Zevenaar, Netherlands
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Brandao E.,Federal University of Santa Maria | Tijs E.,Microflown Technologies | Lenzi A.,Federal University of Santa Catarina | De Bree H.-E.,Microflown Technologies
Acta Acustica united with Acustica | Year: 2011

In this paper the measurement, in situ or under free-field conditions, of the surface impedance and absorption coefficient is investigated. Numerical simulations of the measurement of impedance above a locally reactive surface is performed with the Boundary Element Method (BEM). Experiments are also made in a semi-anechoic chamber and in a regular office room. Three calculation methods used to obtain the surface impedance of an absorbent surface are described and compared, two of them being iterative. The first, referred to herein as the "q-term", relies on an exact description of the sound field above an infinite locally-reactive plane. The second, the "F-term", relies on an approximation for large values of the argument k |r2| in its equations. The third, the "Plane Wave Approximation (PWA)", is a simplification of the spherical wave reflection which considers that the reflected wave has its amplitude and phase changed by a simple planar reflection coefficient. The "F-term" and the "Plane Wave Approximation" methods also assume an infinite locally-reactive plane. The three calculation methods are compared, the differences in the found results are discussed. The three methods are compared mainly for small sound-source to sensor distances (|r2| = 0.3 m) and it is seen that they tend to converge as this distance increase. This comparison is relevant to in situ impedance measurements, since a bad choice of the calculation method may lead to a poor result. © S. Hirzel Verlag. EAA.


Several measurement techniques are available for the determination of the sound absorbing properties of material packages. The Kundt's method and the reverberant room method are the most commonly used techniques and they are standardized. However, both methods cannot be used in situ. In the past it has been shown that the PU in situ method can be used in a broad frequency range (typically from 300 Hz up to 10 kHz), on small samples (typically 0.03 m2 to 0.38 m2 or larger), while hardly being affected by background noise and reflections. Several studies revealed that similar results can be obtained as with the Kundt's tube if the measurements are performed under certain circumstances. A thorough comparison with the reverberant room method has not been conducted yet. In this paper preliminary results are presented of a comparison of the reverberant room method, the PU in situ method, and measurements with PU probes in a reverberant room. Several factors that may cause discrepancies amongst the methods are discussed. In addition, edge effects, which are experienced with the reverberant room method due to the finite size of the sample, are visualized with 3D intensity measurements that are performed in a reverberant room. © 2013 Acoustical Society of America.


Comesana D.F.,Microflown Technologies | Tijs E.,Microflown Technologies
Sound and Vibration | Year: 2016

It is important to apply an effective noise-reduction treatment to determine the contribution of different engine components to the total sound perceived inside an automobile cabin. Although accelerometer or laser-based vibration tests are usually performed, the sound contributions are not always captured accurately with such approaches. Microphone-based methods are strongly influenced by the many reflections and other sound sources inside the engine bay. It has been shown recently that engine radiation can be effectively measured using microphones combined with particle velocity sensors while the engine remains mounted in the car.6 Similar results were obtained from a dismounted engine in an anechoic room. This article focuses on measuring the transfer path from the engine to the vehicle interior to calculate the sound pressure contribution of individual engine sections at the listener's position. To achieve a good signal-to-noise ratio during acoustic transfer paths, a novel monopole loudspeaker was designed. The measurement method and monopole sound source are presented along with results of a validation test.


