Emiel Tijs

An Intensity Method for Measuring Absorption Properties in situ

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 (> 0.1m2) 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 propag tion 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.

A Comparison of Three Methods to Calculate the Surface Impedance and Absorption Coefficient from Measurements Under Free Field or in situ Conditions

Eric Brandão1), Emiel Tijs2), Arcanjo Lenzi3), Hans-Elias de Bree2) 1) Engenharia Acústica. Universidade Federal de Santa Maria, Departamento de Estruturas e Construção Civil, Av. Roraima, 1000, Cidade Universitária, Santa Maria, RS, 97105-900, Brasil. ericbacustica@gmail.com 2) Microflown Technologies, PO BOX 2205, 6802 CE Arnhem, The Netherlands. tijs@microflown.com, debree@microflown.com 3) Laboratório de Vibrações e Acústica, Departmento de Engenharia Mecânica, Universidade Federal de Santa Catarina, Campus Trindade, Florianópolis, SC, 88040-900, Brasil. arcanjo@lva.ufsc.br

Fast, High Resolution Panel Noise Contribution Method

All surfaces of a cabin interior may contribute to the sound pressure at a certain reference position, e.g. the humans ear. Panel noise contribution analysis (PNCA) involves the measurement of the contribution of separate areas. This is an effective method to determine the effect of apparent noise sources at a specific location. This paper presents the latest developments on particle velocity based panel noise contribution analysis. In contrast to the traditional methods, the particle velocity approach is faster; it requires 3 days instead of weeks. While the theoretical base of the procedure in this paper is similar to previously published particle velocity based procedure, here the measurement protocol has now been simplified dramatically, which has reduced the measurement time even more to less than a day. The method and its implementation are explained in the paper and a full measurement procedure is reported. Four steps are required to determine and visualize the pressure contribution of the vehicle interior. In a first step, probes are positioned on predefined interior surfaces. Special probe mounting have been made to decrease the handling time. The second step is a measurement in a certain mode of operation. This step can be done in a laboratory but it is also possible to perform the measurement whilst driving the vehicle on the road. Stationary as well as non stationary running conditions like run ups are accessible and do not limit the applicability of the method. The third step is the determination of the transfer paths from the panels to a certain listening position. This measurement is done assuming reciprocity. A monopole source is placed on the listener position and the sound pressure is measured at the surface. In a fourth and last step the transfer paths are linked with the operational data gathered in step two. The results are then visualized using the predefined geometry model. This paper describes the measurement of a conventional car with a resolution of 137 panels. Since an array of 46 probes was used step 2 and step 3 are repeated 3 times.

Exploration of the differences between a pressure-velocity based in situ absorption measurement method and the standardized reverberant room method

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...

Direct Sound Radiation Testing on a Mounted Car Engine

For (benchmark) tests it is not only useful to study the acoustic performance of the whole vehicle, but also to assess separate components such as the engine. Reflections inside the engine bay bias the acoustic radiation estimated with sound pressure based solutions. Consequently, most current methods require dismounting the engine from the car and installing it in an anechoic room to measure the sound emitted. However, this process is laborious and hard to perform. In this paper, two particle velocity based methods are proposed to characterize the sound radiated from an engine while it is still installed in the car. Particle velocity sensors are much less affected by reflections than sound pressure microphones when the measurements are performed near a radiating surface due to the particle velocitys vector nature, intrinsic dependency upon surface displacement and directivity of the sensor. Therefore, the engine does not have to be disassembled, which saves time and money. An array of special high temperature particle velocity probes is used to measure the radiation simultaneously at many positions near the engine of a compact class car. The particularities of these probes, the mountings used, and the actions taken to cope with disturbances such as airflows are described in this paper. The effective sound pressure is calculated with a particle velocity based transfer path analysis method and a novel sound power based method. To validate these techniques, the data obtained is compared to the results acquired in an anechoic room with a dismounted engine. It is shown that similar results can be obtained with both methods but that the sound power based methodology seems more practical. It is a straightforward and fast approach to characterize the average sound pressure level at certain distance.

In Situ PU Surface Impedance Measurements for Quality Control at the End of an Assembly Line

With PU probes the sound pressure and acoustic particle velocity can be measured directly. Over recent years, the in situ surface impedance method, making use of such a probe, has proven to be an alternative to Kundt’s tube measurements for product development type of work. The in situ method can also be used for the purpose of quality control on the acoustic material, be it during manufacturing or assembly, ensuring the best possible way to monitor the practical effectiveness of the acoustic package designed earlier on. In order to assess the variance of the acoustic package material leaving the assembly line, a relevant number of samples were taken over time. The quality of both the headliners, and the passenger seats were measured, of 25 cars of the same type. The robustness of the measurement method will be discussed, and the results will be presented.

