Conference Papers are technical papers that have been presented at sound and vibration conferences around the world.
Creating the perfect vehicle sound is a critical challenge that needs the buy-in of diverse decision-makers to establish targets, and greater inter-departmental collaboration to realize them. Free-driving sound simulation captures subjective sound preferences with real-time modification. It helps to cascade that target sound down to subsystem or component level, and provides a focal point where all relevant and available data (CAE or test-based) can be assessed for its impact on sound style. In this way, NVH simulators can reduce uncertainty throughout the vehicle development programme: from target setting to delivery. This reduces the need for prototypes by more completely marrying the virtual-world design process with the real-world results, and enabling assessment of NVH predictions from CAE models.
Shale gas has transformed the energy market in the United States where drilling is underway in 17 states, with more than 80,000 wells drilled or permitted since 2005. Deposits elsewhere in the world are under varying degrees of exploration.
The main environmental impacts of the process are water usage, waste water management, methane emissions, groundwater and soil contamination. These are generally addressed as part of national environmental regulations and strong stakeholder engagement is critical in addressing concerns.
close to residential areas there is a significant risk of disturbance triggering complaints.
The unified matrix polynomial approach (UMPA) was developed in order to understand and derive various experimental modal analysis algorithms (which have been developed in isolation) using a common mathematical formulation. Various commercially available algorithms – such as the polyreference time domain, least squares complex exponential, and eigensystem realization algorithm etc. – can be explained using UMPA methodology, which makes it easier to understand both the advantages and limitations of such algorithms. In view of this fundamental characteristic of the UMPA, this paper aims at using the approach to understand, explain and develop the stochastic subspace identification (SSI) algorithm - a popular time domain operational modal analysis (OMA) algorithm. The roots of SSI algorithm lie in the identification of linear dynamic systems, traditionally a communications and controls engineering area. By means of the UMPA, the SSI algorithm’s similarity to a high order time domain OMA algorithm can be shown. It can also be shown that state transition matrices identified using the SSI algorithm and UMPA formulation are related to each other through a similarity transformation, thus characterizing the same system.
It was recently shown that blind source separation (BSS), as originally developed in the signal processing community, can be used in operational modal analysis to separate the responses of a structure into its individual modal contributions. This, in turn, allows the application of simple single-of-degree-freedom techniques to identify the modal parameters of interest. Several publications have recently attempted to give a posteriori physical interpretations to BSS – as initially developed in telecommunication signal processing -- when applied to the field of structural dynamics. This paper proposes to follow the route the other way round. It shows that several separation criteria purposely dedicated to operational modal analysis can be deduced from general physical considerations. Three such examples are introduced, based on very different properties that uniquely characterise a structural mode. The first criterion, coined the “principle of shortest envelope”, conjectures that the envelope of a modal response has, among all possible envelopes, the shortest length. That such a principle leads to the governing differential equation of a single-degree-of-freedom oscillator is proved from calculus of variation. The second criterion, coined the “principle of minimum spectral variance”, conjectures that the frequency spectrum of a structural mode is maximally concentrated around its central frequency. Finally, the third criterion, coined the “principle of least spectral complexity”, states that a structural mode has the lowest possible entropy in the frequency domain. All three criteria can be expressed in terms of a mixing matrix whose columns contain the unknown mode shapes. The recovery of the latter is then trivially achieved by minimising the criteria. Extensive simulations show that the proposed criteria lead to figures of merit very similar to those of the state-of-the-art, while at the same time providing physical insight that other algorithms issued form the signal processing community may dramatically lack.
Second Order Blind Source Separation (SO-BSS) techniques possess several mathematical characteristics making them a viable option for Operational Modal Analysis (OMA). However, on closer scrutiny it is revealed that there are certain subtleties that limit their direct application to OMA applications. This paper continues from past work of the authors, which focussed on understanding SO-BSS techniques from a perspective of OMA applicability and developing SO-BSS based algorithm for OMA. In this paper, a new algorithm is proposed that overcomes the inherent limitations of SO-BSS algorithms with regards to their applicability to OMA. These limitations include applicability to heavily damped systems, identification of complex modes, and applicability to scenarios where number of available sensors is lesser than the number of modes to be estimated, etc. The algorithm’s advantage over original form of SO-BSS is demonstrated by means of an analytical example.
