An investigation of the indirect measurement of broadband force spectra

Timothy John Roggenkamp, Purdue University

Abstract

The motivation of this research is the characterization of a structure-borne acoustical source using the junction forces acting at the interface between the source and the true receiving structure. A method to indirectly determine the broadband junction force spectra is investigated. The matrix of unknown force spectra is synthesized from the responses generated by the unknown forces and the system frequency response functions (FRFs). Synthesized force error bounds and synthesized force error models are developed to identify parameters that indicate the accuracy of the synthesized forces. Guidelines to select the optimum set of response locations based upon the minimization of the FRF matrix condition number are presented. The force synthesis method and the implementation guidelines are experimentally verified using broadband coherent and incoherent multiple random force excitations. The junction forces at the interface between a source and the receiving structure to which it is mounted are dependent on the dynamical characteristics of the receiving structure. Therefore, when source data are collected using a laboratory fixture with dynamical characteristics that are not representative of the true receiving structure, the laboratory junction forces are different from the true junction forces. A force equivalence parameter is developed to identify when the laboratory junction forces are different from the true junction forces. A method to convert between the fixture junction forces and the true junction forces based upon the dynamical characteristics of the fixture, the source and the true receiving structure is presented. For the case when the dynamical characteristics of the source cannot be measured directly, a method to indirectly estimate the dynamical characteristics of the source is developed. The force conversion method and the indirect determination of the dynamical characteristics of the source are experimentally verified on a test structure with two translational degrees of freedom. These methods can be used to characterize a structure-borne acoustical source when the source is mounted to the true receiving structure or when the source is mounted to a laboratory fixture. The source characterization data can then be used to predict the structure-borne noise levels.

Degree

Ph.D.

Advisors

Bernhard, Purdue University.

Subject Area

Mechanical engineering

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