Page 73 - Spring2019

Sound Perspectives
Brian G. Ferguson
Address:
Defence Science and Technology (DST) Group – Sydney Department of Defence Locked Bag 7005 Liverpool, New South Wales 1871 Australia
Email:
Brian.Ferguson@dsto.defence.gov.au
R. Lee Culver
Address:
Applied Research Laboratory Pennsylvania State University University Park, Pennsylvania 16802 USA
Email:
rlc5@psu.edu
Kay L. Gemba
Address:
Marine Physical Laboratory Scripps Institution of Oceanography University of California, San Diego La Jolla, California 92093-0238 USA
Email:
gemba@ucsd.edu
International Student Challenge Problem in Acoustic Signal Processing 2019
The Acoustical Society of America (ASA) Technical Committee on Signal Pro- cessing in Acoustics develops initiatives to enhance interest and promote activ- ity in acoustic signal processing. One of these initiatives is to pose international student challenge problems in acoustic signal processing (Ferguson and Culver, 2014). The International Student Challenge Problem for 2019 involves processing real acoustic sensor data to extract information about a source from the sound that it radiates. Students are given the opportunity to test rigorously a model that describes the transmission of sound across the air-sea interface.
It is almost 50 years since Bob Urick’s seminal paper was published in The Journal of the Acoustical Society of America on the noise signature of an aircraft in level flight over a hydrophone in the sea. Urick (1972) predicted the possible existence of up to four separate contributions to the underwater sound field created by the presence of an airborne acoustic source. Figure 1 depicts each of these contribu- tions: direct refraction, one or more bottom reflections, the evanescent wave (al- ternatively termed the lateral wave or inhomogeneous wave), and sound scattered from a rough sea surface. Urick indicated that the relative importance of each con- tribution depends on the horizontal distance of the source from the hydrophone, the water depth, the depth of the hydrophone in relation to the wavelength of the noise radiated by the source, and the roughness of the sea surface.
https://doi.org/10.1121/AT.2019.15.1.71
The Student Challenge Problem in Acoustic Signal Processing 2019 considers the direct refraction path only. Other researchers have observed contributions of the acoustic noise radiated by an aircraft to the underwater sound field from one or more bottom reflections (Ferguson and Speechley, 1989) and from the evanescent wave (Dall’Osto and Dahl, 2015). When the aircraft flies overhead, its radiated acoustic noise is received directly by an underwater acoustic sensor (after trans- mission across the air-sea interface). When the aircraft is directly above the sensor, the acoustic energy from the airborne source propagates to the subsurface sensor via the vertical ray path for which the angle of incidence (measured from the nor- mal to the air-sea interface) is zero. In this case, the vertical ray does not undergo refraction after transmission through the air-sea interface. The transmitted ray is refracted, however, when the angle of incidence is not zero. Snell’s Law indicates that as the angle of incidence is increased from zero, the angle of refraction for the transmitted ray will increase more rapidly (due to the large disparity between the speed of sound travel in air and water) until the refracted ray coincides with the sea surface, which occurs when the critical angle of incidence is reached. The ratio of the speed of sound in air to that in water is 0.22, indicating that the criti- cal angle of incidence is 13°. The transmission of aircraft noise across the air-sea interface occurs only when the angle of incidence is less than the critical angle; for angles of incidence exceeding the critical angle, the aircraft noise is reflected from the sea surface, with no energy propagating below the air-sea interface. The area just below the sea surface that is ensonified by the aircraft corresponds to the base of a cone; this area can be thought of as representing the acoustic footprint
©2019 Acoustical Society of America. All rights reserved. volume 15, issue 1 | Spring 2019 | Acoustics Today | 71