Sound propagation in air-filled pipes with inhomogeneities

This work is concerned with sound propagation, reflection and absorption in air-filled pipes with inhomogeneities such as an open end, lateral connection, cross-sectional change or porous termination. The focus in this study is made on the sound intensity which is a vector quantity. Because the sound intensity is treated as a complex vector, it gives more information about the local net transport of energy (real part) and local oscillatory transport of energy (imaginary part) and its direction. This provides a possibility to obtain more information on the nature of inhomogeneities in the pipe and their extent. An attempt is made to study the sound intensity in a pipe transmitted through higher-order modes. The results of this work are applied to measure the acoustical properties of porous media and living plants. It is shown that the frequency range of the standard ISO 10534-2 method can be extended significantly to measure the absorption and reflection from a large, inhomogeneous material specimen such as a living plant. The presented experimental data are supported by the hybrid numerical method which is based on the normal modal decomposition and finite element modelling, analytical methods which are based on normal mode decomposition alone and models for the acoustical properties of porous media. It is shown that accurate measurements of the sound intensity in a pipe are problematic. Therefore, this work presents experimental and theoretical evidence together with a sensitivity analysis and discussion on the applicability of this technology for the characterisation of conditions in air-filled pipes

An application of normal mode decomposition to measure the acoustical properties of low growing plants in a broad frequency range

© 2016 Elsevier Ltd This paper presents a new application of the normal mode decomposition to measure the reflection and absorption coefficients of a low growing living plant in a large 300 × 300 mm impedance tube. In this way the higher frequency limit can be extended by a factor of 3 in comparison to that suggested by the standard ISO 10534-2 method for this type of an impedance tube. The adopted method (Prisutova et al., 2014) is based on minimising the difference between the spatial Fourier transform of the measured sound pressure at a range of closely spaced positions along the impedance tube and the predicted transform arising from the normal mode decomposition method. The angular and frequency dependent complex reflection coefficients for the first 5 normal modes are recovered. The acoustical properties of three plants specimen, Pelargonium hortorum, Begonia benariensis and Hedera helix, are measured with the adopted method. These properties are related to the plant morphology through an equivalent fluid model. It is shown that in some cases the predicted and measured data are in close agreement. However, there are cases when the agreement between these data is poor. The possible reasons for this discrepancy are proposed and discussed. This work paves the way for a better understanding of the relations between the plant morphology and its acoustical properties

A method to determine the acoustic reflection and absorption coefficients of porous media by using modal dispersion in a waveguide

The measurement of acoustic material characteristics using a standard impedance tube method is generally limited to the plane wave regime below the tube cut-on frequency. This implies that the size of the tube and, consequently, the size of the material specimen must remain smaller than a half of the wavelength. This paper presents a method that enables the extension of the frequency range beyond the plane wave regime by at least a factor of 3, so that the size of the material specimen can be much larger than the wavelength. The proposed method is based on measuring of the sound pressure at different axial locations and applying the spatial Fourier transform. A normal mode decomposition approach is used together with an optimization algorithm to minimize the discrepancy between the measured and predicted sound pressure spectra. This allows the frequency and angle dependent reflection and absorption coefficients of the material specimen to be calculated in an extended frequency range. The method has been tested successfully on samples of melamine foam and wood fiber. The measured data are in close agreement with the predictions by the equivalent fluid model for the acoustical properties of porous media

On the use of power reflection ratio and phase change to determine the geometry of a blockage in a pipe

Blockages may be detected in pipes by sending acoustic signals down the pipe and measuring the echo from the blockage. This presents a fast and efficient way of determining the presence of a blockage and this method is now being used, for example, to probe the integrity of sewer systems. In this article a method is presented for obtaining both the length and the equivalent cross-sectional area of a blockage using only a single microphone to capture the incident and reflected pulse. The method presented uses the change in phase between the incident and reflected acoustic signals caused by a blockage, as well as the difference in the amplitude of each pulse, to generate two independent equations from which the area ratio and the length of the blockage may be recovered. This requires measurements to be carried out in the plane wave region of the pipe, however it is shown that through appropriate processing of each signal in the frequency domain the area ratio and length of a relatively large number of blockages can be successfully recovered.UK Engineering and Physical Sciences Research Council

On the variation of the acoustic intensity vector near an open end of a pipe

A new laboratory facility for studying of the acoustic intensity field in a round 6m long, 150mm diameter pipe has been developed. It enables to control carefully the boundary conditions at the both ends, positions and the orientation of the acoustics sensors. It has been used to measure the variation of the acoustic intensity near the open end of the pipe with a triaxial intensity probe supplied by Microflown. The measured data have been used to validate a new efficient model based on the normal model decomposition and finite element methods. It is shown that even in the plane wave regime the acoustic intensity distribution near the pipe end is very complex and that the measured intensity vector sensitive to the radial and longitudinal position of the probe. It is demonstrated that a very small variation in the position of the intensity probe can result in a very large change in the intensity vector that is difficult to predict. The results of this work have a strong implication on the design of acoustic intensity sensors that have been developed to characterise the boundary conditions in a pipe

