Wave Which Way
Sensor technology for directional underwater sound
The American Philosophical Society (APS) was founded by Benjamin Franklin in 1743 and stands as the oldest learned society in the United States. Philadelphia native David Rittenhouse served as the society’s president from 1791 to 1796, after holding positions as Professor of Astronomy and Vice-Provost at what is now known as
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the University of Pennsylvania. In 1786, Rittenhouse published an article in the society’s journal, Transactions of the APS, titled, “Explanation of an optical deception.” An expert optical tool maker, Rittenhouse had strung fine hairs between the grooves of two co-linear screws and, by doing so, had invented a crude diffraction grating that spread out the colors of light when one looked through it. After Rittenhouse abandoned the device as an optical oddity, German physicist Joseph von Fraunhofer later reinvented the device when he strung thin wire between the grooves of two screws in 1813 and the diffraction grating has enjoyed continuous improvement ever since.
Dutch scientist Christiaan Huygens had proposed a model for light diffraction almost a century earlier, but it took the works of Fraunhofer, Thomas Young and Augustin-Jean Fresnel to reveal the importance of the wave model of light. Before that time, Sir Isaac Newton had proposed that light was a collection of different-colored particles (a forerunner of our modern model of the photon) that flew through the air in straight lines or rays and reflected from surfaces at predictable angles. With the advent of quantum mechanics in the mid 1800s, it was discovered that both models, the particle-ray model and the wave model, are equally valid ways to view light propagation. The choice of which way to model the propagation often depends on the interactions being observed. Reflection, absorption and scattering are described easily by the ray model, while the wave model is invoked to explain refraction, diffraction and dispersion. These optical models also are applicable to the motion of solids, liquids and gases. The classical Newtonian particle model is used most often to describe the kinematics of solids whereas the wave model is used to describe fluid motion.
Waves themselves can be viewed in different ways depending on their type. Light waves are viewed as primarily transverse since their electric and magnetic fields oscillate side-to-side while the light travels forward. Sound waves in air are modeled as primarily longitudinal since regions of high and low pressures oscillate along the direction in which the wave is traveling. A microphone is used to measure these high-frequency changes in air pressure and to record the sound waves. Underwater vibrations also are modeled as longitudinal pressure waves and can be detected using a similar hydrophone. However, surface waves are obviously transverse. This up and down movement of water is known as shear deformation and can be measured by observing the height and direction of the wave.
Interest in measuring underwater sound was sparked by the sinking of the Titanic in 1912 and the need to detect submarines during World War I. Unfortunately, those prevailing reasons are still with us today. Modern hydrophones can be very sensitive but, because of their reliance on longitudinal pressure waves, are omnidirectional. Hydrophones are arranged in both 2-D and 3-D arrays such that the arrival time of the pressure wave to each sensor can be triangulated to pinpoint both distance and direction to the source of the sound wave. According to the team of Research Engineer Francois Guillot, Professor Peter Rogers and Research Scientist David Trivett at the Georgia Institute of Technology, the U.S. Navy routinely tows hydrophone arrays that are thousands of feet long in order to obtain the desired directional resolution. Working under a grant from the Office of Naval Research, the Georgia Tech team developed a prototype underwater sound sensor able to measure both sound intensity and direction. The new device is sensitive to direction because it is based on the detection of the sound wave’s transverse shear deformation, much like detecting waves on the surface. The device includes a small paddle made of special composite material that has the same density as seawater. The paddle is attached to the main housing by a hinge that permits it to oscillate back and forth as it flutters with the passing wave.
The magnitude of the transverse shear deformation is extremely small and the team needed a sensitive way to measure the paddle vibration. Their method is based on the “optical deception” discovered by Rittenhouse. When light passes through an array of thin parallel obstructions, its waves diffract around them and the resulting constructive and destructive interference spatially separates the wavelengths. In 1978, Dr. Kenneth O. Hill, a scientist at the Communications Research Centre (CRC) in Ottawa, published a method for creating an array of parallel lines within the core of an optical fiber. Named after 1915 Nobel Laureate William Lawrence Bragg, Hill’s “fiber Bragg grating” transmits specific light frequencies while reflecting others back through the fiber. The frequency of the reflected light changes as the line spacing varies due to mechanical strain or temperature effects on the fiber’s linear expansion. The Georgia Tech team attached a single fiber containing two Bragg gratings to their sensor; one situated on the paddle and the other on the main housing. As the paddle oscillates, the distance between the gratings changes and its effect on the reflected light is monitored by a photodetector. While sensitive to its orientation with the wave, an array of these sensors is again needed to pinpoint direction and range of its source; however, the team suggests an array of these sensors will be more than five times smaller, easier to handle and less costly to operate. These are important improvements no matter which way you look at it.
Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at La Salle University. He may be contacted at [email protected].
