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May 1981

Volume 69, Issue S1, pp. 31-S125

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back to top Session J. Shock and Vibration I: Mobility and Ground Vibration
Invited Papers
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The basics of mobility and impedance (A)

Eugen J. Skudrzyk

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S23-S24 (1981); (2 pages)

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Impedances and admittances of lumped parameter systems are described by “positive” functions which have specific properties. Foster's, Cauer's, and other theorems describe their driving‐point behavior and their transmission properties. Mode methods generalize the theory for vibrators with continuously distributed parameters. The mobility turns out to be proportional to the mode contributions, whereas the impedance is found to be a mathematical artifice. The “point mode parameters” make it possible to introduce modal impedances even if force and displacement are spatial variables. Shell vibrators are of special interest; their modes consist of several coupled components with different resonance frequencies. If the resonances of a vibrator are well separated, maxima and minima of their response are determined by the contributions of the two modes whose resonance frequency is nearest to the frequency of the force. The mean line through the driving‐point response is represented by the characteristic or mean value admittance, which is the analog of the characteristic admittance of a wave guide. Examples will illustrate the theory. [This work was sponsored by the Office of Naval Research, Code 474.]
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Mechanical mobility methods for ranking structural transmission path strengths (A)

B. E. Douglas and J. E. Smith

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S24-S24 (1981); (1 page)

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This paper examines the utility of mechanical mobility methods for ranking the vibration transmission path strengths in complex fluid‐structure radiation problems. Such path‐strength determinations are an important factor for selecting appropriate noise control measures and in minimizing the economic impact of corrective action. Emphasis is placed on the direct equivalent force‐moment method which is based on the identification of multiple source inputs (i.e., level, location, and degree of freedom), and the characterization of radiating surfaces (i.e., level, location, degree of freedom, type, and coherence with other outputs). This method is shown to serve as a means of experimentally verifying finite element models and as an aid in modeling source‐forcing functions.
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Mobility of high impedance structures (A)

J. W. Slack

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S24-S24 (1981); (1 page)

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There are a number of ways of measuring mechanical mobility which are commonly used today. These involve differences primarily in transducer instrumentation, excitation signal, and data processing. As part of the author's doctoral research, a number of mechanical mobility measurements were made on a cast iron engine block. The techniques chosen for mobility measurement are strongly influenced by the structures to be studied. The techniques chosen by the author are presented. An unusual technique for measuring the impedance head stiffness is discussed in detail. Examples are given demonstrating reciprocity and reproducibility of data. Some applications of the mobility measurements are presented.
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Assessing the quality of measured mobility data (A)

D. J. Ewins and J. Griffin

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S24-S24 (1981); (1 page)

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A survey has recently been completed in the UK and in France to assess the state‐of‐the‐art in mobility measurement techniques. Specially‐designed test structures were circulated amongst some 30 laboratories whose measurements of specified mobility parameters and modal analysis results were collected and compared. Various forms of presentation of the collected data were explored in order to highlight the trends and the areas of both good and poor agreement. The data were also subjected to various statistical analyses in attempts to quantify the scatter. Results from the survey indicated a significant spread of results in most cases. A review is made of the different measurement and analysis techniques used and their influence on the accuracy of the results. [Work sponsored by Ministry of Defence (P.E.).]
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Measurements and applications of mobility (A)

A. C. Keller

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S24-S24 (1981); (1 page)

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The concept of frequency‐response function measurements has been developed for many years. Originally conceived for use in feedback control systems design and circuit analysis, it is only in the past few decades that it has been applied to problems in structural dynamics. This paper traces the early development of instrumentation for making transfer function measurements, for many years called mechanical impedance plots. The rationale behind many of the analog techniques employed and their relationship to current digital methods is presented. The current state of commercial instrumentation is discussed, some of the uses to which measured functions are put is examined and some conclusions drawn.
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Some practical uses of mobility measurements (A)

R. G. DeJong

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S24-S25 (1981); (2 pages)

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The availability of digital processing equipment makes it possible to collect a large number of impedance and mobility spectra when studying the vibration characteristics of structures. But are they worth more than the paper on which they are plotted? Some examples are given of ways to use mobility measurements to improve system modeling and determine transmission coefficients in connected structures.
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Impedance measurements of joints in mechanical systems with clearances (A)

N. Duke Perreira

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S25-S25 (1981); (1 page)

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Although the motion of mechanical system elements is partially controlled by kinematic constraints, variation in the system motions occur because of the clearances and compliances found in the connections [S. Dubowsky, J. Maatuk, and N. D. Perreira, J. Eng. Ind. 97B, 635–642 (1975); S. Dubowsky and T. Gardner, J. Eng. Ind. 97B, 652–661 (1975)]. In past studies, attempts have been made to model the motions of the system elements when connection compliances and clearances occurred. [R. Herbert and D.C. McWhannel, ASME Paper 76‐Det‐36] Experimental evidence has shown that these models have some degree of accuracy [S. Dubowsky and S. Young, J. Eng. Ind. 97B, 1116–1125 (1975)]. In this work the primary attempt is to experimentally determine if the above dynamical models can be used to determine the force‐motion dependence of the connection clearance and compliance parameters in linkage systems. It is believed that information gained in this study can be used not only in the construction of system dynamical and acoustic radiation models [N. D. Perreira and S. Dubowsky, J. Acoust. Soc. Am. 67, 557–562 (1980)1, but also in developing mechanical wear models for various system joints. [Work supported by NSF.]
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Vibration characteristics of a floor sample (A)

J. H. Rainer and G. Pernica

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S25-S25 (1981); (1 page)

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Vibration tests consisting of heel impacts by a person, shaker impacts, and random shaker excitations were carried out on an 8.5 × 8.3 m steel‐joist‐concrete floor. Structural variations to the floor sample included edge conditions restrained and free; two slab thicknesses; changes in joist stiffness; and inclusion of transverse bracing. The lower four to six natural frequencies and associated mode shapes and damping values are determined. Damping for the sample is significantly increased by the presence of one or more persons on the floor. Viscous damping coefficients for people are evaluated, and comparisons presented between measured and calculated damping ratios, modelling people by simple dashpots. Predicted and measured acceleration responses due to heel impact, shaker impact, and shaker white‐noise excitation are also compared.
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Human response to train vibrations (A)

A. K. Dixit

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S25-S25 (1981); (1 page)

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Preliminary results of a recent Ontario study are presented in which over two hundred residents rated train noise and vibration at seven sites. Models are analyzed for community annoyance as a function of measured vibration levels. Vibration measurement parameters which correlate well with community response are identified for application in the impact assessment procedure. This work forms the continuation of Train Noise Program of Ontario Ministry of the Environment under which railway yard noise has been previously investigated.
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An improved transducer mount for ground vibration measurements (A)

M. Barman and J. E. Coulter

J. Acoust. Soc. Am. Volume 69, Issue S1, pp. S25-S25 (1981); (1 page)

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A simple, convenient, and inexpensive transducer mounting system for soil vibration measurement is suggested. Analysis indicates excellent transducer coupling to soil in all axes of motion. Testing to date indicates an improvement of about two octaves in vertical frequency response over common transducer mounting techniques. At the same time the device avoids some of the shortcomings of other mounting methods.
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