Modal parameters (natural frequency, damping and mode shapes) play an important role in dynamic characterization of a structure. These parameters are estimated using advance parameter estimation algorithms. However, the estimated modal parameters are often quoted without much statistical evaluation of the estimation procedure. Since, modal parameters are estimated from measured data and the estimation procedure itself is often an error minimization procedure (like Least Squares approach), it is necessary to quantify the uncertainty associated with the parameter estimation procedure. One way to achieve this goal is by providing confidence intervals for the estimated modal parameters.
This paper addresses the issue of uncertainty quantification for modal parameters estimated using high order time domain algorithms. A methodology for estimating confidence intervals for the estimated modal parameters is presented and its usage is illustrated by means of simulated and experimental examples.
The presented study continues the work on application of Output Only Modal Analysis (OMA) to operating wind turbines. It is known from previous studies that issues like the time-varying nature of the equations of motion of an operating wind turbine (in particular the significant harmonic components due to the rotor rotation) as well as the considerable aerodynamic damping make OMA of operating wind turbines a difficult task. While in the previous works OMA was based on data provided by sensors mounted on the wind turbine tower and nacelle, we here attempt to improve the results by instrumenting the blades as well. It is believed that the availability of vibration data from the blades will improve the observability of the main global vibration modes (especially the heavily damped out-of-plane modes), and thus will assure a better estimation of modal parameters, especially the damping.
The paper discusses the technical challenges regarding blade instrumentation and data acquisition, data processing applied to eliminate the time-varying nature of an operating wind turbine in the resulting eigenvalue problem and, finally, it presents and discusses the initial results.
Engineers designing wind turbines often use Campbell diagrams as a convenient tool for representing the dependence of wind turbine modal parameters on the rotor (and wind) speed. Experimentally obtained Campbell diagrams are used for tuning wind turbine numerical models, which are used heavily in the design of wind turbine structures and their control systems. Several previous studies showed that Operational Modal Analysis (OMA) can be successfully employed for plotting experimental Campbell diagrams. In this study, OMA is applied to a number of experimental sets of data measured under different wind conditions. However, because several OMA assumptions are partly violated, the resulting diagram is contaminated by computational and noise poles, which cannot be filtered out by conventional means provided by OMA algorithms.
The presented study compares three methods for segregating the physical poles from their computational and noise counterparts. Since the modal frequencies and damping of a wind turbine change with the rotor speed, these modal parameters cannot be utilized for grouping the poles, as is typically done in the case of stabilization diagrams. The mode shape vectors are utilized instead.
The first presented method uses hierarchical clustering to group the poles according to the similarity between the associated mode shape vectors. The second method is based on the Singular Value Decomposition (SVD) performed on an AutoMAC matrix. This method was originally suggested by other authors for cleaning up stabilization diagrams. Finally, the third method utilizes Self-organizing Maps (SOM) for the same purpose.
The methods are applied to the experimental data from the ALSTOM WIND ECO 100 3MW wind turbine; the results are compared and the advantages and disadvantages of the methods are discussed.
The presented study focuses on application of Output Only Modal Analysis to operational wind turbines. Issues like time varying nature of operational wind turbine, significant harmonic components due to rotor rotation and considerable aerodynamic damping make OMA of operational wind turbines a difficult task. The study presents the results of OMA applied to experimental data from ALSTOM Wind 3MW Eco-100 wind turbine. Issues like data handling, using clustering for modal identification and uncertainties analysis are presented and discussed.
The acceleration response of a wind turbine gearbox, tested on a back-to-back test stand of Winergy, is used as input data to a pre-processed operational modal analysis (OMA) or output-only modal analysis. The data is heavily influenced by harmonics and sidebands caused by rotating gear wheels, shafts and bearings. This violates an OMA assumption to have a flat spectrum as input. Therefore, cleaning methods are applied to remove harmonics in the time series. The methods – Time Synchronous Averaging, Periodogram-based and Cepstrum-based – are tested for the applicability to gearbox data concerning their userfriendliness, the quality of the results and possibility of automation.
