No abstract

In a companion paper, a model was derived to predict the vibro-acoustic responses of simplified residential structures exposed to sonic booms. In the present paper, the experimental validation of the numerical model is presented. First, the experimental setup is described, including the structure, instrumentation, and external pressure loading. The structure was a single room made of plaster-wood walls and includes two double-panel windows and a door. The structure was extensively instrumented with accelerometers and microphones to record its vibro-acoustic response. Sonic booms with realistic amplitudes and durations were generated by an explosive technique. Subsequently, numerical predictions on the vibration of the structure and pressures inside the room are compared to experimental data, showing a fairly good agreement overall.

Propagation of impulsive sound around buildings and induced structural loading are investigated experimentally and numerically. Experiments were conducted on a rectangular building at Virginia Tech using sonic booms generated by an explosive technique. Assuming linear-acoustic propagation and acoustically rigid surfaces, these experiments were simulated with a three-dimensional numerical model, in the context of geometrical acoustics, by combining the image source method for the reflected field (specular reflections) with the Biot-Tolstoy-Medwin method for the diffracted field. This numerical model is validated against a boundary element solution and against experimental data, showing a good overall agreement. Some of the key advantages of this modeling approach for this application are pointed out such as the ability to model three-dimensional domains over a wide frequency range and also to decompose the sound field into direct, reflected, and diffracted components, thus allowing a better understanding of the sound-propagation mechanisms. Finally, this validated numerical model is used to investigate sound propagation around a cluster of six rectangular buildings, for a range of elevated source positions.

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.]

This paper presents a numerical model to predict the vibro-acoustic responses of simplified residential structures exposed to sonic booms. The model is validated experimentally in a companion paper. The dynamics of the fluid-structure system, including their interaction, is computed in the time domain using a modal-decomposition approach. In the dynamic equations of the system, the structural displacement is expressed in terms of summations over the in vacuo modes of vibration. The pressures inside the interior volumes are expressed as summations over the acoustic modes of rooms with perfectly reflecting surfaces. The structural modes are computed numerically using the finite element method. A shell element was specifically derived to model the structural components of typical residential buildings, e.g., plaster-wood walls, windows, and doors. The acoustic modes are computed for rectangular geometries using analytical expressions. Using modal decomposition, the dynamics of the fluid-structure system may be formulated by a finite set of ordinary differential equations (modal equations). These equations are then integrated with a Newmark algorithm to solve for the vibro-acoustic response of the system in the time domain. The system response may also be predicted in the frequency domain, by taking the Fourier transform of the time-domain response.