A prototype system for acoustic imaging of gas leaks that emit ultrasound was developed at Kennedy Space Center. Potential government and commercial users concerned with leak imaging and safety expressed interest in this technology.
The prototype system includes an 8 × 8 array of air-coupled acoustic transducers with a peak-response frequency of 40 kHz. As indicated in the figure, the array is placed at the focal plane of a paraboloidal reflector (see figure) with a focal length of about 1 ft (30 cm). Thus, the array functions analogously to focal-plane arrays of photodetectors for imaging optical sources or focal-plane arrays of antenna elements for imaging radio sources, the unique feature of this array being that it provides a low-resolution image of acoustic sources in its field of "view."
The output signals from the transducers are buffered out serially through a 64-to-1 multiplexer, then sent to a microprocessor that includes an analog-to-digital converter. The microprocessor controls the operation of the multiplexer and sends the digitized, processed transducer outputs to a computer, which generates a real-time video display in which the intensities of the acoustic signals are represented by brightnesses in the cells (in effect, acoustic pixels) of a square 8 × 8 grid corresponding to the transducers. By use of a pixel-to-pixel-averaging algorithm, the pattern can be converted to one of 15 × 15 acoustic pixels with blocky appearance of the grid softened somewhat to give an appearance of finer resolution.
As described thus far, the system operates in a passive mode, in which it produces intensity images of leaks or other sources of high-pitch sound. However, the system also offers the more-powerful capability of operation in an active phase-imaging mode, in which the scene is "illuminated" by an ultrasonic signal of specified frequency and phase. Because the microprocessor has access to the phase as well as the magnitude or intensity information for each ultrasonic pixel, it is a relatively simple matter to process the transducer outputs to obtain an image in which the brightness of each ultrasonic pixel represents the difference between the phase of the "illuminating" signal and the signal returned to the corresponding transducer.
Phenomena that alter the speed of sound and thus the phase of the return signal can be imaged in this mode. Tests of the prototype system in the active phase-imaging showed it to be capable of imaging gas leaks, a cold airflow, and moving objects. It should also be possible to image fires and temperature gradients in still or moving air.
Recent improvement included the following:
- Preventing the receiver from being overwhelmed by near-field reflections of the transmitted signal;
- Increasing the resolution by increasing the number of transducers, along with the size of the reflector and/or the frequency; and
- Designing alternative systems with wider fields of view.
This work was done by William D. Haskell, James P. Strobel, Robert C. Youngquist, John S. Moerk, and Robert B. Cox formerly of I-NET for Kennedy Space Center.
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