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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.

Operational Modal Analysis -the technique where modal parameters are estimated without known input forces- has been an accepted tool in advanced mechanical engineering. The technique has its advantages were the modal parameters have to be estimated during the operational conditions as in flight ad on road testing. The paper will discuss the recent state of the art in vibration data acquisition as well as in modal parameter estimation enabling easy use of Operational Modal Analysis. A new data optimisation method, which will enhance the speed of calculation, will be shown Examples from both laboratory set-up and practical examples from automotive applications will be shown.

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.

Operational Modal Analysis, also known as Ambient Modal Analysis, has an increasing interest in mechanical engineering. Especially on structures where the excitation and not less important the determination of the forces constitutes a problem In this paper modal analysis is made on the exhaust system of a passenger car. For estimation of the modal parameters two scenarios were used: Estimation using only the responses when excited by the engine during operation and classical input output modal analysis. The results from the two different methods are compared. When using the response measurements only to estimate the modal parameters different types of dedicated estimators for Operational Modal Analysis are used and compared.

The non-contact measurement methods used in laser-based vibrometers offer some unique advantages compared to using traditional, surface mounted accelerometers. Applications that were previously difficult or even impossible to perform can now be accomplished. This includes measurements on small lightweight, delicate or hot structures, inaccessible parts and measurements in high-voltage or radioactive environments. Furthermore, the frequency range can be quite high. When using a Scanning Laser Doppler Vibrometer (SLDV), additional benefits are gained. This includes high spatial resolution in the measurement and fast and flexible data acquisition. The paper itself describes the major features of such Scanning laser Doppler Vibrometer in general, but emphasis will be given during the oral presentation to its benefits in the automotive industry specifically.

This paper shows how the impulse response of each individual resonance in a frequency response function is calculated via a series of weighting functions and then used for damping calculations. The technique is not new (1)* but previously it has been indicated (2, 3) that when using pseudo-random excitation, T > 2.5τ must be fulfilled in order to make accurate damping measurements, where T is the Fast Fourier Transform record length and τ is the decay constant of the damping. It is shown that tolerable damping estimates can be made even if the record length T is as small as 0.05τ. This is because the short record length produces an aliasing phenomena on the amplitude of the impulse response function only - not on the slope of the decay.