Understanding and characterization of wind turbine dynamics, especially when operating, is an important though challenging task. The main problem is that an operating wind turbine cannot be truly modeled as a time invariant system, which limits the applicability of conventional well-established modal analysis methods. This paper compares two experimental techniques that characterize the dynamic behavior of an operating horizontal axis wind turbine (Vestas V27, 225kW, rotor diameter 27m, 12 accelerometers on each blade). The first method uses a multiblade coordinate transformation to convert the time periodic system into a time invariant one, assuming that the system is perfectly isotropic. Conventional operational modal analysis then can be applied to identify the modal parameters of the time invariant model. The second method processes the periodic response directly based on an extension of modal analysis to linear time periodic systems. It utilizes the harmonic power spectrum, which is analogous to the power spectrum for a time invariant system, to identify a periodic model for the turbine. This work demonstrates both of these methods on measurements from the operating turbine and discusses the challenges that are encountered. The procedure is demonstrated by using it to extract the time-periodic mode shapes of the first edge-wise modes, revealing that this turbine apparently has non-negligible blade-to-blade variations and hence the dynamics of these modes are considerably different than one would expect for an anisotropic turbine.
Modal parameters (natural frequency, damping and mode shapes) play an important role in dynamic characterization of a structure. These parameters are estimated using advance parameter estimation algorithms. However, the estimated modal parameters are often quoted without much statistical evaluation of the estimation procedure. Since, modal parameters are estimated from measured data and the estimation procedure itself is often an error minimization procedure (like Least Squares approach), it is necessary to quantify the uncertainty associated with the parameter estimation procedure. One way to achieve this goal is by providing confidence intervals for the estimated modal parameters. This paper addresses the issue of uncertainty quantification for modal parameters estimated using high order time domain algorithms. A methodology for estimating confidence intervals for the estimated modal parameters is presented and its usage is illustrated by means of simulated and experimental examples.
The use of single layer and double layer microphone arrays, both hand-held as well as robot operated, has been greatly extended within the last decade. This paper summarizes how a small double layer array with typically 128 microphones can be used for interior cabin measurements for mapping various acoustical properties. There are four major applications. The first one is general patch holography (or conformal mapping) of basic acoustical quantities like sound pressure, particle velocity and sound intensity. Optionally sound quality (SQ) metrics for describing human annoyance like loudness, sharpness, fluctuation strength and roughness, etc. can also be mapped. Other applications are in-situ absorption measurement - for example inside a car cabin, intensity component analysis (e.g. incident, reflected, scattered, net intensity, etc. can be separated) and finally sound pressure contribution from various panels inside cabins to an operator/driver's position. Some measurements are done in operational condition and some are reference laboratory measurement of typical frequency response functions.
A measurement technique is described for the localization and visualization of noise sources on moving rail vehicles using beamforming. The Delay-And-Sum (DAS) beamforming is often used on stationary (fixed) sources. However, the method can also be applied to moving sources such as rail vehicles, road vehicles and aircraft fly-overs, as well as rotating blades on wind turbines. Recently, deconvolution techniques have been introduced as post-processing after DAS to improve spatial resolution and reduce the level of ghost sources in the calculated noise maps. This paper describes a commercially available system which includes DAS and deconvolution techniques, dedicated to the rail vehicle industry. Special consideration is paid to the configuration of the test site and its influence on the measurement results. The advantages of various microphone array designs for measurements on bogies, rails and pantographs are discussed. Guidelines are given for a selection of appropriate array (half-wheel, logarithmic wheel) for the source of interest and illustrated with practical results from noise emission measurements on regional trains.
The 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 the present paper, covering also a Doppler correction to be made in the shading function, when that function is used in the PSF calculation.
The number of noise source identification (NSI) techniques available to engineers working on noise, vibration and harshness problems has increased considerably in recent years. The choice of the most appropriate technique depends upon the application and the information required. This paper reviews techniques for noise source identification and quantification ranging from simple hand-held sound intensity systems, hand-held array systems to large ground based microphone arrays. The methods include Beamforming, Spherical Beamforming and Acoustic Holography. Guidelines are given to help the engineer choose a suitable technique based on the frequency range of interest, the distance from the measurement array to the test object and the resolution required. Practical application examples ranging from hearing aids to wind turbines are presented to illustrate the various NSI techniques.
