Application Notes

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A pass-by noise measurement is defined as the method of measuring the noise emission of a road vehicle under acceleration conditions, with various gear positions in a certain measurement range. These measurements are mandatory for automotive manufacturers in terms of product certification. For this reason, ISO (International Organization for Standardization) regulates the measurement and analysis procedures, as well as the reporting format.
Pass-by noise measurements are the most important part of troubleshooting in the mass production of cars, for example in the analysis of engine, intake and exhaust, and tyre noise. The pass-by noise measurement method is designed to meet the requirements of simplicity, as far as it is consistent with the reproducibility of the results.
The pass-by noise measurement should be performed in a large open space, for type approval of commercial vehicles, or measured during the official test station’s manufacturing stage. Therefore, it is very important that the certification of emission noise measurements is performed before mass production starts.
In some cases, however, pass-by noise measurements cannot be taken out in ‘the field’ because of bad weather or poor test-track conditions. In such cases, the indoor simulated pass-by noise measurement is often used. The indoor simulated pass-by noise measurement does offer a number of advantages such as good repeatability, flexibility, ease of use, etc., and is being considered as the conformance test, together with the field pass-by test, by ISO standard.

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Dynamic sensitivity calibration of pressure sensors using steady sine wave excitation is very rare. Pressure sensors are typically calibrated with static pressure only or, in addition to this, with a single pulse using a dynamic or hydraulic impulse calibrator. Until now no system has been available for dynamic calibration using steady sine excitation.

Experiments have been made using the Brüel&Kjær High Pressure Microphone Calibration System Type 9719 for this purpose. This system was originally designed to measure the dynamic linearity and the harmonic distortion of measurement microphones in the dynamic range of 94 to 171 dB_RMS re 20 μPa at a single frequency, 500 Hz. The distortion of the applied pressure is less than 0.5% up to the maximum SPL of 171 dB_RMS.

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Brüel & Kjær is a world-leading manufacturer and supplier of sound and vibration solutions. We help our customers solve their sound and vibration problems, whether measuring traffic noise, car engine vibration, evaluating building acoustics or performing quality control. The wide range of business areas we cover gives us the capability to offer complete vibration testing solutions from a single supplier, providing our customers with a better platform and an expanded team to service their requirements.

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The Application Note verifies the new Symmetrical Pinnae for Head and Torso Simulator (HATS) Type 4128-C, by comparing measurements obtained using the original pinnae to those obtained using the new symmetrical pinnae.

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Brüel & Kjær is a world leading manufacturer and supplier of sound and vibration solutions. We help our customers solve their sound and vibration problems – from measuring traffic noise and vibration in car engines to evaluating building acoustics and performing quality control. This combined business adds capability in offering complete vibration test solutions from a single source, providing our customers with a greater platform and an expanded team to service their needs and requirements.

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Brüel & Kjær is a world leading manufacturer and supplier of sound and vibration solutions. We help our customers solve their sound and vibration problems – from measuring traffic noise and vibration in car engines to evaluating building acoustics and performing quality control. This combined business adds capability in offering complete vibration test solutions from a single source, providing our customers with a greater platform and an expanded team to service their needs and requirements.

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L'analisi Envelope o Demodulazione di Ampiezza consente di estrarre il segnale di modulazione da un segnale modulato in ampiezza ed il risultato è la storia temporale del segnale modulante. Questo segnale può essere studiato così com'è nel dominio del tempo oppure può essere soggetto ad una successiva analisi di frequenza. L'analisi Envelope è lo spettro FFT (Fast Fourier Transform) del segnale modulante e può essere impiegata per la ricerca e la diagnosi di difetti che determinano la modulazione di ampiezza della frequenza caratteristica di un elemento rotante quali, ad esempio, riduttori, turbine e motori elettrici; è, inoltre, un mezzo diagnostico eccezionale per la verifica di fessurazioni e di impronte nei cuscinetti a rotolamento.

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Kurtosis is a statistical parameter used to characterize a signal. In essence it provides a measure of the “peakedness” of a random signal. Signals that have a higher kurtosis value have more peaks that are greater than three-sigma; that is, peaks that are greater than three times the RMS value of the signal. In the real world many kinds of vibration environments are characterized by signals that have high kurtosis value (relative to Gaussian random). The fatigue and damage potential for these vibrations are higher than for a pure Gaussian replication of the vibration environment. Hence using a traditional Gaussian random signal as the test signal will actually under test the product for its service environment.

