Modal testing is a form of vibration testing of an object whereby the natural (modal) frequencies, modal masses, modal damping ratios and mode shapes of the object under test are determined.
There are several ways to do modal testing but impact hammer testing and shaker 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 amplification of the response, clearly seen in the response spectra. Using the response spectra and force spectra, a transfer function can be obtained.