摘要:

When implementing a discrete computational method to predict the vibro-acoustic behavior of a larger structure one must decide how much effort and refinement to expend on modeling smaller attached structural components. In representing these attachments, or features, it is important to know what scales are important with regard to a valid prediction of the system response. To aid in the study of the impact of feature scales, sensitivity relationships for important system variables are developed in this paper. The sensitivity analysis will, for one, consider the change in the radiated power with respect to changes in the scale of an attached mass. These relations are developed analytically and do not require finite difference methods or eigenvalue derivatives. It is shown that computing the sensitivity of the radiated acoustic power only requires evaluating one additional term as all of the other terms are found during the normal course of modeling the system. Furthermore, the terms that include fluid-loading on the structure need only be reevaluated when the excitation frequency is changed. It is also noted that the sensitivity relations developed here can easily be used to investigate the effects of modifying a number of structural parameters.

ISSN:0001-4966    

DOI:10.1121/1.420104

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper investigates the modeling of an elastic plate coupled to a reverberant acoustic enclosure and subjected to convected fluid loading. The primary objective of this work is to quantify the effects of variations in external fluid convection velocity on power flow from the plate. The plate power flow constituents consist of injected power due to turbulent boundary layer (TBL) pressure disturbance, sound power radiated to the convected flow, sound power radiated to a reverberant cavity, and power dissipated through structural damping. Results indicate that variations in the convected fluid loading can significantly alter the balance in power flow. These results demonstrate the importance of including the effects of convected fluid loading in modeling the sound transmission through plates. Results also indicate that convected fluid loading serves to decrease the sound power radiated to the cavity as the external flow velocity increases. This effect, however, is overwhelmed by the increase in turbulent boundary layer pressures with the square of the flow speed. This work concludes with a discussion on the potential effectiveness of dissipative control strategies (be they active or passive) toward minimizing the sound transmitted to the cavity.

ISSN:0001-4966    

DOI:10.1121/1.420073

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

For sonar imaging systems and other situations where scattering amplitudes are resolved spatially (e.g., ultrasonic microscopy and nondestructive testing) approximations of outgoing leaky wave amplitudes are needed as a function of position on the imaged surface. The approach developed here approximates the amplitude of a leaky wave pole contribution to the total scattering as a spatial convolution of the local incident pressure with a spatial response function. Leaky rays to a surface point of interest follow a Fermat path having a stationary phase whereas the pole contribution becomes a surface integral that includes defective paths. Increased curvature of the surface or of the incident wavefront ordinarily cause more rapid dephasing along defective paths and a corresponding reduction in size of the Fresnel coupling patch. Examples given include leaky wave excitation on a partially coated cylinder at normal incidence and regular helical leaky wave excitation on tilted cylinders. A helical wave is found to be excited by diffraction at the edge of an idealized coating truncated along the cylinder’s axial direction. The leaky wave amplitude becomes proportional to a Fresnel integral of complex argument which accounts for the partial blockage of the Fresnel coupling patch.

ISSN:0001-4966    

DOI:10.1121/1.420074

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A method of formulating a model to evaluate the aeroelastic structural acoustic response of a panel subjected to turbulent boundary layer (TBL) noise sources and coupled with full potential flow aerodynamics is presented. Reduced-order models of both the aerodynamics and the structural acoustic coupling are presented such that a state-variable realization of the entire system dynamics can be developed for future active control system design and synthesis with modern and robust control theory. Results from this study demonstrate the importance of including aeroelastic coupling in modeling the structural acoustic response of panels for interior noise control on modern aircraft. At subsonic flow conditions, the aeroelastic coupling serves to increase the transmission loss across the panel with increasing Mach number; however, the power spectrum of the TBL noise source increases with increasing Mach number as well and thus offsets this benefit to some degree. Results from this study also serve to demonstrate that for future analysis of robust stability and performance, variations in the plant dynamics due to variations in flow conditions must be considered in the design of broadband, feedback, active structural acoustic control systems.

ISSN:0001-4966    

DOI:10.1121/1.420075

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper considers the active power flow control of a distributed-parameter planar structure, with particular emphasis on a vortex power flow which has the potential to confine the vibrational power into a restricted area of the structure. Without letting the vibrational power, a cause of exciting structural modes, disperse into the structure, control of the structural response can be achieved. This paper begins by deriving the necessary condition for producing a vortex power flow in the vicinity of a disturbance point force at an arbitrary exciting frequency. Then, with a wave visualization system newly developed, it becomes possible to observe the wave propagation (the vortex power flow) taking place in the structure. In order to investigate the contribution of structural modes to the vortex power flow configuration, an energy stream function as well as a vorticity function is derived in a general form. By using these functions, the generation mechanism of a vortex power flow actively induced around a disturbance force location is quantitatively studied.

