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Apr 1980

Volume 67, Issue S1, pp. S1-S103

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back to top Session D. Shock and Vibration I: Practical Vibration Problems and Solutions
Invited Papers
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Method for predicting the development of vibration‐induced white finger (A)

A. J. Brammer

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S9-S10 (1980); (2 pages)

Online Publication Date: 11 Aug 2005

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Habitually exposing the hands to vibrations may lead to recurring episodes of finger blanching and numbness, with reduction in manual dexterity, tactile sensitivity, and damage to soft tissue occurring in severe cases. By systematically analysing retrospective studies of workers occupationally exposed to vibration, it can be shown that the duration of employment (and hence exposure) determines, on the average, both the appearance of the initial vascular symptoms, and the transition from reversible to irreversible disorders in groups using similar tools. A power law relating vibration amplitude and duration of exposure can then be deduced for occupations involving near‐daily exposure, provided an equinoxious contour is selected to frequency‐weight vibration spectra. In this way both the appearance and progression of subjectivity reported symptoms can be related to the vibration experienced by the hands. If it is then assumed that an individual's susceptibility remains unchanged during the course of the disorders, tolerable vibration exposures for the whole population can be predicted. The vibration limits so derived compare favorably with those proposed for day‐long exposure by the ISO.
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Random response of identical, one‐dimensional coupled subsystems (A)

P. W. Smith, Jr.

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S10-S10 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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A precise statistical analysis has been carried out for the steady‐state response of a general one‐dimensional wave‐bearing system formed of two identical subsystems coupled together at an end. The statistics correspond approximately to averaging in frequency. Cross correlation between the wave fields incident on the coupling from the two subsystems (neglected in most statistical analyses) strongly affects the magnitude of the coupling power and, for dissipative couplings, the dissipative power. Asymmetry of the wave field in each subsystem (also usually neglected) can have a significant additional effect even when the dissipative loss factor is small. These results from a scattering analysis are compared with prior results [T. D. Scharton and R. H. Lyon, J. Acoust. Soc. Am. 43, 1332–1343 (1968)] from a modal analysis of a particular system.
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Random vibration of an impacting oscillator; the distribution of peaks (A)

Huw G. Davies

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S10-S10 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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Random vibration of a two‐degree‐of‐freedom oscillator with mass impacting springlike stops is discussed. The excitation is stationary Gaussian white noise. The mean‐square acceleration of the impacting mass remains finite. An exact expression is obtained for the joint probability density function of displacement, velocity, and acceleration of the impacting mass. The pdf of positive peaks is obtained, at least to within a constant, which must be determined numerically. Increasing concentrations of peaks occur as the strength of the stops is increased. The exact peak pdf is compared with an approximate expression obtained by assuming that the response is a narrow‐band process. The comparison covers a wide range of values of the ratio of expected frequency of peaks to expected frequency of zero up‐crossings. Marked differences between the exact and approximate expressions are obtained when the frequency ratio even slightly exceedes unity.
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A statistical energy analysis of the vibration of piping structures (A)

Richard G. DeJong

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S10-S10 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The vibration characteristics of piping structures were investigated using statistical energy analysis. Previous analytical expressions for the modal density and impedance of cylindrical shells [M. Heckl, J. Acoust. Soc. Am. 34, 1553‐1557 (1962)] were expanded to obtain more accurate results in the frequency range below the ring frequency. The case of a pipe connected to a plate was considered, and an expression for the coupling loss factor between the two structures was developed for a wide frequency range extending from well below to well above the ring frequency of the pipe. Laboratory measurements were made on three pipe‐plate structures having different geometric ratios. The results indicate the importance of the various order cylinder modes in the vibration characteristics of piping structures. [Work supported in part by DTNSRDC/Annapolis and EPA‐ONAC.]
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Equivalent circuit for radial vibrations of thick walled hollow cylinders (A)

W. D. Wilder

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S10-S10 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The exact closed form distributed equivalent circuit and four‐pole parameters are derived for radial vibrators of thick walled hollow cylinders. The formulation applies to cylinders which are either very short (free) or very long (clamped) in the axial direction, and to lostless or viscoelastic materials.
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The frequency response of vibrators with coupled modes, such as cylindrical and spherical shell (A)

