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Near-field Acoustical Holography (NAH) is a powerful, high-resolution noise source location technique for low-to-medium frequencies, requiring however often too many measurement positions at high frequencies. Beamforming provides too poor resolution at low frequencies, but very good resolution at high frequencies with relatively few measurement points. A combination of the two techniques therefore seems attractive. But where traditional NAH requires a regular array, Beamforming can provide the attractive high-frequency performance only by the use of irregular array geometries. The present paper describes a system that enables NAH and Beamforming to be supported with the same irregular array, and which can provide comparable scaling of the output from the two types of array measurement.

Noise levels and noise characteristics in a car cabin are important competitive discriminators in the automotive industry today. Efficient tools for mapping and analyzing the underlying source distributions are therefore very important for automotive manufacturers. The present paper describes a system based on a hand-held microphone array with integrated position measurement and using the Statistically Optimal NAH (SONAH) calculation method to perform patch holography. The SONAH method has the big advantage as compared to traditional NAH that spatial window effects are small even when the measurement area does not fully cover the source area. The paper also presents practical measurements.

Some of the frequencies of transmission gear whine noise reach up to several kHz. High-frequency gear whine noise is mostly transmitted by air (airborne); therefore, it is critical to reduce transmission radiation noise. This paper presents how to solve the problem of high-frequency noise in the range of 2.0 - 4.1kHz by experiment using Inverse Boundary Element Method (IBEM) and by computer simulation using Boundary Element Method (BEM).

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.

In recent years the increase of computer data processing power has facilitated the use of several multi-channel techniques for noise source location and ranking. These techniques include Spatial Transformation of Sound Fields (STSF) and Inverse Boundary Element Method (IBEM) for reconstruction of the sound field on the source surface from near field measurements and Beamforming for far field measurements. STSF and IBEM are suited for the low to medium frequency range whereas Beamforming is suited for the medium to high frequency range. The array methods are briefly introduced and a discussion of the advantages and limitations of the different techniques is presented. Benefits of combination of different techniques are also considered and exemplifying measurements are presented.

To ensure that vehicles meet the ISO 362 pass - by requirements before they are introduced in the market their transient noise radiation must also be tested under laboratory conditions. The paper describes a Time Domain Holography based method for localization, quantification and ranking of sound generating regions on vehicles under run-up (i.e. non-stationary) conditions. A case study on a tyre illustrates some of the possibilities with the new technique.

Ever stricter legislation is compelling vehicle manufacturers to reduce external vehicle noise. There is, for example, a recommendation before the European Commission that vehicle drive-by noise be reduced to 74 dB(A) for cars and light trucks to take effect from October 1994. Engine manufacturers are therefore exposed to an increasing demand to produce quieter products. This paper explains how certain companies are employing the Spatial Transformation of Sound Fields (STSF) technique to address this problem. A brief introduction to the main principles of STSF is given followed by a number of practical examples of measurements on engines in test cells.