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Shock and Vibration
Volume 9 (2002), Issue 4-5, Pages 165-175

On the Force Drop Off Phenomenon in Shaker Testing in Experimental Modal Analysis

Paulo Sergio Varoto and Leopoldo Pisanelli Rodrigues de Oliveira

Dynamics Laboratory, Mechanical Engineering Department, School of Engineering of Sao Carlos, University of Sao Paulo, Brazil

Received 15 November 2002; Revised 15 November 2002

Copyright © 2002 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The Electrodynamic Vibration Exciter (shakers) has been one of the most employed excitation sources in modal tests. The shaker is an electromechanical device that provides a mechanical motion due to the input signal sent to its coil. Despite being widely used, it is well known that the shaker interacts with the structure under test. In particular, when the structure passes through a given resonance, the force delivered by the shaker abruptly decreases, causing the so called drop off phenomenon. This paper aims to study this force drop off phenomenon in the single shaker modal testing. Analytical models are developed to help in understanding the physical principles involved in the interaction between the shaker and the structure under test. Experimental analyses are performed using different shakers as well as excitation signals, in order to evaluate the effects of the input signal, as well as the power amplifier operational modes, on the structure dynamics. Preliminary tests revealed that significant distortions might occur during vibration tests using shakers and these distortions significantly affect the determination of the structure response.