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DIRAC PC software is used for measuring a wide range of room acoustical parameters. Based on the measurement and analysis of impulse responses, DIRAC supports a variety of measurement configurations. For accurate measurements according to the ISO 3382 standard, you can use the internally generated MLS or sweep signals through a loudspeaker sound source. Survey measurements are easily carried out using a small impulsive sound source, such as a blank pistol or even a balloon. Speech measurements can be carried out in compliance with the IEC 60268-16 standard, for male and female voices, through an artificial mouth-directional loudspeaker sound source or through direct injection into a sound system, taking into account the impact of background noise. DIRAC is not only a valuable tool for field and laboratory acoustics engineers, but also for researchers and educational institutions.

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The aim of this application note is to give a brief overview of the mobility measurement based modal analysis methods and give some practical explanations and hints for use of the different techniques. This includes multi-reference and Multiple-Input Multiple-Output measurement techniques.

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Non-stationary Spatial Transformation of Sound Fields (NS-STSF) is a revolutionary technique for mapping noise in 3D-space and time. NS-STSF in effect allows you to film the noise emanating from a test object as a sequence of snapshots of instantaneous sound pressure using a microphone array. You can then map the noise distribution in time and space by transforming the measured sound-field to other planes, parallel to the measurement plane. All common soundfield descriptors such as SPL, intensity and spectral content are available from a single measurement. This system is the only truly transient implementation of the Acoustic Holography technique.

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The Multi-analyzer System PULSE™ includes Envelope Analysis Type 7773. This application note briefly describes the ideas behind Envelope Analysis of local bearing faults, how Envelope Analysis is implemented in PULSE™, practical considerations, and two case studies.

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This application note presents a case and explains the concepts and features of tone assessment in Sound Level Analyzer Type 2260 H. The description is based on the JNM2 method, since this is the de facto standard and the one specified in most detail. For computer-based tone assessment, the JNM2 method includes guidelines which are implemented in Type 2260 H.

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Operational Modal Analysis, also called Ambient Modal or Output-only Modal, is a technique where modal parameters are estimated from response data without knowing the input loading force. The method has, for over a decade, been used for parameter estimation on civil structures such as bridges and towers where artificial excitation and determination of forces present a problem.

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Until now it has been fairly difficult to measure accurately automobile aerodynamic noise in a wind flow using a microphone, because of disturbances such as the microphone self-generated wind noise and background noise in a wind tunnel.
This Application Note presents the results of checking the validity of the Spatial Transformation of Sound Fields (STSF) technique for noise measurements in a wind flow.

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The modal parameters of simple structures can be easily established by the use of PULSE™, the Multianalyzer System Type 3560. This application note describes how to measure the modal frequencies by inspection of frequency response functions, how to determine the modal damping with the aid of the frequency weighting function included in the analyzer, and how to establish the mode shapes by examining the value of the imaginary part of the frequency response function.

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The Dynamic Sound Test System performs dynamic and static sound testing on earth-moving machinery according to ISO standards. The major components of the system are the AudiBel software and the PULSETM Multi-Analyzer platform. Due to an advanced system design, only one person, the machine operator, is required to perform a complete test.

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In this application note, it is demonstrated how to use the order tracking facility of PULSE™, the Multi-analyzer System Type 3560. The data to be analysed is a coast-down of a large turbogenerator in a power station. Critical speeds were confirmed during the measurement. In addition, the 37th order component (and multiples thereof) were identified, caused by loose blades in the cooling fan system.

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When making vibration measurements, it may happen that not all the signal measured comes from vibration of the object. Some of it may derive from electrical pollution in the environment. This application note describes some possible sources of spurious measurements together with recommendations on how to avoid them.

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Sound Level Meters are often used for monitoring in remote places, and you may want to “monitor the monitor” or transfer measured data to your homebase PC, without going to the measurement site. A telephone connection — wired or wireless — is a convenient way to transfer measurement data. And modems are a vital link in the chain of transmission. This application note explains what equipment you need and how to use it.

