IMPEDANCE TUBE FACILITY

The impedance tube is a well known acoustic entity, and it has been used to measure the acoustic properties of materials in air for at least 100 years. When one wishes to measure the impedance of materials in water, the problem becomes much more difficult. Here and in the following pages we will review some of the basics of impedance tube techniques, the difficulties when using water, and how to overcome them.

The basic idea of the impedance tube is to utilize a rigid walled tube, of constant cross-sectional area, with a piston type source at one end and a sample (whos impedance is desired) at the other end. Once the source is turned on, standing waves develop in the tube, and an interference pattern is produced. From the position of the peaks and nulls in this pattern, and from the ratio of the maximum pressure to the minimum pressure, the acoustic impedance and absorption of the sample can be obtained. (For more quantitative information, see any standard acoustic text, such as Pierce's Acoustics, or Kinsler & Frey's Fundamentals of Acoustics.) Alternatively, one can incorporate a more modern impedance determination method, the two sensor transfer function technique, which has become a standardized measurement. This is the technique employed at BU. A photo of the BU impedance tube facility is shown below.


The bubbly fluid generator can be seen on the left of the tube, with a microscope positioned for sample bubble sizing. The tube itself is sitting on the red dolly. The hydrophones are near the left end of the tube. The source is mounted on the right end. The electonics are shown to the right.

A typical result is shown below. The plot shows the measured impedance of a transmission line with a pressure release termination and a theoretical prediction of the same.

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This page is maintained by Preston Wilson
Last Updated March 13, 2002