Page 73 - Spring 2019
P. 73

International Student Challenge
Problem in Acoustic Signal
Processing 20 ‘I 9
Brian G. Fgrgusfin The Acoustical Society of America (ASA) Technical Committee on Signal Pro-
Addrm: cessing in Acoustics develops initiatives to enhance interest and promote activ-
Defencg Science and Technology (DST) ity in acoustic signal processing. Onegof these initiatives is to pose international
Gmup A Sydney student challenge problems in acousoc signal processing (Ferguson and Culver,
Dgpmment m.DEfencE 2014). The -lntemational Student Challenge Problem for 2019 involves processing
Locked Bag 7005 real acoustic sensor data to eittract information about a source from the sound
Liverpool‘ New Scum wales 1871 that it radiates. Students are given the opportunity to test rigorously a model that
Australia describes the transmission of sound across the air-sea interface.
Email: It is almost 50 years since Bob Urick’s seminal paper was published in The lournal
BrianFergusOn@dsmdefmceggwau 0}-' the Acoustical Society ofAmericiz on the noise signature of an aircraft  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
R. L“ Culver presence of an airborne acoustic source. Figure 1 depicts each of these contribu-
tions: direct reiraction, one or more bottom reflections, the evanescent wave (al-
Addms’: ternatively termed the lateral wave or inhomogeneous wave), and sound scattered
Apphed Research Labnmlnry from a rough sea surface. Urick indicated that the relative importance ofeach con-
Pennsylvania 5”“ Universiw tribution depends on the horizontal distance of the source Eton: the hydrophone,
U“‘V”=“7 Pm“ 1’“““Y‘"““ 158” the water depth, the depth of the hydrophone in relation to the wavelength of the
USA noise radiated by the source, and the roughness of the sea surface.
Email: The Student Challenge Problem in Acoustic Signal Processing 2019 considers the
rlcS@p5"""'d“ 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
K371" Gemba wave (Dall’Osto and Dahl, 2015). When the aircraft flies overhead, its radiated
Address: acoustic noise is received directly by an underwater acoustic sensor (after trans-
Marine Physical Laboratory mission across the air-sea interface). When the aircraft is directly above the sensor,
Scripps Institution of Oceanography the acoustic energy from the airborne source propagates to the subsurface sensor
University of California, San Diego via the vertical ray path for which the angle of incidence (measured from the nor-
La Iolla, California 92093-0238 mal to the air-sea interface) is zero. In this case, the vertical ray does not undergo
USA refraction after transmission through the air-sea interface. The transmitted ray is
Email: refracted, however, when the angle of incidence is not zero. Snel.l’s Law indicates
gemb2@uEsd_Edu that as theangle of incidence is increased from zero, the angle of refraction for
the transmitted my 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 Amumml Society u[Amerim. All right: reserved. volume 15, issue 1 1 Spring 2019 | Acoulclcn Thday | 71

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