Revues techniques

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This article investigates the capability of an array containing two parallel layers with 8 × 8 microphones in each layer and with 3 cm microphone pitch to measure sound intensity at low frequencies in sound fields with strong reactive or diffuse components. This is done basically by extending the concept of pressure-residualintensity (p-RI) index from two-microphone intensity probe measurements to array measurements and by estimation of that index based on both simulated and real measurements. Good agreement is achieved between p-RI indices obtained from simulated measurements and from real measurements in a large standing wave tube. It is shown that for typical array microphones it is essential to correct for the differing frequency responses, typically based on response data stored in the Transducer Electronic Data Sheets (TEDS) of the individual microphones.

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Time Selective Response, TSR, is a frequency response measurement method based on linearly swept sine signals. Because TSR can be used for free-field measurements in ordinary rooms and is fast, accurate and relatively insensitive to background noise, it is convenient for measurements of microphone and sound level meter free-field responses. However, the method’s combination of time and frequency weighting can make it complicated to estimate the uncertainty of the measured response. This paper briefly summarizes the method and presents some experience with and guidelines for choosing measurement and weighting parameters and considerations on the associated uncertainty on the results. The results are discussed on the basis of practical measurements of microphone and sound level meter free-field responses at Brüel&Kjær.

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This paper describes the design of a Volume Velocity Source and verifies its use in the measurement of p/Q FRFs. Two designs of Volume Velocity Source are presented, one for low-mid frequencies, and one for mid-high frequencies, both taking advantage of the two-microphone technique for estimating volume velocity at the orifice during operation.

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Laser interferometry – using counters for fringe counting and determination of zeroes at higher frequencies – has been used for accelerometer calibration since the late 1960s. As digital techniques evolved during the 1980s and 90s, it became possible to make a more sophisticated approach towards interferometer detectors’ complex output signals. This evolution was described in a revision of the previous standard, ISO 5347.1, which resulted in the new ISO 16063.11 in 1999. In the new standard, the sine approximation method was introduced. As the input is known to be a sine, a least square fitting of the results calculated from the interferometer output gives good results. The fitting is essentially a filtering at the known frequency. Therefore, with the advanced high-resolution FFT analysers available today, it is logical to use such analysers to do the filtering.

A system following these concepts has been realised. This system will be described, and an evaluation of the system’s sensitivity to different imperfections (e.g., noise and gain differences in the interferometer output) will be reported. The evaluation is made using computer-generated test signals, and the technique can also be used to verify the system whenever needed.

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A method for measuring the normal incidence transmission loss and related acoustical properties of a sample placed in a four-microphone standing wave tube is described.

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While traditional spectral analysis techniques based on Fourier Transform or Digital Filtering provide a good description of stationary and pseudo-stationary signals, they face some limitations when analysing highly non-stationary signals. These limitations are overcome using Time-frequency analysis techniques such as Wavelet Transform, Short-time Fourier Transform, and Wigner-Ville distribution. These techniques, which yield an optimum resolution in the time and frequency domain simultaneously, are described in this article and their advantages and benefits are illustrated through examples.

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Dyn-X Technology, Order Tracking, and Comparison of IBEM PNAH and LDV Methods

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Acoustical Solutions in the Design of a Measurement Microphone for Surface Mounting; Combined NAH and Beamforming Using the Same Array; Patch Near field Acoustical Holography Using a New Statistically Optimal Method

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This article explains the basic principles of Beamforming, including the main performance parameters Resolution and Sidelobe Level. Special attention is given to the influence of array design and to cross-spectral beamforming. Different array designs, including Brüel&Kjær’s newly patented wheel array design, are described and compared, and the basic principle of Brüel&Kjær’s geometry optimisation method is outlined. A new, improved version of cross-spectral beamforming used in Beamforming Software Type 7768 is introduced and its benefits are verified. The article also provides some guidelines for performing good measurements and finally, describes a set of measurements representing typical applications.

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In a previous paper (SENSOR 1997), the principle of a triaxial, piezoelectric accelerometer was described.

This paper describes a similar design but with the piezoelectric ring suspended on only three simple supports. The basic theory behind the principle is explained and measurement results from a very compact, practical unit are also described.

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The Influence of Environmental Conditions on the Pressure Sensitivity of Measurement Microphones; Reduction of Heat Conduction Error in Microphone Pressure Reciprocity Calibration; Frequency Response for measurement microphones – a question of Confidence; Measurement of microphone random-incidence and pressure-field responses and determination of their uncertainties

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This article gives a general introduction to the principles and applications of the Brüel & Kjær Non-stationary STSF measurement technique, with special emphasis on the new functionality included with Version 2.0 of the software package Type 7712. After a general introduction, the principles of Bad Measurement Point Interpolation and Envelope Intensity are descibed. Two case studies of typical applications are then outlined: the first one, which is measurement of brake squeal, is an example of transient noise source localisation. The second case study is mapping and analysis of microphenomena and orders in engine noise radiation.

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In this article the filter characteristics of the Vold-Kalman Order Tracking Filter are presented. Both frequency response as well as time response and their time-frequency relationship have been investigated. Some guidelines for optimum choice of filter parameters are presented. The Vold-Kalman filter enables high performance simultaneous tracking of orders in systems with multiple independent shafts. With this new filter and using multiple tacho references, waveforms, as well as amplitude and phase may be extracted without the beating interactions that are associated with conventional methods. The Vold-Kalman filter provides several filter shapes for optimum resolution and stop-band suppression. Orders extracted as waveforms have no phase bias, and may hence be used for playback, synthesis and tailoring.

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Danish Primary Laboratory of Acoustics (DPLA) as Part of the National Metrology Organisation; Pressure Reciprocity Calibration – Instrumentation, Results and Uncertainty; MP.EXE, a Calculation Program for Pressure Reciprocity Calibration of Microphones