Output-Only Modal Testing of Simple Residential Structures and Acoustic Cavities Using the Response to a Simulated Sonic Boom and Ambient Excitation

Structural modal properties of single-room and two-room rectangular structures built with typical residential construction are extracted using only the vibration responses to (1) a sonic boom simulated with a linear distribution of detonating cord and (2) ambient excitation. Then, the acoustic modal properties of the cavities enclosed by the residential structures are extracted using the pressure responses to the same two types of excitation. The technique involves cross-correlating the responses measured at many points to the responses at a few different reference points on the structure or in the cavity. This allows for the calculation of impulse responses at many points around the structure or cavity due to excitation at the reference points. Structural natural frequencies, modal damping ratios, and mode shapes obtained from the output-only analyses are presented and compared to the results from conventional impact hammer modal testing and from a finite element model, demonstrating good agreement. Acoustic natural frequencies and modal damping ratios extracted in the output-only analyses are presented and compared with room pressure spectra measurements made with speaker noise excitation and analytically calculated results for the single room and the two-room configurations, also showing good agreement between analyses. [Work supported by NASA Langley.].