Noise from large construction projects can be a nuisance for nearby communities. Vibration from activities like pile driving, concrete crushing and tunneling can also create nuisance but may also risk structural damage. Increasingly, both noise and vibration are significant factors surrounding a construction project. If not managed properly they can lead to project delays, further prolonging the nuisance and significantly increasing project cost. To help mitigate these risks, contractors are now using continuous noise and vibration monitoring to ensure that the impact from construction activity is kept within guidelines.This paper looks at how continuous real time monitoring of noise and vibration can be used to help manage community impact and reduce the risk of structural damage. It will look at legislation and best practice both in Canada and in other parts of the world. Presenting the concept of environmental capacity it also shows how engaging communities, setting expectations and building trust can be effective way of mitigating impact.The paper goes on to highlight new technology that is in use in Canadian construction sites which uses the approaches discussed above to help manage compliance and reduce risk.
Since its first mention in 1999, Integrated Environment Noise Management has promised new possibilities for managing environment noise through the interaction between measurements and calculations. This has lead to the technique of Dynamic Noise Mapping, where measurements are used to update calculated noise maps, which was primarily intended to improve local noise maps and supplement noise monitoring results. However, with the ever-increasing availability of internet communication and the rise of managed services which enable the efficient exploitation of new technology, integrating measurements and calculations for more pro-active environment noise management, where noise issues can be avoided, is becoming potentially interesting and possible. So, the link between calculation software and real-time noise monitoring solutions enables the automated creation of noise maps based on real-life noise and weather monitoring, giving feedback to enable a more optimal planning of operations within the noise limits. This dynamic mapping can be done with updates as regularly as every hour enabling industry to schedule operational activity, quickly assess the noise impact and ensure that they are doing the utmost whilst complying with noise limits. This paper describes how Dynamic Noise Mapping for Pro-Active Environment Noise Management works and its possibilities, challenges and limitations based on current knowledge and technology. The paper will also identify areas of future research.
The construction of infrastructure and new buildings risks causing significant impact on the neighbourhood, particularly for major infrastructure projects. Due to local community concerns, construction activities are often subject to operational restrictions. To effectively operate within these restrictions, instrumentation is often deployed to monitor noise and / or vibration. An alternative approach to the purchasing and operating costly equipment to monitor compliance is available through Managed Services offering technology innovation enabling simultaneous noise and vibration monitoring and manpower and cost reductions in monitoring resulting in a more economically attractive approach to traditional noise monitoring.
Long term monitoring of noise from airports, industrial facilities and construction projects has been well established over many years. The systems monitor noise levels at various points around the facility, feed the data back to a central system where data can be summarized and any breaches in compliance criteria are reported. These systems are passive by nature; they simply report what happened giving little opportunity to do anything to prevent breaches in the first place. Should breaches occur, attendance at the site is typically necessary to investigate before advice on mitigation can be given to prevent future breaches of compliance.
Technology advances in how data is captured, what data is captured and how it is accessed now means that in a large number of cases investigation can be achieved remotely without the need for site visits. This makes the process more efficient, lower cost, and much more immediate. Technology can now deliver results in real time that can help to prevent breaches occurring in the first place.
This paper outlines the technology advances and suggests how they will help to change the way noise consultants can deliver more monitoring services over a wider geographical area with fewer staff whilst simultaneously providing a better quality result.
The two microphone acoustic impedance tube is used to measure the acoustic impedance and absorption coefficient properties for absorptive materials. A commonly followed test method for this is described by the standard ASTM E1050. This test standard is popular compared to alternative test methods due to its repeatability, speed of test and small sample size requirements. The two microphone broadband noise source based test method was introduced in 1985 and was an update to the single microphone sinusoidal excitation method given by ASTM C384. The ASTM E1050 standard was updated in 1998 to include changes in the required physical dimensions of the tube. Specifically, the tube length was said to be increased to be sufficiently long to meet the requirement that plane waves be fully developed before reaching the microphones and test specimen. Further, a minimum of three tube diameters was specified between the sound source and the nearest microphone to allow for sufficient distance for the subsiding of any non-plane waves propagating within the tube. Using two different tube lengths meeting the requirements of the two versions of the standard, this study investigated experimentally whether any differences resulted in the measured normal incidence absorption for multiple test samples as a result of the prescribed dimensional changes. The precision of the measured results are compared using the repeatability and reproducibility requirements defined in Table 2 of the E1050 standard.
