Tech Briefs

Sound wave reflections distinguish between various substances.

A novel outdoor synthetic aperture acoustic (SAA) system consists of a microphone and loudspeaker traveling along a 6.3-meter rail system. This is an extension of a prior indoor laboratory measurement system in which selected targets were insonified while suspended in air. Here, the loudspeaker and microphone are aimed perpendicular to their direction of travel along the rail. The area next to the rail is insonified and the microphone records the reflected acoustic signal, while the travel of the transceiver along the rail creates a synthetic aperture allowing imaging of the scene.

At left is a photograph of the SAA Measurement System mounted on a 6.3-m rail. The transceiver is mounted on a tripod that is supported by a motorized platform that traverses the rail. At right is a closeup view of the acoustic transceiver.
Ground surfaces consisted of weathered asphalt and short grass. Several surface-laid objects were arranged on the ground for SAA imaging. These included rocks, concrete masonry blocks, grout-covered foam blocks; foliage-obscured objects and several spherical canonical targets such as a bowling ball, and plastic and metal spheres. The measured data are processed and ground targets are further analyzed for characteristics and features amenable for discrimination.

The system supports a desired standoff detection capability by looking perpendicular to the direction of travel. The sensor’s field of view is different from forward-looking radar or other opto-acoustic systems, but addresses the lateral view of the standoff detection objective. Opto-acoustic and other excitation systems have been used successfully to detect buried landmines.

An acoustic transceiver (quasi-monostatic) was constructed and mounted on a 6.3-meter portable rail system to facilitate controlled outdoor measurements (ultimately, the objective is a teleoperated vehicular mounted version). The direct measurement of the transceiver is the acoustic reflectivity found in the synthetic aperture or “acoustic scene.” The acoustic absorption of some materials is frequency-dependent, and the frequency spectrum of the backscatter is also strongly influenced by the target geometry when the target dimensions are comparable to or greater than the acoustic wavelength. A number of targets were imaged, and variations in acoustic reflectivity and phase versus aspect angle were observed.

By moving along the rail, the transceiver can continuously collect data, allowing the generation of synthetic aperture. Several canonical targets were placed on a weathered asphalt surface and image formation, i.e., wavefront reconstruction, was performed. This allows for the image processing techniques to include target and ground surface scattering analyses. Additionally, a digital spotlighting technique was performed on selected targets to produce aspect angle versus frequency plots. This allows for subsequent spatial and spectral analyses to be performed.

Digitally spotlighted and broadside position preliminary assessment of the selected targets indicate different relative magnitudes and frequency responses. As expected, the spherical targets have lower acoustic cross-section magnitudes, and the blocks’ flat surface perpendicular to transceiver travel have significantly larger scattering magnitudes. For example, a manufactured concrete block shows lower broadside-acoustic magnitude compared to three grout-covered foam blocks.

This work was done by Steven Bishop, Jay Marble, and Pete Gugino of the Army RDECOM CERDEC NVESD; Therese-Ann Ngaya, Joseph Vignola, and John Judge of the Catholic University of America; and Erik Rosen and Mehrdad Soumekh. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. ARL-0105

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Outdoor Synthetic Aperture Acoustic Ground Target Measurements (reference ARL-0105) is currently available for download from the TSP library.

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