IMPEDANCE TUBE FACILITY: Preliminary results
Shown in the following figure is the measured complex reflection coefficient of an air termination, obtained in our heavy walled (4" O.D. x 2" I.D. 304L stainless steel) impedance tube, using the standard two-sensor technique, as described in the ASTM Standard Designation E 1050-90. The only deviation we make from the standard is the use of a reduced sound speed, as described previously, and the way we cross calibrate the two sensors.
The standard calls for the use of a non-reflecting termination in order to perform a cross calibration of the two sensors. This is required because of the different responses of the two sensors. (Ideally, one would have two identical sensors, with exactly the same response, but in reality, there are phase and amplitude mismatches between the two real sensors.) A non-reflecting termination was not possible in our lab. Instead, we used an inverse technique and backed out the cross calibration from a known impedance termination, namely air. Then we proceeded forward and obtained the above results.
The measurements and calculations which produced the above results are performed in real time with a HP 89410A Vector Signal Analyzer and two wall-mounted hydrophones of our own design. Certain consideration was taken to minimize the effect of the sensors on the soundfield, and effect of wall motions on the sensors. Since this inverse and then forward technique is really not very convincing, we will further ground truth the impedance measurement facility using a thick steel plate. This will give us the two limiting cases. Additionally, we will measure the impedance of the open tube submerged in a large body of water, and use for comparison the classic results of
H. Levine and J. Schwinger, "On the radiation of sound from an unflanged circular pipe," Physical Review 73 (4), 383-406 (1948).
If all goes well with the remainder of the ground truthing, we will then be prepared to make measurements of bubbly fluids. Presently, we are still perfecting the techniques of bubble production and cloud characterization.
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This page is maintained by Preston Wilson
Last Updated March 13, 2002