Two mounts were added to a helicopter making it possible to carry different payloads. To validate the structural effects of these modifications, modal tests were performed on-ground on the helicopter in its standard configuration as well as in its modified configuration with the added payloads. In addition, an in-flight test was performed to verify the impact on the existing flight envelope. For all tests, Operational Modal Analysis was used. The obtained results allowed for updating the flight procedures and operating profiles for the helicopter and provided added flexibility with respect to the best possible helicopter configuration to obtain the mission objectives, while maintaining optimum safety for the flight crew.
In this work, the noise sources on three types of construction equipment are imaged with a beamforming array, while simultaneously the radiated sound powers are determined by a sixmicrophone hemisphere per ISO 6393 or ISO 6395. Of particular interest are: noise induced by turbulent flow at the exit of an exhaust stack, the effect of a noise reduction package in the engine compartment, and crawler track noise during motion. The absolute levels of the mapped source regions are compared with the total radiated sound power.
Over the last 10 years, the use of Sound Quality (SQ) has become an accepted automotive industry standard.This “newer” field of acoustics has proven to be quite useful in many areas, such as; new product design, quantifying subjective opinions on products with objective metrics, troubleshooting current models, and acoustic modeling of products. This paper will cover the basics in sound quality and apply some of these fundamental concepts in a few examples of the use of sound quality in a practical engineering environment.
A new ASTM standard has been adopted for characterizing acoustical materials in a tube. This new standard is ASTM E2611-09, Measurement of Normal Incidence Sound Transmission of Materials Based on the Transfer Matrix Method. This test method describes the use of a tube, four microphones, and a digital frequency analysis system for the measurement of normal incidence transmission loss and other important acoustical properties of materials by determination of the sample’s acoustic transfer matrix. In this paper the two and four microphone methods for calculating the reflection coefficient and the related normal incidence sound absorption coefficient will be compared with each other and with the results predicted using finite element software.
In a former paper (“Second Order Blind Identification (SOBI) and its relation to Stochastic Subspace Identification (SSI) algorithm”, 28th IMAC, 2010), the authors established the link between the popular SSI algorithm used in output-only modal analysis and the Second Order Blind Identification (SOBI) algorithm developed for blind source separation in the field of signal processing. It was concluded that the two algorithms, although seemingly very different, are actually jointly diagonalizing the same covariance matrix over a range of time-lags. This is explicit in SOBI and implicit in SSI. One main difference, however, is that SOBI focuses on estimating the (real) modal matrix as a joint diagonalizer, but without taking advantage of the specific structure of the covariance matrix formed by the Markov coefficients and by incorrectly assuming no-damping or very low damping. On the other hand, SSI specifically exploits the covariance matrix structure so as to estimate complex modes, but puts less emphasis on the “joint diagonalizing” property of the modal matrix. The aim of this communication is to introduce a new algorithm based on Alternating Least Squares (ALS) approach that combines advantages of both SOBI and SSI in order to return improved estimates of modal parameters. It is shown in this work that this algorithm is capable of identifying complex modes, closely spaced modes and heavily damped and can also be expanded to deal with the cases where there are less number of sensors available than the number modes to be estimated. The suggested approach therefore is a step towards expanding the applicability of BSS based approaches to Operational Modal Analysis applications.
The authors previously presented an extension of operational modal analysis to linear time periodic systems. This paper builds on that work, revealing how more advanced operational modal analysis methods can be extended to linear time-periodic systems. These extensions are found to provide more accurate estimates of the damping of the modes of the time-periodic systems, and to provide good estimates of the mode shapes of the systems so long as the measurements stand out clearly above the noise. Application of the complex mode indicator function an the EMIF algorithm makes it possible to separate the forward and backward whirling modes of a wind turbine, which is difficult since each of these modes is manifest at several harmonics due to the anisotropy in the tower supporting the turbine.