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When using the new Type 3.2 ear simulators, the measurement results obtained will in some cases be different (more realistic) than results obtained with the Type 1 ear simulator. This application note explains the reason for these differences. It also suggests some basic rules for designing handsets which are less sensitive to differences in ear simulators and hence also less sensitive to the different conditions under which the handsets are used. Eventually, using the new ear simulator types may lead to an improved performance of the telephone handset, in terms of a better perceived sound quality.

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In industrial products, vibration and the noise it creates is often a major parameter to be taken into account. It is therefore usually very important to measure the material properties i.e. stiffness and damping, that determine how vibrations are generated, transmitted and damped. This application note describes a method developed by Mr.Fuglsang Nielsen which will allow the standard non-resonant method for the determination of complex modulus to be used at higher frequencies than otherwise possible. It is also shown how this method can be implemented using the Multichannel Analysis System Type 3550.

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To obtain precise information about the noise radiation from tyres it is desirable to measure with the tyre in actual operation on a road. The STSF technique for tyre noise measurements is able to present detailed information about both the acoustical near-field, as intensity radiation mappings very close to the surface of the tyre, and farfield calculations such as pass by simulations and radiation patterns.

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In Europe, a new "Machinery Directive" has been written, coming into force from 1/1/95. The purpose of this directive is to ensure the health of people using machines. The directive states, among other things, that equipment sold in Europe must fulfil specified noise demands. The equipment covered by this directive is everything from jacks and garden tractors, to chain saws and plastic moulding machines. What should be stated in the instruction manual for the equipment? If the A-weighted sound pressure level is below 70 dB(A), this must be indicated (Lp < 70 dB(A) re 20 Pa). Above 70 dB(A), the A-weighted sound pressure level should be stated. If the sound pressure level exceeds 85 dB(A), the A-weighted sound power level should also be stated. If the machine is very large, the sound power level is replaced by the sound pressure level measured at significant positions. If the noise contains peaks exceeding 130 dB(C), this must also be stated.

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With Measuring Amplifier Type 2525, Brüel & Kjaer has not introduced just a simple amplifier, but a sophisticated measurement system. Besides its ability to amplify signals and to display the signal's RMS and peak values, the Type 2525 has a number of other facilities. In this application note, we are going to focus on its capability to perform mounted resonance tests of piezoelectric accelerometers. With this feature the Type 2525 is able to measure the resonance frequency of the accelerometer, and by comparing the measured frequency with the one specified on the calibration chart, it is possible to determine whether the accelerometer is correctly mounted or not.

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This application note shows how to store data on the built-in 3550 disk drive, and how to use the stored data in commercially available IBM-PC application software such as spreadsheets or word-processors.

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This is a typical case story regarding noise labelling of a tape streamer, made in cooperation with Tandberg Data Storage A/S, Oslo.

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The acceptable exterior noise level from passenger cars is constantly decreasing through national and international regulations. Most recently, the acceptable level within the European Union has been lowered to 74 dB(A). To fulfill these demands, still more advanced measuring techniques have to be used in the development process and for troubleshooting. In this example, traditional measuring techniques suggested that the main noise source was the exhaust orifice. Detailed analysis of the sound field using Spatial Transformation of Sound Fields showed that the engine air intake system was dominant. As a result the overall sound pressure level was reduced by minor adjustments to the intake system as opposed to major modifications to the exhaust system.

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The Brüel & Kjaer Signal Analyzer Type 3550 and 2140 families implement the Hilbert transform to open up new analysis possibilities in the time domain. By means of the Hilbert transform, the envelope of a time signal can be calculated, and displayed using a logarithmic amplitude scale enabling a large display range. Two examples which use the Hilbert transform are presented here: - The determination of the damping or decay rate at resonances, from the impulse response function. - The estimation of propagation time, from the cross correlation function.

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Vibrations in rotating and reciprocating machinery are caused by the acceleration of the moving parts and by impact activity in the machinery. Order tracking is applied to relate vibration to the various moving parts. Gated tracking is used to identify where in the engine cycle the vibration is generated. Gated tracking is especially interesting when dealing with reciprocating machines like combustion engines, pumps and compressors.

