Page 60 - 2016Fall

TECHNICAL COMMITTEE REPORT
Structural Acoustics and Vibration
for each, understanding the concepts of vibration and acous- tic response has wide-reaching importance.
Many of these systems may also act as excitation sources for the technical area of building vibration and noise as well. For example, ground-borne wave propagation and radiated sound power resulting from large public transit systems such as trains often propagates into nearby buildings, resulting in unwanted mechanical vibration and noise within those structures. Building design is thus studied from the stand- point of structural acoustic and vibration in these structures due to excitation by external sources (trains), general me- chanical systems (HVAC systems), and even human-gener- ated sources. Ground-borne vibration is not always unde- sirable, however. As an example, structural acoustics and vibration researchers are responsible for the many recent advances in utilizing the purposeful generation of in situ shear waves into the ground as a means for characterizing soil composition at building construction sites and for other seismic/geotechnical purposes (Foti et al., 2015).
Specific commercial industries involving structural acous- tics and vibration include a wide and varied range of con- sumer products from copy machines to wind turbines to pumps and piping systems to musical instruments. Even a variety of biomedical devices and instruments such as hear- ing aids and ultrasonic diagnostic equipment are increas- ingly reliant on structural acoustic and vibration scientists and engineers to provide a comprehensive understanding of wave-structure interaction. Finally, this technical area also encompasses many more general topics that span many of the aforementioned applications such as active vibration and noise control, structural health monitoring, metamaterials and phononic crystals, acoustic transducers and sensors, and fluid flow-induced vibration and noise.
Regardless of the specific application area above, the typical structural acoustics and vibration practitioner will likely uti- lize one or more of the following modern scientific methods of investigation in his/her workday activities: (1) theoreti- cal/analytical calculations, (2) experimental examination, or (3) computational physics-based modeling and simula- tion. Using the theoretical method involves the assembly and coupling of the governing equations of motion for a given vibroacoustic system and solving them in a largely closed-form manner, incorporating mathematical methods as required for the solution. Alternatively, empirical inves- tigation of the structural acoustics and vibration behavior of a given system typically requires the fabrication and test-
ing of the system, with significant attention given to proper instrumentation selection and the calibration required for the collection of accurate measurement data. Scientists and engineers involved primarily in experimental investigation typically spend a substantial part of their workday in work and machine shops, laboratories, and possibly specialized test facilities such as anechoic or reverberant acoustic cham- bers, transmission loss chambers, or shock machines. Last, a large variety of physics-based computational mechanics and acoustics algorithms, solvers, and software tools have been developed that allow for the virtual solution and examina- tion of complex structural acoustics and vibration problems. Details on the more commonly used numerical approaches in everyday use and also continually being researched and improved for use in simulating structural acoustics and vi- bration problems is described below.
Although structural acoustics and vibration researchers are generally interested in understanding the physics and pre- dicting the fundamental mechanisms and signature levels of structural acoustics and vibration in complex systems, per- haps the majority of the time they are investigating the struc- tural acoustics and vibration response to attempt to limit and/or control it in some manner (see article by Schnitta in this issue of Acoustics Today). That is, the primary focus of structural acoustics and vibration can likely be divided into two basic approaches, control and reduction of structural acoustics and vibration. The idea of control can be either to limit the response (reduction) or to tailor the response to a desired reaction by enhancing or frequency shifting of select desired natural vibratory resonances or modes. The meth- ods used to control vibration and acoustical responses vary from simple and inexpensive so-called passive treatments to complex active and adaptive control. Passive treatments can vary from those incorporated a priori during the design of structures themselves to produce a desired acoustic re- sponse (e.g., including in the original design a set of resilient passive isolation mounts between a motor and its adjacent mounting structure) to retrofitted or add-on treatments that alter the existing structural response (e.g., sound insulation or damping coatings that may be sprayed on retroactively once an undesirable acoustic environment arises). The sim- plest examples of ubiquitous passive treatments are those utilizing softer isolation- and/or damping-based properties such as foams and rubbers, which are used to mitigate the acoustic or vibration response in a wide variety of applica- tions from industrial machinery to transportation systems.
58 | Acoustics Today | Fall 2016