E. J. Skudrzyk

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S10-S10 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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In the standard theory, the mode masses are multiplied by a mode shape factor to account for the energy of coupled‐in‐modes. This procedure represents an excellent approximation in the frequency range near the resonance peaks, but it is not exact. If, for instance, a spherical shell is driven by a point force, the stretching modes are not excited directly since the point of attack is a node for these modes; nevertheless, they all become excited because of their coupling with the transverse vibration of the shell. Because of the distributed nature of their excitation, they can be excited with a negative sign. For instance, in the case of a spherical shell at low frequencies, the stretching modes counteract the transverse motion. It is proved in network theory, that coupled systems can be represented by parallel connections of simple tuned circuits with frequency‐independent elements, and that, because of the coupling, some of their signs can be negative. Similar results apply to shells. As a consequence, the driving point admittance does exhibit not only sharp antiresonance, but exhibits also some shallow minima. The deviations between the exact and the standard mode solution will be discussed and the frequency response of coupled vibrators will be illustrated by the mean‐value method for cylindrical and spherical shells. [This work was sponsored by the Office of Naval Research (Code 474).]
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Dynamic response of fluid coupled coaxial cylinders to seismic excitation (A)

M. L. Chu

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S11-S11 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This study deals with the overall dynamic behavior of concentric flexible cylinders coupled with fluid in the annular region, subjected to both sinusoidal and seismic excitations. Three different combinations of acrylic cylinders are used, i.e., one outside cylinder and three inside cylinders with different inside diameters. The cylinders have the same thickness and length, which when paired with the outer cylinder provide three annular gap sizes. To determine the sinusoidal response of the system the outside cylinder is point‐excited and the natural frequencies and corresponding mode shapes are determined. For the seismic tests, the three coaxial setups are mounted on a rigid steel support frame. The support frame is suspended from the ceiling by flexible rods and excited by a hydraulic shaker with a simulated seismic input. Two distinct responses are observed, i.e., either in‐phase or out‐of‐phase breathing modes between the inner and outer cylinders. The out‐of‐phase modes approach some limiting value as the gap size decreases, while the in‐phase modes increase in frequency as the gap becomes smaller. Comparison between the inner and outer cylinder displacement responses from the seismic loadings shows the ratios of the averaged peak responses varies as a function of the gap between the inner and outer cylinder. Similar trends were observed on the spectral densities of the responses. It also appears that the gap size has a strong influence in the frequency response of the system.
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On the nonlinear response of a submerged cylindrical shell to transient acoustic loading (A)

M. E. Giltrud

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S11-S11 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This work gives the nonlinear solution for the response of isotropic cylindrical shells to transient acoustic loading. The approach is numerical and in particular applies the USA‐STAGS code which combines the finite element method for the structure with the Doubly Asymptotic Approximation (DAA) for the fluid. We assess the influence of both large strain and plasticity on the response of the structure and the fluid. One important finding is that the modal responses are dependent upon the loading intensity, a situation not observed for the linear case.
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Effect of a fluid‐viscoelastic composite baffle on the signal pressure (A)

Sung‐Hwan Ko

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S11-S11 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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A theoretical model was developed to evaluate the effect of a fluid‐viscoelastic composite baffle on the signal received by an array of hydrophones. The composite baffle placed between the vibrating plate and the semi‐infinite fluid medium (water) is designed primarily for reducing the pressure fluctuations in the water originated from the vibratory motion of the plate. Due to their acoustic softness, noise reduction baffles for underwater use degrade the signal being received by producing a partially out‐of‐phase reflected wave. Therefore, it is desirable to improve the signal‐to‐noise ratio for a given type of the baffle. The viscoelastic portion of the composite baffle (which may be called the signal conditioning plate) is a rubberlike material that has a loss factor associated with the shear modulus. The present model is compared to the case where the viscoelastic signal conditioning plate is replaced by an elastic thin plate. Effects of various parameters on the signal pressure are discussed. [Work supported by Naval Material Command and Naval Sea Systems Command.]
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