Technical Reviews
Each issue of Technical Review contains a collection of technical, scientific articles including theory, measurement techniques, and instrumentation for acousticians and vibration engineers.

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Technical Review No. 1 2015: Fast Wideband Acoustical Holography

Patch near-field acoustical holography (NAH) methods like SONAH and ESM are limited to relatively low frequencies, where the average array-element spacing is less than half of the acoustic wavelength, while beamforming provides useful resolution only at medium-to-high frequencies. With adequate array design, both methods can be used with the same array. But for holography to provide good lowfrequency resolution, a small measurement distance is needed, while beamforming requires a larger distance to limit side lobe issues. The wideband holography (WBH) method of the present paper was developed to overcome that practical conflict. Only a single measurement is needed at a relatively short distance and a single result is obtained covering the full frequency range. The method uses the principles of CS: A sparse sound field representation is assumed with a chosen set of basis functions, a measurement is taken with an irregular array, and the inverse problem is solved with a method that enforces sparsity in the coefficient vector. Instead of using regularization based on the 1-norm of the coefficient vector, an iterative solution procedure is used that promotes sparsity. The iterative method is shown to provide very similar results in most cases and to be computationally much more efficient.

Technical Review
(2015)

Size:
3199 kb

Authors:
Jørgen Hald, A. Schuhmacher, E. Varricchio

Technical Review No. 1 1989: STSF - a unique technique for scan-based Near-field Acoustic Holography without restrictions on coherence

The Spatial Transformation of Sound Fields (STSF) technique involves a scanning over a (planar) surface close to the source under investigation. From cross spectra measured during the scan, a principal component representation of the sound field is extracted. Any power descriptor of the near field (intensity, sound pressure, etc.) can then be investigated by means of Near-field Acoustic Holography (NAH), while the more distant field can be determined by application of Helmholtz' integral equation. The present paper outlines the theoretical foundation of the cross spectral principal component technique implemented in STSF, and relates it to various other NAH techniques. It is demonstrated that compared to these techniques, the cross spectral formulation as implemented in STSF has the advantage that no restrictions are placed on the coherence and bandwidth of the sound field, without requiring simultaneous measurements.

Technical Review
(2015)

Size:
3258 kb

Technical Review No. 1 2014: Experimental CHaracterization of Operating Bladed Rotor Using HPS and SSI Techniques

The dynamic response of mechanical systems with rotating elements, such as operating wind turbines, cannot be described using a classical linear time-invariant (LTI) formulation because the mass and stiffness matrices can vary periodically during rotation. Such systems belong to the class of linear periodic time-variant (LPTV) systems and require special treatment for their experimental identification. For instance, the Harmonic Power Spectra (HPS) method, which is based on Floquet theory, can be applied. Afterwards, the experimental responses are modulated exponentially using rotational frequency. The HPS matrix is computed between the modulated responses and is used as the input for Operation Modal Analysis (OMA). OMA provides the frequencies of the modes and the Fourier coefficients for reconstructing the time-periodic mode shapes. In the authors' prior publications, the HPS method is applied to the frequency domain. This study extends the HPS method to the time domain and makes it possible to use powerful stochastic subspace identification (SSI) techniques for modal identification. This leads to more accurate parameter estimates and can be used on modes with close frequencies. The advantage of the suggested approach is that it allows for the use of existing implementations of SSI and thus provides a simple tool for modal identification of periodic systems.

Technical Review
(2014)

Authors:
D. Tcherniak, S. Yan, M.S. Allen

Technical Review No. 1 2013: Noise test of revised notched nozzle using a jet engine

This paper describes engine noise tests conducted in an outdoor environment using a revised notched nozzle. A notch is a small dent formed at the nozzle edge that penetrates into the primary jet. The notched nozzle is expected to improve the acoustic performance with less deterioration in aerodynamic performance relative to that of a conventional nozzle. The slight penetration of the notch causes small disturbances immediately after the nozzle, driving the subsequent mixing process in the shear layer. The mixing process helps surpress both large-scale vortices in the far downstrream regiuon and excessive shear near the nozzle.

Technical Review No. 1 2012: High-resolution Fly-over Beamforming This paper describes a commercially available fly-over beamforming system based on methodologies already published, but using an array that was designed for quick and precise deployment on a concrete runway rather than for minimum sidelobe level. Time domain tracking Delay And Sum (DAS) beamforming is the first processing step, followed by Deconvolution in the frequency domain to reduce sidelobes, enhance resolution, and get absolute scaling of the source maps. The system has been used for a series of fly-over measurements on a Business Jet type MU300 from Mitsubishi Heavy Industries. Results from a couple of these measurements are presented: contribution spectra from selected areas on the aircraft to the sound pressure level at the array are compared against the total sound pressure spectrum measured by the array. One major aim of the paper is to verify that the system performs well although the array was designed with quick deployment as a main criterion. The results are very encouraging. A second aim is to elaborate on the handling of the array-shading function in connection with the calculation of the Point Spread Function (PSF) used in deconvolution. Recent publications have used a simple formula to compensate for Doppler effects for the case of flat broadband spectra. A more correct formula is derived in this paper, also covering a Doppler correction to be made in the shading function, when that function is used in the PSF calculation.

