An acoustic modem that uses a separate phased-array transmitter and receiver has been conceived that allows the communication of high-frequency acoustic waves sideways to the transducer along the fluid-filled pipe. The transducer allows directing waves between the transmitter and the receiver with potentially minimal interferences with the piping walls. In addition, another method of directly sending communication signals up- and down-hole has been conceived that uses angled piezoelectric ring transducers.

General schematic view of the Acoustic Modem system for fluid-filled pipes.
Because a liquid-filled pipe forms a waveguide, different modes of waves may propagate in the liquid core with different phase and group velocities. In order to receive clear, less distorted signals, it is better to operate at the frequency that only allows a fundamental mode to propagate through the fluid. For an ideal, hard-walled, liquid-filled pipe of 5-inch (≈13-cm) diameter, the lowest cutoff frequency for the higher mode is ≈7 kHz, assuming the velocity in the liquid is 1,500 m/s.

The phased array consists of at least two ring transducers. The active piezoelectric ring is covered by an electric isolation layer, and has an acoustic backing to prevent acoustic energy from radiating outside of the pipe. The frequency of the transducer may be adjusted to be lower than any high mode cutoff frequencies to avoid signal distortion that can be caused by different phase and group velocities of the high-propagating modes in the liquid-filled pipe, and get the benefit of relatively low-propagating attenuation of the fundamental (0,0) mode in the pipe.

The inner surface of the transducer conforms to the inner diameter of the pipe to avoid interference of the flow or creation of any local, noise-causing turbulence. The electronic transmitter or receiver sends or receives signals in an upward direction. The height of the transducer should be less than the half-wavelength of the (0,0) mode wave for high transmission efficiency. The transmitted acoustic signals from the transducers are added when propagating upward in the pipe, but can create phase cancellation at a downward direction in the working frequency range. A similar response can be achieved when the array functions as an acoustic receiver. The cancellation can be optimized by adjusting the number and spacing of the transducers. In case the working bandwidth is relatively narrow, the time delay stages can be replaced by phase shifters.

This work was done by Yoseph Bar-Cohen, Eric D. Archer, Xiaoqi Bao, and Stewart Sherrit of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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Refer to NPO-48720.