Index of content:
Volume 104, Issue 4, October 1998
- STRUCTURAL ACOUSTICS AND VIBRATION 
104(1998); http://dx.doi.org/10.1121/1.423734View Description Hide Description
In contrast to a thin plate having no shear component of deformation, a Timoshenko–Mindlin plate cannot support a concentrated line dipole moment exerted by a pair of equal and opposite line forces acting normal to its lateral surface. In this paper, it is argued that, in order to account for all the connectivity between a rib and the plate, it is necessary to assume a nonzero width footprint for the rib upon the plate. This footprint is idealized by a connection between the rib and plate along two lines parallel to the rib. The problem posed is one dimensional, with plate responses invariant in the direction of the rib; therefore, along each line of contact, the rib exerts upon the plate only a normal force and a tangential (shear) force perpendicular to the rib. These four initially independent forces combine to produce resultant opposing forces acting on the end of the rib which generate velocities of the rib that can be related to those of the plate. The response quantity of interest is the structural power reflected from the rib due to a wave propagating freely in a direction normal to the rib and incident upon it. It is shown that the rib footprint cannot be set equal to zero if the rib’s rotational properties are to have a meaningful influence on the calculated reflected power.
104(1998); http://dx.doi.org/10.1121/1.423735View Description Hide Description
A new, flexural wave solution is derived for the Kirchhoff, thin plate equation on a semi-infinite domain with a free edge. This wave is a freely propagating wave that travels parallel to the edge, i.e., the curves of constant phase are straight lines perpendicular to the edge, while the wave amplitude falls off exponentially with distance from the edge, so the curves of constant amplitude are straight lines parallel to the edge. For a fixed frequency its speed is proportional to that of the standard bending wave speed. The constant of proportionality depends on the Poisson ratio only. Its value is slightly less than 1.
104(1998); http://dx.doi.org/10.1121/1.423736View Description Hide Description
The leaky waves generated on plates driven in bending above the bending coincidence frequency radiate as extended sources; therefore, the standard saddle point evaluation of the radiated pressure, which yields a far-field result, may not be appropriate at the range of interest. The calculation can be modified by removing the contribution of the leaky waves from the original integrand, evaluating the remaining regular part by the saddle point method, and evaluating the effect of the leaky waves in terms of error functions and residue. Results showing range dependence are presented for radiation by a steel plate in water driven by normal and tangential line forces. The correctness of the results is made plausible by comparing independent calculations of acoustic power passing through a semicircle at finite range.
104(1998); http://dx.doi.org/10.1121/1.423737View Description Hide Description
There are two mainstream methods for the active control of noise in enclosures: active noise cancellation(ANC) and active structural acousticcontrol (ASAC). ASAC systems are most effective when the primary noisesource is the radiation from a vibrating structure, but cannot efficiently controlsound from other sources.ANCsystems are effective when other noisesources are present, but will sometimes require a much higher dimensionality than ASAC systems to control structural radiation. This paper proposes a practically motivated implementation strategy for ANC actuator placement. Specifically, it proposes to implement an array of acoustic controlsources in front of vibrating boundaries to form an acoustic boundary control layer. This provides low-power performance comparable to ASAC systems for controllingsound radiation from the boundary, but also an efficient system for controllingnoise from other disturbances. Numerical simulations are presented to demonstrate the ability of the acoustic boundary control to suppress interior noise due to acoustic and structural sources. Because of the expense of implementing large active control systems, there is great interest in developing an actuator grouping strategy to reduce the control system dimensionality. A preliminary feasibility study, to establish under which conditions actuator grouping may be successful, is presented. A strategy in which actuators are grouped through an off-line analysis of the control problem is studied.
The influence of frequency-shaped cost functionals on the structural acoustic control performance of static, output feedback controllers104(1998); http://dx.doi.org/10.1121/1.423738View Description Hide Description
This paper investigates the level of modeling necessary to design static, output feedback controllers for structural acousticcontrol. The acoustic performance of three active control techniques is compared: active structural acousticcontrol (ASAC); active vibration control (AVC); and discrete active vibration control. The ASAC cost functional is created by the use of frequency-dependent models, which relate the structural response to radiated acoustic energy. A broadband measure of control signal power is developed and compensators are designed such that their power requirements are equal. These investigations are conducted on three different transducer arrangements. Results demonstrate that frequency-shaped cost functionals influence the design of frequency-independent controllers when the order of the control system is greater than one. With more than one sensor-actuator pair, the control energy bandwidth is shifted by the ASAC cost functional to provide increased low-frequency performance.