Technical Review
(2012)

Size:
3889 kb

Technical Review No. 1 2011: Dual-layer Microphone Array, Acoustic Intensity Probe Calibrator, Multi-field Microphone 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.

Technical Review
(2012)

Size:
2587 kb

Technical Review No. 1 2010: Time Selective Response Method, In situ Measurement of Absorption Coefficient, Transverse Motion in Accelerometer Calibration

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.

Technical Review
(2010)

Size:
1723 kb

Technical Review No. 1 2009: Use of Volume Velocity Sound Source in FRF Measurements, Turnkey Free-field Reciprocity System

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.

Technical Review
(2010)

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5343 kb

Technical Review No. 1 2008: Primary Calibration using Laser Interferometry, Infrasound Calibration of Microphones, High-temperature IEPE Accelerometer

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.

Technical Review
(2009)

Size:
2899 kb

Technical Review No. 1 2007: Measurement of Normal Incidence Transmission Loss and Other Acoustical Properties of Materials Placed in a Standing Wave Tube 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.

Technical Review No. 1 2006: Dyn-X Technology Order Tracking; Comparison of IBEM, PNAH, and LDV Methods Dyn-X Technology, Order Tracking, and Comparison of IBEM PNAH and LDV Methods

Technical Review No. 1 2005: Surface Microphone; NAH and Beamforming using the same Array; SONAH 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

Technical Review No. 1 2004: Beamforming 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.

Technical Review No. 1 2002: New Design Principle for Triaxial PZ Accelerometers, FE models for Optimizing Accelerometer Designs, High Pressure Microphone Calibration System

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.

Technical Review
(2002)

Size:
483 kb

Technical Review No. 1 2001: Properties and Calibration of Laboratory Standard Microphones; Uncertainties in Microphone Frequency Responses 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

Technical Review No. 1 2000: Non-stationary STSF

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.

Technical Review No. 1 1999: Characteristics of the Vold-Kalman Order Tracking Filter 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.

Technical Review No. 1 1998: Danish Primary Laboratory of Acoustics, Microphone Reciprocity Calibration, Calculation Program for Reciprocity, Calibration 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

Technical Review
(1998)

Pages:
40

Technical Review No. 1 1997: A New Design Principle for Triaxial Piezoelectric Accelerometers; A Simple QC Test for Knock Sensors; Torsional Operational Deflection Shapes (TODS) Measurements 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.

Technical Review No. 2 1996: Non-stationary Signal Analysis using Wavelet Transform, short-time Fourier Transform and Wigner-V ille Distribution

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.

Technical Review
(1996)

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3413 kb

Technical Review 1996-2 Non-stationary Signal Analysis using Wavelet Transform, Short-time Fourier Transform and Wigner-Ville Distribution

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.

Technical Review No. 1 1996: A Sound Intensity Probe for Measuring from 50 Hz To 10 kHz; Measurement of Microphone Free-field Corrections and Determination of their Uncertainties; Reduction of Non-linear Distortion in Condenser Microphones by Using Negative Load Capacitance; In Situ Verification of Accelerometer Function And Mounting 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.

Technical Review No. 2 1995: Order Tracking Analysis This article introduces the theory behind digital order tracking, which is implemented by oversampling, interpolation and resampling techniques in Brüel&Kjær instrumentation. As application examples order tacking is applied to the vibration signal from a coast down of a turbogenerator from a power plant station, a run-up of a large marine diesel engine, gated measurement on an automobile engine and finally tracking is combined with time domain averaging on a single cylinder, four stroke engine.

Technical Review No. 1 1995: Spatial Transformation of Sound Fields (STSF) in the Automotive Industry; Pass-by Noise Measurements Ever stricter legislation and competition compel vehicle manufacturers to reduce external vehicle noise. In order to simulate the actual working conditions of vehicles, acoustic testing must be performed not only under stationary but also under non-stationary conditions, e.g. run-up. This article deals with the use of Brüel&Kjær's Spatial Transformation of Sound Fields (STSF) technique as a tool for assessing and understanding the noise radiation phenomena, both qualitatively and quantitatively. Two different implementations of STSF will be described: the well known Cross spectral approach enabling a scan measurement to be applied on a stationary sound source and a new Time domain approach requiring all measurement positions to be acquired simultaneously. While the Cross-spectral STSF is a well established commercially available technique, the Time domain method is still under development.

