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Identification of noise sources that relate to the actual physical geometry of a structure such as an engine, a gearbox, etc. has seen an increasing interest during recent years. New techniques such as the Inverse Boundary Element Method (IBEM) has emerged with capability of displaying surface velocity estimates on the conformal shape of the object using non-contact acoustic measurements around the structure. This paper compares IBEM with the more traditional Laser Doppler Vibrometry (LDV) method. LDV provides a point-by-point sampling of the surface of a structure, thereby producing a vibration map where the IBEM method makes use of acoustic measurements around the structure. These measurement data are fed into an IBEM allowing for reconstruction of the structural surface vibration. The quality of the IBEM surface vibration reconstruction is assessed and discussed for an experiment using a vibrating panel on a closed box and compared to the results from the laser measurements.

The present paper focuses on methods for estimating equivalent source positions or “hot spots” on an object to be modelled acoustically. This procedure is the first step in the derivation of an acoustic equivalent source model to be used e.g. in connection with measured acoustic transfer paths. Methods based on Near-field Acoustic Holography as well as the Inverse Boundary Element Method are described and compared. The use of the different methods is illustrated by actual measurements and calculations on a real passenger car exhaust line system.