Variation in subcomponents and assembled products

High quality manufactured products are expected to behave essentially identically. However, the measurement of a frequency response function on nominally identical products will typically show significant variation in the dynamic behavior. The dynamic behavior of manufactured products in the frequency range of interest for most noise control applications as measured by frequency response functions are highly sensitive to uncontrolled aspects of manufacturing processes and uncontrolled aspects of the experiment. In this paper, data from well controlled experiments on nominally identical pickup trucks, diesel engines and compressor housings are reported in order to illustrate the variation found in typical manufactured products. The variation in the frequency response functions is also examined in order to identify statistical properties that can be expected of manufactured products. Post-processing of the coefficient of variation for this data is studied as a metric to characterize these product variations.