A TECHNIQUE FOR EXPERIMENTAL ACOUSTIC MODAL-ANALYSIS

A technique developed to carry out experimental acoustic modal analysi s using commercially available structural modal analysis software is d escribed. This uses a finite difference calculation to determine the s patial variation in pressure. In order that the resulting function exh ibits the same nodes and antinodes as the actual pressure distribution at resonance, a second-order finite difference calculation is perform ed to obtain the second spatial derivative. This is implemented in pra ctice using a three side-by-side microphone probe with an analogue dif ferential amplifier. The technique is verified by measuring the natura l frequencies and mode shapes of a bare rectangular office. These resu lts are compared with analytical calculations and output from a finite element model. The results show very good agreement for all modes in the frequency range of interest.