A PU Probe Array Based Panel Noise Contribution Analysis Whilst Driving

This paper presents new developments on hot wire anemometer based panel noise contribution analysis. The used sensor allows the direct measurement of particle velocity. Some historical remarks are given and the latest developments of the technique are reported. Four steps are required to determine the panel noise contribution of the interior of a vehicle and to visualize the results in 3D. In a first step the probes are positioned on the interior surfaces and their x,y,z coordinates are measured. Based on these data a 3D geometry model is created. The geometry data are acquired using a specially designed 3D digitizer. The second step is a measurement in a certain mode of operation. This step can be done in a laboratory but it is also possible to perform the measurement whilst driving the vehicle on the road. Stationary as well as non stationary running conditions like e.g. run ups are accessible and do not limit the applicability of the method. The third step is the determination of the transfer paths from the panels to a certain listening position. This measurement is done reciprocally. In a fourth and last step the transfer paths are linked with the operational data gathered in step two. The results are then visualized using the 3D geometry model. This paper describes the measurement of a conventional car with a resolution of 180 panels. Since an array of 45 probes was used step 2 and step 3 had to be repeated 4 times. The complete measurement typically takes approximately 3 days.

Mapping 3D Sound Intensity Streamlines in a Car Interior

Sound source localization techniques in a car interior are hampered by the fact that the cavity usually is governed by a high number of (in)coherent sources and reflections. In the acoustic near field, particle velocity based intensity probes have been demonstrated to be not susceptible to these reflections allowing the individual panel contributions of these (in)coherent sources to be accurately determined. In the acoustic far field (spherical) beam forming techniques have been used outdoors in the free field, which analyze the directional resolution of a sound field incident on the array. Recently these techniques have also been applied inside cars, assuming that sound travels in a straight path from the source to the receivers. However, there is quite some evidence that sound waves do not travel in a straight line. The Maritime Institute of Stetting in Poland made numerous 3 D sound intensity measurements demonstrating an erratic pattern of sound intensity streamlines [1], [2] His approach was transferred from a lab to an actual car cabin upon request of a larger European car manufacturer. At 900 positions inside the car the 3D intensity is measured with a 3 D sound intensity probes using three particle velocity sensors. Such a probe is not susceptible to the pressure-intensity index. Several speakers that are driven in sequence are used as controlled sound sources. The results demonstrate that even with a single sound source, the 3D intensity streamlines are strongly bending, suggesting that far field techniques do not point towards the sound source. INTRODUCTION A number of different types of experimental approaches exist to localize and quantify sound sources in cabin interiors, all having their specific strengths and weaknesses. The aim is always to detect the acoustically weak part of the vehicles in order to define appropriate counter measures. In the near field several techniques are used such as window-based methods [3], [4], intensity measurements [5], laser scanning vibrometry measurements [6], and holographic technologies [7]. In the far field, spherical beam forming [8] has been introduced. The concept assumes that sound waves travel in a straight line from the sound source to the receiver. This assumption was verified in this study. For that purpose, a four channel three dimensional sound intensity probe is used to measure the intensity levels inside a vehicle at a large number of measurement positions. Several loudspeakers are placed inside the car and powered in sequence to study the influence of different the locations of the source. To get a good impression of the location of the measurement points, the inner geometry of the car is digitized with a specially designed tool. As it can be difficult to display 3D intensity information in a 3D environment, an attempt is made to make the results more understandable using various visualization techniques.

An Ultra Miniature Measurement Tool to Measure the Reflection Coefficient of Acoustic Damping Materials in Situ

A novel measurement tool is developed that is capable to measure the reflection coefficient of acoustic materials in situ and thus in real live situations such as a car. The measurement tool is a combination of two novel methods, the surface impedance method [1], [2], and the mirror source method [3]. The surface impedance method measures the acoustic impedance close to the surface of an acoustic absorbing material. The method is very sensitive for highly reflective surfaces [1], [2], [6]. The mirror source method uses a miniature monopole sound source that is placed close by the acoustic reflecting material. A particle velocity microphone (a Microflown [4], [5]) is placed close to the monopole source in such way that its sensitive direction is aiming at the acoustic reflecting material and its non sensitive direction is aiming at the source. This way it is only measuring the ‘mirror source’: the reflected image of the monopole sound source. From the ratio with a reference measurement on a fully reflecting plate the reflection coefficient can be determined. In this paper some results of both methods are shown and it also shown that there is a minor effect of reflecting surfaces in the environment on the measured absorption curves. This observation paves the way for application of the described method in cars.

Integration of an End-of-Line System for Vibro-Acoustic Characterization and Fault Detection of Automotive Components Based on Particle Velocity Measurements

The automotive industry is currently increasing the noise and vibration requirements of vehicle components. A detailed vibro-acoustic assessment of the supplied element is commonly enforced by most vehicle manufacturers. Traditional End-Of-Line (EOL) solutions often encounter difficulties adapting from controlled environments to industrial production lines due the presence of high levels of noise and vibrations generated by the surrounding machinery. In contrast, particle velocity measurements performed near a rigid radiating surface are less affected by background noise and they can potentially be used to address noise problems even in such conditions. The vector nature of particle velocity, an intrinsic dependency upon surface displacement and sensor directivity are the main advantages over conventional solutions. As a result, quantitative measurements describing the vibro-acoustic behavior of a device can be performed at the final stage of the manufacturing process. This paper presents the practical implementation of an EOL system based on data acquired with a single 3D probe containing three orthogonally placed acoustic particle velocity sensors. Aspects such as installation process, feature extraction, classification, fault detection and diagnosis are hereby discussed. The presented results provide experimental evidence for the viability of particle velocity-based solutions for EOL control applications.