Fernandez Comesana D.,Microflown Technologies | De Bree H.-E.,Microflown Technologies
Proceedings of Forum Acusticum | Year: 2014

Sound visualisation can be used as a powerful approach to study a great variety of acoustic and vibro-acoustic problems. Traditional methods, such as step-by-step measurements or simultaneous multichannel systems, have a strong trade-off between time requirements, "exibility and cost. However, if the sound field can be assumed time stationary, scanning methods enable the assessment of variations across space with a single transducer, as long as the position of the sensor is known. The sound visualisation techniques reviewed in this paper are developed for the measurement method "Scan & Paint". Scan & Paint is based on the acquisition of sound pressure and particle velocity by manually moving a p-u probe (pressure-particle velocity sensors) across a sound field whilst filming the event with a camera. The sensor position is extracted by applying automatic color tracking to each frame of the recorded video. It is then possible to visualise sound variations across the space in terms of sound pressure, particle velocity or acoustic intensity. In this paper the practical capabilities of this measurement methodology are evaluated for multiple applications such as near-field and far field source localisation, vibro-acoustic assessment, characterization of material properties and acoustic intensity vector field mapping.


Pousa G.C.,Microflown Technologies | Comesana D.F.,Microflown Technologies | Wild J.,Microflown Technologies
INTER-NOISE 2015 - 44th International Congress and Exposition on Noise Control Engineering | Year: 2015

The detection of faulty parts is of fundamental importance during the manufacturing process. Most traditional acoustic techniques for fault diagnosis are based on analysis of the sound pressure emitted by the device. However, the performance of such methods are strongly limited for industrial scenarios due the presence of high levels of background noise. Furthermore, to carry out fault detection in rotating machinery, the exact measure of rotational speed of the shaft is required. The placement of a specific tachometer for this purpose is not often possible at the end of the assembly line due to time or spatial constraints. Particle velocity sensors are a non-contact solution that are able to capture surface vibration and can therefore be used to simultaneously quantify the vibro-acoustic behavior of a device and to perform order tracking. In addition, they provide better signal to noise ratio as they are less affected by background noise when measurements are performed close to the radiating surface. This paper presents the application of a single 3D acoustic particle velocity sensor for fault detection in rotating machinery under factory conditions with high levels of background noise. The implemented method is able to perform tachless order analysis and make use of Gaussian mixture models for the fault classification. The proposed method provides evidence for the viability of particle velocity-based solutions for end of line control applications in noisy conditions. © 2015 by ASME.


Tijs E.H.G.,Microflown Technologies | Druyvesteyn W.F.,Microflown Technologies
Acta Acustica united with Acustica | Year: 2013

In this paper, a new procedure is investigated to measure bulk properties of acoustic absorbing samples that are not clamped on the sides. Tests have been performed with a PU probe and with a sound source that is emitting spherical sound waves above a large slab of a homogeneous material. Multiple tests have been performed for different configurations, i.e. with and without a backplate as well as with two sample thicknesses, for several sound source heights. A method is presented that uses a combination of tests performed under different conditions in order to obtain the complex characteristic impedance and complex wavenumber of the sample. The method utilizes a plane wave model to calculate the bulk properties for each sound source height. Erroneous estimates are found since this model does not correct for spherical waves. However, depending on the height of the sound source the results are affected by near field effects differently. Tests at different heights are combined, and the plane wave values are obtained using an extrapolation technique. The results of the method are investigated through tests and simulations. © S. Hirzel Verlag · EAA.


Tijs E.H.G.,Microflown Technologies
Proceedings - European Conference on Noise Control | Year: 2012

The properties of acoustic absorbing samples can best be measured in the way in which they are installed. For several years PU probes are utilized for in situ measurements that are performed on small samples, with high resolution, in the presence of background noise and reflections. In order to calculate the absorption of the sample a sound field model is made. Usually these models incorporate a spherical sound field from a point source. However, because monopoles are limited in frequency range a piston-on-a-sphere loudspeaker is often applied instead. In free field conditions the radiation impedance of these sound sources is similar to that of a true point source. Here the impact of using a piston-on-a-sphere loudspeaker also for the in situ absorption measurement on the sample itself is investigated. © European Acoustics Association.