Work presented in the current paper in an extension of the pervious work and describes the details of the measurement campaign aimed at identifying modal parameters of ALSTOM’s ECO 100 wind turbine. Since measuring on an operational wind turbine is a challenging job in itself, the paper also describes measurement planning and execution phases. The paper illustrates various key aspects related to practical measurements on an actual wind turbine and underlines the importance of proper planning and experiment design. The importance of a priori nowledge provided by finite element model based simulations is also underlined.
The influence of different sound fields on the measurement error is discussed in some detail with practical examples and it is shown how a worst-case error exceeding 10 dB @ 20 kHz is a real risk. After a brief discussion of a condenser microphone which drastically reduces the error caused by influence of an unknown sound field or varying angle of incidence. Finally, test results from production samples of the new microphone are shown.
Aerodynamic noise is particularly important for passenger car driving comfort and high speed train community comfort. This paper describes the application of acoustic array systems in wind tunnels to minimise this noise exposure.
Aerodynamic noise is particularly important for passenger car driving comfort and high speed train community comfort. This paper describes the application of acoustic array systems in wind tunnels to minimise this noise exposure.
The study demonstrates the method using simulated vibrational responses of operational 3MW wind turbine. The responses of the tower and blades were obtained from the simulation of operational wind turbine dynamics under realistic wind load using commercial aeroelastic code.
The study suggests a novel approach which is intended to solve a known TMM weakness due to noncausality of transmissibility functions. The study compares the results obtained using the new approach with two other implementations found in the literature.
The paper is organized as follows: the first section explains the application of the transmissibility matrix method to TPA; next section compares different interpretations of the indicator signals. Section 3 introduces a simple mechanical system and discusses results obtained by different implementations of TMM.
Operational Modal Analysis (OMA) is one of the branches of experimental modal analysis which allows extracting modal parameters based on measuring only the responses of a structure under ambient or operational excitation which is not needed to be measured. The present study shows that the aeroelastic phenomena due to rotor rotation dramatically changes the character of aerodynamic excitation and sets limitations on the applicability of OMA to operational wind turbines. The main purpose of the study is to warn the experimentalists about these limitations and discuss possible ways of dealing with them.
The paper gives a brief introduction into the theory of ONPA and highlights the main assumptions the method is based on. Focusing on structure borne noise paths, the study demonstrates that, interpreting the measured signals in special way, the true noise contributions can be obtained. Then the approach is being extended to systems with rotational DOFs and demonstrated on a simple mechanical system.
The current study continues investigation of the method accuracy and applicability for structure borne cases. The method is applied to simulated data, which makes the validation against exact results possible. A way to improve method accuracy is suggested. The results of the improved method are compared with traditional methods results.
Traditionally, the design of control algorithms for wind turbines is performed based on (linearized) models of the wind turbine dynamics. Control performance is strongly dependent on the accuracy of these models and for this reason validation of the dynamics is essential for achieving optimal control. The aim of this work is to identify, at different wind speeds, the dynamic model of a wind turbine in operation by means of two different system identification techniques. This work has been partly performed within the SenternNovem long-term research project "SusCon: a new approach to control wind turbines" (EOSLT02013) and partly within the InVent project-ACC1Ó (CIDEM | COPCA).
The goal of this paper is to address these issues in order to make OMA more applicable and suitable for wind turbines. The paper first shows the application of OMA to a parked turbine and then subsequently shows its application to operational wind turbine. It is demonstrated how incorporating techniques like Coleman transformation and proper test planning, OMA can act as a viable tool for characterizing the dynamics of an operational wind turbine.
Time Selective Response, TSR, is a frequency response measurement method based on linearly swept sine signals. This paper briefly recollects 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 at Brüel & Kjær of microphone and sound level meter free field responses.
Practically all reference microphone calibrations that are performed by national metrology institutes are pressure response calibrations. This is the case even if most practical measurements are carried out under free- or diffuse-field conditions. This paper describes the technical aspects of the elaborate system, which has developed by the Danish Technical University (DTU) and is now offered by Brüel & Kjær with software and technical support from the university staff.