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Operational Modal Analysis, also known as Output Only Modal Analysis or Ambient Modal Analysis, has for over a decade been used for extracting modal parameters from civil engineering structures and is now also being used for mechanical structures, such as on-road and in-flight testing. The advantage of this method is that no artificial excitation needs to be applied to the structure and if artificial loading is required, the force does not need to be measured. All parameter estimation is based on the response signals, thereby minimising the amount of work required for test preparation. As the loading force is unknown in Operational Modal Analysis, specially designed modal parameter estimation techniques need to be used. In classical modal analysis many validation tools are based upon the known force, but if this is not known, other validation tools must be implemented. In addition, different estimation techniques are used, so that results can be compared.

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.