De Bree H.-E.,Microflown Technologies
38th European Rotorcraft Forum 2012, ERF 2012 | Year: 2012

In general, avionics is a source of noise pollution. It is possible to predict the noise pollution if the sound radiation of the individual aircraft is known. In general the noise radiation of aircraft is strongly directional. Prediction tools only can predict the noise pollution if these directional noise emissions are known. Once the noise pollution predictions are reliable one is able to predict what flight path is optimal with respect to noise pollution. There are two ways to determine the directivity of an aircraft. One can simulate the noise radiation and verify simulations with measurements. Secondly, it is possible to directly measure the complete directional noise emission of an aircraft. In this paper the last option is presented. Usually, acoustic measurements that involve aircraft are very expensive, since airtime in a conditioned environment is very expensive. The method presented here does not rely on test flight in a conditioned environment. It can be applied anywhere desired, which saves a lot of effort. The measurement system in this paper is able to acoustically detect, classify and locate aircraft. Once the 3D location and heading of an aircraft is known, it is possible to calculate its sound radiation in a certain direction. Because the aircraft is moving, the sound radiation is obtained at various angles. If the same or similar aircraft are passing the measurement system at different flight paths it is possible to compare the directional sound radiation. The core of the system is an acoustic vector sensor (AVS), which measures pressure and acoustic velocity[1]. Such sensors can be used to detect, classify and locate various noise sources. In early years, the AVS was used in automotive industry for locating interior noise problems. In that area, the sensors are mainly used in laboratory conditions. In order to make it possible to measure rotary wing aircraft, the AVS is developed further. They are ruggedized, windscreens are developed and the system is made to be battery powered and remotely operated. The AVS systems showed to be reliable for weeks of outdoor use. Apart from improving the hardware, algorithms are developed that enable to detect and classify rotary wing aircraft. With the classified signals, aircraft can be localized, meaning that the projected position on the ground, the height and the heading of the aircraft can be calculated. The system is shown to be operational outdoors, detecting and locating rotary wing aircrafts.


Grosso A.,Microflown Technologies | Verbeek J.,Microflown Technologies
Sound and Vibration | Year: 2010

The Microflown PU Mini probe provides new capabilities that are suitable for specific acoustic measurements on cars, aircraft, trains, helicopters and space structures. The Microflown sensor based on the Micro Electro Mechanical Systems (MEMS) uses two extremely sensitive heated platinum wires that have very little thermal resistance. A PU Mini measures the particle velocity in one direction therefore in a diffuse sound field, a PU Mini picks up only one third of the sound field, while a pressure microphone measures the total sound field. PU probes are broad-band transducers (20 Hz-20 kHz) and do not require spacers between sensors. The small size of the sensor allows testing on small objects with high spatial resolution such as computer hard drives or cell phones. A combination of a Microflown PU Mini with scan-and-listen hardware allows human ears to be capable of listening to particle velocity. PU probes are very small and cover a broad frequency range without any spacing requirements.


Tijs E.,Microflown Technologies | Druyvesteyn E.,Microflown Technologies
Acta Acustica united with Acustica | Year: 2012

The well-known Kundt's tube and reverberant room method are often used for measurement of acoustic absorption properties of samples under laboratory conditions. Several in situ measurement methods exist, but most of them are limited in frequency range, require large samples and/or are vulnerable to background noise or reflections. The PU in situ impedance method [1, 2] has been used successfully on relatively small samples (> 0.1 m 2 ) in a broad frequency range (300 Hz - 10 kHz) under reverberant conditions (e.g. a car interior or a concert hall), see e.g. [3, 4, 5, 6, 7]. The small source-sample and probe-sample distance are the main reasons for the relative small sample size requirement and the low influence to background noise and reflections. However, in some cases the procedure shows artefacts because all the reflection at the top of the sample is considered, not taking into account wave propagation in the material. In this research the principle of measuring intensity instead of impedance is investigated. To eliminate near field effects an extrapolation technique is introduced that combines several measurements. The result is a technique to measure the absorption coefficient without knowledge of the material. The methods are examined theoretically and verified with experiments. © S. Hirzel Verlag EAA.

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