An acoustic method of blockage characterisation in a pipe based on the cross-sectional mode analysis

Copyright © (2014) by the International Institute of Acoustics & Vibration All rights reserved. This paper presents an acoustic method to characterise a blockage of an arbitrary shape in an air filled pipe. Two sets of experiments are conducted using the transient pulse signals. For the first experiment, the frequency range extends beyond the plane wave regime so that three first cross-sectional modes are excited and analysed with a horizontal microphone array in the absence and presence of a blockage. The 2D Fourier transform is then applied to transfer the acquired sound pressure data into the frequency and wavenumber space from which the modal reflection and transmission coefficients can be determined. For the second experiment, the microphone array is placed at a single position and the frequency range is below the first cross-sectional mode which makes the analysis relatively simple and robust. A set of equations is derived and employed to calculate the area ratio and the length of a blockage making use of a reflection coefficient from a blockage and a phase change between incident and reflected signals

Use of fibre-optic sensors for pipe condition and hydraulics measurements: a review

The combined length of the sewerage and clean water pipe infrastructure in the UK is estimated to be about 800,000 km. It is prone to failure due to its age and the inadequacies of the current pipe inspection methods. Fibre-optic cable sensing is an attractive way to continuously monitor this infrastructure to detect critical changes. This paper reviews the existing fibre-optic sensor (FOS) technologies to suggest that these technologies have better sensing potential than traditional inspection and performance monitoring methods. This review also discusses the requirements for retrofitting an existing pipeline with an FOS. It also demonstrates that there is a need for further research into methods applicable to non-pressurised pipelines, as there is very little existing literature that focuses on partially filled pipes and pipes with gravity fed flows

Teaching creativity to undergraduate engineering students

Over the recent years, it has become more and more apparent that creativity is a skill equally important for both technical and artistic careers. However, methods for teaching creativity that work for arts students are not always appropriate for engineering students. The present study outlines the adaptation of a creativity development session from an artistic degree curriculum (Mascareñas, 2019), to make it suitable for teaching to engineering students. The session was run three times with 1st and 2nd year engineering students at a Russell Group university in the north of England, and both qualitative and quantitative feedback was collected from students after the session. The main findings indicate the importance of a trusting relationship between students and the educator, the need for balance between delivering a memorable experience and offering support, and the significance of subsequent reflection.</p

Measurement of complex acoustic intensity in an acoustic waveguide

Acoustic intensity is normally treated as a real quantity, but in recent years, many articles have appeared in which intensity is treated as a complex quantity where the real (active) part is related to local mean energy flow and the imaginary (reactive) part to local oscillatory transport of energy. This offers the potential to recover additional information about a sound field and then to relate this to the properties of the sound source and the environment that surrounds it. However, this approach is applicable only to multi-modal sound fields, which places significant demands on the accuracy of the intensity measurements. Accordingly, this article investigates the accuracy of complex intensity measurements obtained using a tri-axial Microflown intensity probe by comparing measurement and prediction for sound propagation in an open flanged pipe. Under plane wave conditions, comparison between prediction and experiment reveals good agreement, but when a higher order mode is present, the reactive intensity field becomes complicated and agreement is less successful. It is concluded that the potential application of complex intensity as a diagnostic tool is limited by difficulties in measuring reactive intensity in complex sound fields when using current state of the art acoustic instrumentation.The authors would like to thank the U.K. Engineering and Physical Sciences Research Council (Grant Nos. EP/H015280/1 and EP/H015469) for their support of the work reported in this article

Local and international: richly contested concepts in practitioner research (a patchwork paper)

This paper is a co-written paper by masters' students at a university in the Midlands region of England. In it the authors, all practicing teachers, explore via a series of interlinked research stories how they conceptualise: 'international’ and ‘local’ in their research. This paper by focusing, sharply on this aspect of their research positioning, reveals that this was in complex, nuanced and idiographic ways. The Midlands is one of the most multi-cultural and multi-ethnic regions of Britain and is therefore one where embracing and interpreting intersectionality is at the heart of navigating personal and professional life. One interpretation of these terms that emerged therefore interwove ‘the local’ and ‘the international’ and emphasised using research that focuses on the localised detail of classroom practice to find paths towards building inclusive classrooms within a multicultural context. However, another perspective that emerged drew conceptual boundaries between these terms, seeing internationally conducted research as a distinct but illuminating route into understanding local issues; while a third took as its starting point ‘the local’ and from this built an aspirational goal that good practice grounded in locally conducted research could inform international debates. These are just three illustrative examples of different ways that these teacher researchers interpreted the dichotomy of the global and the local. As is common and appropriate with patchwork presentations there is no attempt to impose a single methodological framework on the multiple authors and the research vignettes are rich and varied. This paper reveals that to practitioner researchers the concepts of international and local are richly contested and are utilized in varied ways. It will contribute to building an understanding of how practitioner research, too often perceived as localised and distinct from international research, can define its place in the international research community