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Beamforming with an array of microphones on a sphere is an attractive tool for doing noise source localization in cabin environments. In order to achieve acceptable angular resolution, the array must have some minimum diameter, implying that many microphones are needed to obtain low sidelobe level over the frequency range of interest. For electric cars there is an increased need to cover high frequencies. The present paper describes a method to significantly reduce the sidelobe level over a broad frequency range relative to Spherical Harmonics Beamforming (SHB). For each focus point, a set of Finite Impulse Response (FIR) filters are optimized to minimize the highest sidelobe in a Filter And Sum (FAS) beamformer, while maintaining sensitivity at the focus point and limiting the White Noise Gain (WNG).

This paper investigates the ability of Nearfield Acoustic Holography to “focus” on a source from the not-so-nearfield. While the theory of Nearfield Acoustic Holography (NAH) has been around for half a century, practical commercial products using NAH are less than three decades old, with application in just about every field that measures noise from on-highway to industrial/construction to agriculture to marine to aerospace to white goods to electronics and more. With the wide variety of applications, in many cases it is not possible to measure in the nearfield over the entire measurement area. With the advent of new algorithms, one approach is to create a “conformal” array; another might be to perform patch measurements. This paper instead investigates the use of a planar array measuring at a distance from the calculation plane to identify sources. Since evanescent waves are not measured, using this technique to feed into CAE models is probably not applicable.

Beamforming based on measurements with a spherical microphone array is a recognized technique for localization of noise sources. The method is very efficient for getting a quick overview of the positions of the most dominant sources. However, when more detailed information about the radiation from a source is needed, acoustical holography is normally more suitable, because it provides the actual acoustical quantities (pressure, velocity and/or intensity) at positions on or near the source surface, whereas beamforming only gives a directional map of contribution at the array position. Even though spherical arrays are most often used for beamforming they can also be applied for holography purposes. This means that with the same hardware beamforming can be used to identify a noise source, and then the array can be moved closer to the source to get a clearer picture of the radiation by using holography.

The paper describes an array-based method for measurement of panel contributions in a cabin. Both the FRF measurements and the operational measurements are taken with a dual layer microphone array, and the required sound field data on a panel surface mesh are calculated using the SONAH patch holography algorithm. Multiple array positions can be measured to cover the needed mesh area, and area-integration of contributions is then performed on the surface mesh. Measurement or alignment of surface mesh geometry and measurement of array positions is performed using an integrated 3D position measurement system. After a theoretical description the method is evaluated through simulated and actual measurements.

In some cases it is important to be able to measure not only the total sound intensity on a panel surface in a vehicle cabin, but also the components of that intensity due to sound radiation and due to absorption from the incident field. For example, these intensity components may be needed for calibration of energy flow models of the cabin noise. Two different methods are introduced in the present paper: one based on surface absorption coefficient and one based on surface admittance. The two methods are compared in terms of underlying assumptions and through simulated and real measurements. The method based on absorption coefficient appears to be the more robust.

The present paper describes a set of array-based methods that can provide both a snapshot overview of problematic areas across the panels of a car cabin and perform efficient analysis of details of a noise problem. The snapshot overview is obtained with Beamforming, while detailed analysis of problematic areas is performed with a small planar single-layer or double-layer array in combination with SONAH holography. Using the SONAH algorithm for patch near-field acoustic holography, all sound field parameters can be estimated directly on the irregularly shaped panel surfaces. All the array measurements can be performed very efficiently by the use of a position measurement system integrated with the array. Panel geometry can also be measured using the position measurement system. The paper gives an overview of the different methods and presents results from a case study.

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.

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.

Disc brake squeal can be a major problem during development of new brake systems. Squeal can be loud and persistent or fugitive but nevertheless annoying. Increasing the knowledge of the mechanisms generating squeal is one important contribution to the extensive research and development work being performed in order to solve the problems. The vibration motions of the brake components during squeal, especially the disc, have been studied intensively, and the existence of standing or traveling waves and the direction of such waves have been debated. Several measurement techniques have been employed in order to reveal the nature of the disc motion, including holography, scanning laser vibrometry and rowing accelerometers in the disc. Also acoustic holography has been employed previously - see for example [2] - but this paper documents the ability of acoustic holography to create new knowledge about disc brake squeal through measurement of the disc motion.

Localization and ranking of sound radiating regions on tyres under transient conditions is very difficult both because of the transient nature and because it is difficult to measure sufficiently close to the sound radiating regions. Using Near-field Acoustic Holography it is possible to measure at some distance and then extrapolate back to the surface of the tyre. With the Time Domain Acoustical Holography it is further possible to study the transient nature of the sound generation in any desired detail. As an example, the present paper compares the sound radiation under stationary and under accelerating conditions.