FRF’s should only be averaged as moduli and not with respect to phase and coherence.
When analysing a waterfall or performing order analysis it is important to consider the frequency resolution or the frequency spacing.
There is often a desire to increase the resolution to finer and finer detail. But that is a process of diminishing returns, and actually fraught with danger. And that danger is waterfall smearing. Continue reading What is “waterfall smearing”?
When engineers talk about the ‘Load Spectrum’ what do they mean?
There is no simple answer, simple terms like load and spectrum can be used in different situations and therefore to mean different things. However the most common definition of load spectrum is as follows… Continue reading What Is A Load Spectrum?
Much confusion revolves around linear and non-linear numbers. The following outlines the mathematical process to convert from a number expressed in dB to a linear quantity. How do we convert to decibels and back again? Continue reading How Do I Convert To Decibels?
The Auto Spectral Density or Auto RMS spectrum analyses uses Fourier Transforms to process optionally overlapped sections of the input data. The result of each Fourier analysed section is called a periodogram. We then process all the resulting periodograms to produce a spectral result. Continue reading What is Auto Spectral Density?