Methods for Accurate Determination of Acoustic Two-Port Data in Flow Ducts [Elektronisk resurs]

• 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. 

Ämnesord

Engineering and Technology  (hsv)

Mechanical Engineering  (hsv)

Fluid Mechanics and Acoustics  (hsv)

Teknik och teknologier  (hsv)

Maskinteknik  (hsv)

Strömningsmekanik och akustik  (hsv)

TECHNOLOGY  (svep)

Engineering physics  (svep)

Acoustics  (svep)

TEKNIKVETENSKAP  (svep)

Teknisk fysik  (svep)

Akustik  (svep)

Genre

government publication  (marcgt)

Indexterm och SAB-rubrik

Two-port

duct acoustics

measurements

flow noise supression