Reply by January 15, 20082008-01-15
Dear scholars,

My present work centres on extraction of modal parameters from the plots of magnitude of frequency response response functions, FRFs, versus frequency. FRF is simply the ratio of the FFT of the response (output) to the FFT of the excitation (input). My questions can be summarised as follows:

1. The time histories of the response and excitation are 250Hz and 500Hz respectively. How best can I determine the FFT of the two time histories taking their different sampling frequencies into consideration.

2. For the condition described in Question 1 above, the excitation is made at a point and the response monitored at 4 different points. Hence the excitation time series is a vector while the response time series is a matrix. Kindly guide me on how to determine the FRFs and also plot the frequency spectrum (i.e magnitude of the FRF versus frequency). Which sampling frequency will dominate - the response or excitation?

Thank you.