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For several years, Toyota Motor Corporation have used a traditional manually operated back-to-back vibration transducer calibration system based on older Brüel & Kjær equipment. However, continuously adding vibration transducers, Toyota Motor Corporation developed a need for a more efficient means of calibrating these transducers. To ensure continued optimum vibration measurement results, reliability, high precision and high accuracy were vital factors when the Calibration Laboratory began looking for a suitable calibration system. Vibration Transducer Calibration System Type 9610, providing automated FFT-based sensitivity and phase calibration of vibration transducers over a wide frequency range, was chosen for reasons of accuracy, confidence and durability. This Application Note introduces the concept of FFT-based vibration transducer calibration and discusses the features, advantages and benefits of the Vibration Transducer Calibration System Type 9610 in connection with its use at Toyota Motor Corporation.

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This Application Note introduces a new excitation technique for exciting mechanical structures, in order to make mobility measurements. A small hand-held exciter or shaker has been developed for field measurements yielding the same advantages as those obtained using an impact hammer as well as those obtained using a shaker for the frequency response function measurements. The main advantages are: easy to use in the field, no elaborate fixturing, and best linear approximation of system under test as well as no leakage, assuming an appropriate excitation signal has been selected. The only disadvantage is that some degree of freedom (DOF) jitter may be introduced using this method, and the use of the hand-held exciter is only applicable for relatively small structures. A complete review of advantages/disadvantages of various excitation techniques is included in this Application Note in order to put the method into the right perspective.

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In Europe, a new "Machine Directive" has been written, coming into force from 1/1/95. The purpose of this directive is to ensure the health of people using machines. The directive states, among other things, that all equipment sold in Europe must fulfil specified noise demands. The equipment covered by his directive is everything from toothbrushes, washing machines, office machines and handheld drills to cars and heavy vehicles. What should be stated in the instruction manual? If the A-weighted sound pressure level is below 70 dB(A), this must be indicated (< 70 dB(A) re 20uPa). Beyond 70 dB(A), the A-weighted sound pressure level should be stated. If the sound pressure level exceeds 85 dB(A), the A-weighted sound power level should also be stated. If the machine is very big, the sound power level is replaced by the sound pressure level measured at significant positions. If the noise contains peaks exceeding 130 dB(C), this must also be stated. Note that all sound pressure levels should be measured under "free field" conditions at the typical position of the operators head. If that is not defined, the sound pressure level must be measured at a distance of 1 metre from the surface of the machinery at a height of 1.6 metres from the floor.

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This application note is an introduction to the large number of international standards existing regarding sound power determination. The standards mentioned are mainly ISO (International Organization for Standardization), IEC (International Electrotechnical Commission), ECMA (European Computer Manufacturers Association), DIN (Deutsches Institut for Normung e.V.), ANSI (American National Standards Institute) and some of the European "Council Directives".

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Being modular and expandable, the Brüel & Kjaer Multi-channel Analysis System Type 3550 makes fast, easy and accurate mobility measurements on structures. In its standard dualchannel configuration, and equipped with only a force hammer and an accelerometer, even the novice can begin to explore the dynamic behaviour of structures. This note gives an introduction to this important application area and outlines the theoretical background and experimental techniques.

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Apart from spectrum averaging, there is another averaging method that can be used when measuring periodic signals, namely synchronous time domain averaging. The main advantage of this method is that unwanted noise components are averaged out. On the 3550 analyzer, time domain averaging can be combined with order tracking analysis to yield a very powerful method for measuring on reciprocating engines (internal-combustion engines, pumps, and compressor).

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In operation, the propulsion shafting of a ship is deformed axially and torsionally. Both deformations cause axial vibrations, which at critical RPM may become unacceptable. The vibrations are damped by an integral axial vibration damper. The critical RPM are calculated based on a model of the shafting, and measurements are performed to verify the calculated natural frequencies and the effect of the damper. The verification is required by some of the shipbuilding classification societies. It is demonstrated that Type 3555 fitted with Extended Analysis Software Type 7639 is suited for the measurements.

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This Application Note introduces the MRIT concept and demonstrates the techniques on a bi-symmetrical structure which has repeated roots in all the modal peaks. These are decomposed using the three-reference data set, measured with the Brüel & Kjaer Type 3557 four-channel analyzer, and Polyreference curvefitting as implemented in the CADA-PC modal analysis package.

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This Application Note describes and examines the use of two Torsional Vibration Meters Type 2523 in a critical, severely time-limited, trouble-shooting situation where the performance of a torsional vibration elastic coupling, fitted to the propulsion system of a new series of smaller navy vessels, had to be thoroughly investigated in order for the propulsion system to be cleared for operational service.