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Methods for Accurate Determination of Acoustic Two-Port Data in Flow Ducts
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Helwan University, Egypt.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.ORCID iD: 0000-0002-8474-8563
2005 (English)In: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005Conference paper, Published paper (Other academic)
Abstract [en]

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Accurate measurement of the acoustic two port data can be used not only to determine the transmission loss but also to determine physical properties like flow resistivty as well as speed of sound and impedance. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. Techniques to improve the acoustic two-port determination have been tested in this paper. A number of possible configurations for connecting loudspeakers to the flow duct have been investigated. It was found that using a perforate pipe section with about 50% porosity between the loudspeaker side branch and the duct gave the best signal-to-noise ratio out of the studied configurations. Different signal processing techniques have been tested for reducing the adverse effects of flow noise at the microphones. The most successful techniques require a reference signal which can be either the electric signal being input to the loudspeakers or one of the microphone signals. As a reference technique stepped sine excitation with cross-spectrum based frequency domain averaging was used. This technique could give good results for most cases. Using a periodic signal (saw-tooth) and synchronised time domain averaging good results could be obtained if a sufficient number of averages was used. At flow velocities higher than M=0.2 about 10000 averages were needed. Random excitation together with cross-spectrum based frequency domain averaging also gave good result if the same number of averages was used. Ordinary frequency domain averaging is not sufficient at high flow velocities. It was also shown that using cross-spectrum based frequency domain averaging an improvement could be obtained if the microphone with the highest signal-to-noise ratio at each frequency was used as the reference microphone rather than a fixed microphone.

Place, publisher, year, edition, pages
2005.
Series
International Congress on Sound and Vibration, ICSV, Vol. 3
Keyword [en]
Two-port, duct acoustics, measurements, flow noise supression
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-62396Scopus ID: 2-s2.0-84881574417ISBN: 978-162748149-6 (print)OAI: oai:DiVA.org:kth-62396DiVA: diva2:480247
Conference
12th International Congress on Sound and Vibration 2005, ICSV 2005, Lisbon, Portugal, 11-14 July, 2005
Note

QC 20140825

Available from: 2012-01-19 Created: 2012-01-19 Last updated: 2014-11-17Bibliographically approved

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Bodén, Hans

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