Experimental and numerical investigation of the acoustic absorption coefficient at very low frequency

D. W. Y. Khoo - Y. A. Abakr

Department of Mechanical, Manufacturing and Materials Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia

Abstract
Sound attenuation had always been an interesting topic of investigation for many reasons. The range of frequency investigated by many researchers is usually of the order of tens of kHz, the lower frequency range investigated by some researchers, typically a few hundreds to few thousands of Hz, still falls far away from the interesting zone for the low frequency thermoacoustic engine, which is operating in a frequency range of 30--150 Hz, hence, this is considered a very low frequency range. Passive attenuation of acoustic waves was one of the important issues when creation of a travelling wave is intended. In order to experimentally simulate a thermoacoustic engine designed to utilise waste heat from cooking stoves as a source of electrical energy, a setup has been developed to represent the driver section of the engine. As a first step of the investigation, an acrylic straight tube configuration with a square cross-section is used to investigate the effectiveness of different absorbing materials by using multi-microphone impedance tube technique. Different passive attenuation options were tested. A hard end constructed of an 8 mm aluminium plate was used for validation. Different noise signals were tested, the uniform white noise was chosen over Swept sine signal because of its steady behaviour in the low frequency region. A Matlab program was created to calculate the absorption coefficient of the tested materials. The current study also includes the comparison with the results produced by the numerical simulation using DeltaEC. Results show the elastic aluminium termination end has better attenuation characteristics compared with the other alternatives. The outcomes of this work will be used for further ongoing investigations on loss assessments of the low frequency thermoacoustic engine. Moreover, the validated DeltaEC model will be used to investigate a wider range of alternative for better optimisation. The absorption coefficient can be improved by using a more elastic end and the optimum elasticity requirement will be optimised using DeltaEC simulation. A compliance volume integrated with an elastic end is also expected to work as a better alternative at very low frequency. Figs 5, Refs 21.