ISSN:0001-4966    

DOI:10.1121/1.420076

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Noise measurements and a questionnaire survey were carried out in air-conditioned landscaped offices in the present study in an attempt to find out what existing noise index gives the best correlation with the auditory sensation feeling of office workers. Results from statistical analyses show that theEquivalent Sound Pressure Levelis the best among the 14 commonly used noise indices for this purpose. It is also found that the less commonly usedZwicker’s loudness levelperforms better than the commonly adoptedNoise Criterion curvesandNoise Rating curvesin predicting auditory sensation of office workers in air-conditioned landscaped offices. The performance of the 14 noise indices in predicting auditory comfort are compared.

ISSN:0001-4966    

DOI:10.1121/1.420077

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Recently, active noise control technology has been used to increase the sound transmission loss of double-wall structures. Several approaches have been put forward and explored individually. However, no comparative study on those approaches has been conducted to show which approach is more effective for given circumstances. In this paper, three different approaches for controlling sound transmission through double-panel partitions into a room, i.e., applying acoustic control sources in the air gap between the two panels (cavity control), applying vibration control sources on the radiating panel (panel control), and applying acoustic control sources in the receiving room (room control), are studied and compared to each other experimentally. The mechanisms involved in each approach are illustrated and the conditions for effective noise attenuation are examined. The results show that the modal overlap of each subsystem (the cavity, the radiating panel, and the room) and the control mechanisms involved are the two most important factors that determine the effectiveness of each corresponding approach.

ISSN:0001-4966    

DOI:10.1121/1.420105

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The active control method was used to suppress the sound diffracted by an outdoor barrier. This method operated by the cancellation of the sound pressure at the diffraction edge of the barrier, which normally behaves like the virtual source of the diffracted field. The results of two experiments are shown in this paper. In the first experiment, we employed two independent controllers that utilized multi-channel adaptive signal processing to minimize the sum of the mean square of the sound pressure at four and six points along the diffraction edge. Measurement of sound pressure levels at various distances from the barrier showed effective sound suppression, with about 6-dB excess attenuation over the barrier’s insertion loss at the receiver at a distance of 50 m. A practically realistic noise source, a fan blower, was used as a primary source in the second experiment and the moderate attenuation could be achieved at almost all the receiver points. The results obtained in these two experiments provided the experimental verification of the strategy for the active suppression of sound diffracted by a noise barrier.

ISSN:0001-4966    

DOI:10.1121/1.420078

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The acoustic attenuation of acoustic fiber materials is mainly determined by the dynamic resistivity to an oscillating air flow. The dynamic resistance is calculated for a model with geometry close to the geometry of real fiber material. The model consists of parallel cylinders placed randomly. Two cases are treated: flow perpendicular to the cylinder axes, and flow parallel to the axes. In each case two new approximate procedures were used. In the first procedure, one solves the equation of flow in a Voronoi cell around the fiber, and averages over the distribution of the Voronoi cells. The second procedure is an extension to oscillating air flow of the Brinkman self-consistent procedure for dc flow. The procedures are valid for volume concentration of cylinders less than 0.1. The calculations show that for the density of fibers of interest for acoustic fiber materials the simple self-consistent procedure gives the same results as the more complicated procedure based on average over Voronoi cells. Graphs of the dynamic resistivity versus frequency are given for fiber densities and diameters typical for acoustic fiber materials.

ISSN:0001-4966    

DOI:10.1121/1.420079

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper deals with noncoherent discrete-time detection of a narrow-band signal subject to slow and nonselective fading and embedded in correlated non-Gaussian noise modeled as a spherically invariant random process whose modulating random variable is continuous. At first, an asymptotic sufficient statistic for an arbitrary fading law is derived; then, the asymptotically optimum detector for Rayleigh-distributed amplitude fluctuations is synthesized. The detection structure implementation requires the knowledge, but for a scale factor, of the correlation function of the noise, but is independent of the distribution function of the modulating random variable. The performance of the asymptotically optimum detector synthesized for Rayleigh fading is assessed via computer simulations. The results show that the performance degradation with respect to the fully optimum performance is scarcely significant, even for low values of the sample size. Moreover, in highly non-Gaussian noise the proposed detector largely outperforms the fully optimum detector synthesized under the correlated Gaussian noise assumption.

ISSN:0001-4966    

DOI:10.1121/1.420080

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP