Extended Frequency Range of Impedance Tube Measurement

Abstract:

The paper presents a new method for extending the frequency range of impedance tube measurement. The method is based on decomposing and measuring higher order mode waves in ducts and picking up (0, 0) mode of the incident and reflected wave for the material of uniform surface impedance. Therefore the normal incidence absorption coefficient and the acoustic impedance can be determined. Compared with previous research, the limit of cut off frequency in the tube can be avoided and the frequency range of measurement can be increased greatly. Finally, the verification test with the proposed method is carried out by adopting the single microphone axial transmission system in a large vertical impedance tube. From the test results, the upper limit frequency of the tube is extended from 280 Hz to 2 000 Hz or higher.

Key words:cut off frequency; modes decomposition; duct; impedance tube measurement; acoustic properties