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There are several ways to do modal testing but impact hammer testing and shaker (vibration tester) testing are commonplace. In both cases [[energy]] is supplied to the system with a known frequency content. Where structural resonances occur there will be an [[Amplifier|amplification]] of the response, clearly seen in the response spectra. Using the response spectra and force spectra, a [[transfer function]] can be obtained. The transfer function (or [[frequency response function]] (FRF)) is often [[curve fitting|curve fitted]] to estimate the modal parameters; however, there are many methods of modal [[Estimation theory|parameter estimation]] and it is the topic of much research.
== Impact Hammer Modal Testing ==
[[File:Modal hammer.jpg|thumb|Model hammer with
An ideal impact to a structure is a perfect impulse, which has an infinitely small duration, causing a constant amplitude in the frequency ___domain; this would result in all [[modes of vibration]] being excited with equal energy. The impact hammer test is designed to replicate this; however, in reality a hammer strike cannot last for an infinitely small duration, but has a known contact time. The duration of the contact time directly influences the frequency content of the [[force]], with a larger contact time causing a smaller range of bandwidth. A [[load cell]] is attached to the end of the hammer to record the force. Impact hammer testing is ideal for small light weight structures; however as the size of the structure increases issues can occur due to a poor [[signal-to-noise ratio]]. This is common on large [[civil engineering]] structures.
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