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Near-field Acoustical Holography (NAH) can perform source location with high spatial resolution even at low frequencies by measuring very close to the sound source and by reconstructing part of the evanescent near-field. But a measurement grid with less than half wavelength spacing is required, and the measurement area should cover the main radiating regions to avoid windowing effects. These requirements make the method impractical at higher frequencies, typically above 3-5 kHz. At those higher frequencies, however, Beamforming can provide good resolution with typically 40-90 measurement points, because it is possible to use irregular arrays at intermediate measurement distances. Various array designs exist that can provide good suppression of ghost images up to frequencies, where the average element spacing is much larger than half a wavelength. The problem to be addressed here is that it is not practical to have to use two different arrays to perform the two types of measurement.

In the automotive industry there is a growing need for measurement of acoustical transfer functions in connection with transfer path analysis, the main outcome being acoustical source contribution analysis. These transfer functions are from monopole Volume Velocity at a source location to the resulting sound pressure at a receiver (listener) position. In most cases it is an advantage to make use of reciprocity, which allows the monopole source position and the pressure response position to be interchanged. The source to be used for these measurements must be powerful and omni-directional, and the frequency range of interest is typically 50-6300 Hz. So the Brüel & Kjær OmniSource™ is in many ways perfectly suited for the application. This paper will discuss the design criteria for a Volume Velocity Source as well as the verification of the performance. Also the use of Volume Velocity Source in Transfer Path Analysis often called Source Path Contribution is described.

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.