Technical Reviews

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.

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

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

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.

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

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.

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.

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

A simple set-up for quick and purely electrical QC testing of knock sensors is described in this paper. The main goal is to measure the “impedance” frequency response function of the sensors. The advantages of the test procedure described in this paper are as follows: Simple “pseudo” one channel measurement, from the operator’s point of view. Electrical testing, i.e. no need for shakers, power amplifiers, elaborate fixtures etc.. Pulse excitation, thus very fast — in the order of 100ms for one measurement test. No averaging needed. Standard frequency response function display is used, whereby no complex postprocessing is required. The only requirement is a small junction box with the necessary transformer and some electronics. The power supply for the junction box is fed from the analyzer. The testing method can be extended to other types of transducers such as accelerometers.

The upper frequency limit of a p-p sound intensity probe with a certain microphone separation distance is generally considered to be the frequency at which an ideal probe would exhibit an acceptably small finite difference error in a plane wave of axial incidence. This article shows that the resonances of the cavities in front of the microphones in the usual ‘face-to-face’ configuration give rise to a pressure increase that to some extent compensates for the finite difference error. Thus the operational frequency range can be extended to an octave above the limit determined by the finite difference error, if the length of the spacer between the microphones equals the diameter.