Technical Review No. 2 1994: The use of Impulse Response Function for Modal Parameter Estimation; Complex Modulus and Damping Measurements using Resonant and Non-resonant Methods (Damping Part II) This article discusses the errors that are introduced when calculating the impulse response function from the frequency response function using DFT/FFT analysis technique. This article also represents an expansion of papers presented at the Modal Analysis Conference ISMA 15, held in Leuven, Belgium 1990 and included in the Proceedings of IMAC 9, Firenze, Italy 1991.

Technical Review No. 1 1994: Digital Filter Techniques vs FFT Techniques for Damping Measurements (Damping Part I) In this article several methods for measurements of damping are summarized with respect to their advantages and disadvantages. Especially the use of Digital Filters (DF) and Fast Fourier Transform (FFT) are compared. In general FFT analysis is best suited for heavily damped structures although with proper memory and postprocessing facilities, lightly damped structures may also be covered, while it is advantageous to use DF analysis when dealing with lightly damped structures only. The use of Time-frequency analysis techniques such as the Wavelet Transform and the Short-time Fourier Transform is also demonstrated.

Technical Review 1990-2 Optical Filters and their Use with the Type 1302 & Type 1306 Photoacoustic Gas Monitors The Brüel & Kjær Multi-gas Monitor Type 1302 and Toxic-gas Monitor Type 1306 both utilize interchangeable optical filters to select a band of infrared light at an appropriate wavelength for monitoring some particular gas. This article describes the range of optical filters available: their construction, specification and their selection and use with Type 1302 and/or Type 1306 for a particular monitoring task.

Technical Review 1990-1 The Brüel & Kjaer Photoacoustic Transducer System and its Physical Properties The Brüel and Kjær Multi-gas Monitor Type 1302 and the Toxic-gas Monitor Type 1306, both used for monitoring gases, are based on photo-acoustic spectroscopy. In photoacoustic spectroscopy (PAS) the gas to be measured is irradiated by intermittent light of pre-selected wavelength. The gas molecules absorb some of the light energy and convert it into an acoustic signal which is detected by a microphone. PAS is an inherently very stable method for detecting very small concentrations of gas. In this article the Brüel & Kjær PAS transducer system is described, the acoustic signal generated by the gas is analyzed, and the influence of noise and other disturbing signals, which are likely to influence the performance of the system, are discussed.

Technical Review 1989-2 STSF - Practical instrumentation and applications; Digital Filter Analysis: Real-time and Non Real-time Performance The Spatial Transformation of Sound Fields (STSF) technique involves a scan using an array of transducers over a (planar) surface close to the source under investigation. From cross spectra measured during the scan, a principal component representation of the sound field is extracted. Any power descriptor of the near field (intensity, sound pressure, etc.) can then be investigated by means of Near-field Acoustic Holography (NAH), while the more distant field can be determined by application of Helmholtz' integral equation. The theoretical foundation of the cross spectrum principal component technique implemented in STSF has been discussed by J. Hald in [1] and the advantages of STSF compared to a number of acoustical holography techniques have been demonstrated. In this article, the practical implementation of STSF is discussed and some applications are described.

Technical Review 1989-1 STSF - a unique technique for scan-based Near-field Acoustic Holography without restrictions on coherence The Spatial Transformation of Sound Fields (STSF) technique involves a scanning over a (planar) surface close to the source under investigation. From cross spectra measured during the scan, a principal component representation of the sound field is extracted. Any power descriptor of the near field (intensity, sound pressure, etc.) can then be investigated by means of Near-field Acoustic Holography (NAH), while the more distant field can be determined by application of Helmholtz' integral equation. The present paper outlines the theoretical foundation of the cross spectral principal component technique implemented in STSF, and relates it to various other NAH techniques. It is demonstrated that compared to these techniques, the cross spectral formulation as implemented in STSF has the advantage that no restrictions are placed on the coherence and bandwidth of the sound field, without requiring simultaneous measurements.

Technical Review No. 2 1988: Quantifying Draught Risk

Draught, defined as an unwanted local cooling of the human body caused by air movement, is one of the most common causes of complaints in ventilated or air-conditioned spaces. The high ventilation rate required to establish acceptable air quality, often leads to uncomfortably high air velocities. Because of the high level of pollution in modern buildings, caused by increased internal loads from office machines (computers, printers, etc.), there will be an ever increasing demand for higher ventilation rates. In most existing standards the requirements for the air velocity in a room are based on the mean air velocity limit, which depends on the existing air temperature. New draught studies show, however, that high turbulent air flow causes more complaints of draught than low turbulent air flow at the same mean air velocity and temperature. Many field studies show that turbulence intensity in rooms varies from 10-70%, indicating that it should be taken into account during revision of the existing standards. This paper discusses how to deal with the draught problem, and describes the measurements necessary for estimating draught risk.

Technical Review
(1988)

Size:
2579 kb

Technical Review No. 4 1987: Use of Weighting Fuctions in DFT/FFT Analysis (Part II); Acoustic Calibrator for Intensity Measurements Systems Part II of the article "Use of Weighting Functions in DFT/FFT analysis" contains the following Appendices referred to in Part I of the article A: Analogy between filter analysis and DFT/FFT analysis, B: Windows and figures of merit, C: Effective Weighting of overlapped spectral averaging D: Experimental Determination of the BT product for FFT-analysis using different weighting functions and overlap, E: Examples of User Defined Windows, F: Picket Fence Effect

Technical Review No. 3 1987: Use of Weighting Fuctions in DFT/FFT Analysis (Part I); Signals and Units This article demonstrates how the analogy between DFT/FFT (Discrete Fourier Transform/Fast Fourier Transform) analysis and filter analysis (analogue or digital) can be used to better understand the applications of different weighting functions used in DFT/FFT. The filter characteristics of the most commonly used weighting functions (also called windows) are illustrated and discussed with respect to their use in various practical applications of system and signal analysis. The mathematical formulations of the analogy as well as rigorous details of the article will be given in the Appendices in Part II of this article to be published in Technical Review No. 4-1987.

Technical Review 1987-2 Recent Developments in Accelerometer Design; Trends in Accelerometer Calibration Two-channel FFT analysis is used to evaluate the performance of various accelerometer designs. The measurements demonstrate the importance of using the frequency spectrum for evaluation of the environmental performance of transducers. Furthermore, the superior environmental-performance characteristics of the Briiel & Kjaer Delta Shear® piezoelectric accelerometer design as compared to other designs are demonstrated.

Technical Review No. 2 1987: Recent Developments in Accelerometer Design, Trends in Accelerometer Calibration

Two-channel FFT analysis is used to evaluate the performance of various accelerometer designs. The measurements demonstrate the importance of using the frequency spectrum for evaluation of the environmental performance of transducers. Furthermore, the superior environmental-performance characteristics of the Brüel & Kjær Delta Shear^{®} piezoelectric accelerometer design as compared to other designs are demonstrated.

Technical Review
(1987)

Size:
2792 kb

Authors:
Torben R., Licht, Henrik Andersen, Henrik Brill Jensen

Technical Review No. 1 1987: Vibration Monitoring of Machines

All rotating machinery generate vibration, the analysis of which renders valuable information about the condition of the machines. With the advent of battery-operated portable FFT Analyzers and Desk-Top Calculators, it is now possible to detect incipient faults in the most common machine elements several months before repair becomes imperative. This article describes the type of vibration signals to be expected for faults in typical elements and the analysis techniques to be used for early detection. Furthermore, diagnostic methods are described which permit estimation of the remaining life of an element once a fault has been detected.

Technical Review
(1987)

Size:
3483 kb

Technical Review 1987-1 Vibration Monitoring of Machines All rotating machinery generate vibration, the analysis of which renders valuable information about the condition of the machines. With the advent of battery-operated portable FFT Analyzers and Desk-Top Calculators, it is now possible to detect incipient faults in the most common machine elements several months before repair becomes imperative. This article describes the type of vibration signals to be expected for faults in typical elements and the analysis techniques to be used for early detection. Furthermore, diagnostic methods are described which permit estimation of the remaining life of an element once a fault has been detected.

Technical Review No. 4 1986: Sound Insulation using Intensity Microphones for Intensity Probes and their Calibration

The measurement of sound intensity is becoming increasingly popular, and the range of applications is broadening accordingly. The sheer size and cost of the equipment has, however, been an impediment to the widespread use of the sound intensity technique. This paper presents a small, battery-operated intensity analyzer which allows field measurements to be done conveniently in octaves and broadband. To illustrate the usefulness of the analyzer, a measurement of sound insulation between two rooms in a building is described.

Technical Review
(1986)

Size:
6301 kb

Technical Review No. 3 1986: Deconvolution for Modal Decoupling Oblique DOF

The problem of modal coupling encountered in vibration response, leads to obscurity of the discrete nature of the system's dynamic properties. The coupling effect can be significantly reduced by deconvolution, which is obtained by weighting the response signal from an impact excitation or a measured impulse response. The transform of the weighted function is called the mode spectrum, the properties of which will be discussed, indicating some guidelines for practical application. The theory of deconvolution will be described together with experimental results from analytical data and from measurements made on a real structure with coupled modes.

Technical Review
(1986)

Size:
6036 kb

Technical Review No. 2 1986: Spectral Match

The article reviews some methods for obtaining spectral match, and calculations of measurement errors are made for some transducers of differing match, exposed to different types of light sources. The new CIE method for defining the error in spectral match is explained and compared to other error definitions used in USA and Japan.

Technical Review
(1986)

Size:
5616 kb