Sound is essentially a mechanical disturbance that travels through a fluid (1). The human hearing is likewise mechanical in nature and is a feat of evolutionary engineering. Our auditory system picks up the minute differences in the time and intensity of the arrival of sound waves at each ear, and in a synergistic combination of our mechanical and neurological functions, our brain is able to deduce their directionality (2, 3).
Our hearing works adeptly in air, but the transmission characteristics of water, in which sound travels approximately four to five times faster, and the biophysical mechanics of the human auditory system make the perception of sound more difficult underwater (3). While submerged, the density of our eardrums is too close to the density of the water to impede the sound wave. Instead, our cranium provides the physical impedance and carries the vibrations to both of the inner ears via bone conduction and enables the perception of sound, but at the loss of directionality (3).
Given our ill-equipped hearing under water, humans have crafted machines that have overcome—and even taken advantage of—the characteristics of sound in water far beyond what nature alone has enabled us.
History of Research in Underwater Acoustics
Underwater acoustics research has been practiced since the early nineteenth century. In 1826, Swiss physicist Jean-Daniel Colladen and French mathematician Charles-Francois Sturm used a bell apparatus for measuring the speed of sound in the waters of Lake Geneva, Switzerland, which yielded a respectable value of 1435 m/s at 8°C, coming within 2% of currently accepted values (4,1).
Interest in underwater sound propagation in the ocean, however, surged only after the sinking of the Titanic in 1912. Five days following the incident, English mathematician and meteorologist L.F. Richardson filed an application at the British Patent Office for echo ranging with airborne sound; a month later, he returned with yet another proposal for an underwater analog (5).
The outbreak of World War I unleashed a flurry of activity in underwater acoustics research, especially for use in unrestricted submarine warfare. In 1916, under the British Board of Invention and Research, Canadian and British physicists formed the Anti Submarine Division (ASD) of the British Naval Staff. Working under strict secrecy, they affixed the codename “asdic” to refer to their aquatic sound experimentations and succeeded in using piezoelectric crystals to produce the world’s first underwater sound detection apparatus in mid-1917. World War I came to a close, however, before artificial underwater echolocation could be applied in underwater submarine warfare against German U-Boats (5).
Shortly after World War I, German scientists revealed their own extensive research in underwater acoustics by publishing the first scientific paper on oceanic acoustics, theoretically describing the bending of sound rays produced by slight temperature and salinity gradients in the sea and underscoring their importance in determining sound ranges. The significance of their research, however, would remain unrecognized for nearly six decades (5).
The interwar years of peace also saw slow but inexorable advances in applying underwater sound for practical and military needs. American engineers began their own examinations in underwater acoustics and began to recognize what their German colleagues had discovered years before: the vagaries of sound propagation in the sea (5). For instance, those involved in the operation of early sonar devices realized the correlation between performance and the time of day. Operators obtained good echo readings in the morning but obtained poor or almost no readings during the afternoon. With the use of temperature measuring equipment, scientists were able to correlate characteristics of sound waves with thermal gradients (5). By 1938, several adequate sonar systems were developed, and by the outbreak of World War II, numerous American ships were well equipped for underwater listening and echo ranging.
The onset of World War II heralded yet another period of intense research on underwater acoustics. While still officially neutral, America welcomed delegations of British scientists through the British Technical and Scientific Mission (a.k.a “Tizard Mission”), who confidentially imparted their scientific knowledge in exchange for making use of America’s industrial and R&D capabilities (6). The Americans coined the word “sonar”—a sonic counterpart to the then-glamorous “radar”—to describe their own underwater sound experiments and was only retroactively given a full name, as “SOund Navigation And Ranging.” Many of the most fundamental concepts of underwater acoustics, such as sonar equations, target strength, noise output, and underwater sound in the human ear were first quantitatively described during World War II (5).
Characteristics of Underwater Sound Propagation
Since the nascent beginnings of underwater acoustics research in the nineteenth century, researchers began to see the fluctuations of sound propagation underwater. Water, compared to air, has a higher viscosity, heat capacity, and conductivity of sound waves. These characteristics make the propagation of sound more complex, particularly in real-life systems like the ocean.
Speed of Sound Underwater
While the accepted speed of sound in air is 340 m/s, the speed of sound underwater is more difficult to ascertain because the speed of sound is much more affected by temperature, dissolved impurities (usually salinity), hydrostatic pressure, and mass density. In an empirically derived formula, the speed of sound underwater in a simplified equation is:
C(T,P,S) = 1449.2 + 4.6T – 0.055T2
+ 0.00029T3 + (1.34 – 0.01 T)(S – 35) + 0.16z,
where is T represents temperature in ˚C, S represents salinity in parts per thousand, and z represents the depth in meters. More complex equations have been published, with the most accurate equation to-date involving 19 terms and coefficients up to 12 significant figures (7). Generally, an increase in temperature and salinity will increase the speed of sound in water. Usually, ocean salinity, S, is estimated around a constant 35 ppt, so the sound speed formula reduces to a function of temperature and depth, with the temperature of the water as a greater parameter than salinity. (5, 7).
American researchers also found that the derivation of the speed of sound in the ocean is further complicated by the existence of the sound speed gradient affected by varying temperature and pressure levels in ocean systems. A particular example of a sound speed gradient arises in the thermocline, a thin zone of water within which temperature decreases rapidly with depth between the surface zone above or the deep zone below. The depth and the thickness of a thermocline layer are affected by heating and cooling of surface zone during the day, the changing of the seasons, weather variations, and local environmental conditions (8).
General Frequency Changes underwater
Water is also known to manipulate a sound wave’s frequency content—the number of occurrences of a repeating event per unit time. According to Urick, “water, with or without contaminants such as air bubbles, is to some extent nonlinear. That is, the change in density caused by a change of pressure of a sound wave in water is not linearly proportional to the change in pressure […] frequencies different from the input frequency occur at the output. For sinusoidal acoustic waves, a variety of additional frequencies in water are found to be generated” (5).
Scattering and Reflection in Water Surface
The nonlinearity of sound waves in water may be attributed partly to scattering, a physical process in which any form of radiation—such as light or sound—or moving particles is forced to deviate from its constant trajectory by the existence of non-uniformities in the medium through which they pass (3). The types of non-uniformities in water include particles such as salt, debris, air bubbles, micro-bubbles, droplets, and density fluctuations in fluids. Besides minute particles, the fluctuating sea surface can also act as a “scatterer.” On a smooth water surface, water forms an almost perfect reflector of sound; when the surface is rough, the surface acts as a scatterer, “sending incoherent energy in all directions” (5).
Scattering and Reflection in Seafloor
Scattering can also be observed at the seabed. In the simplest model with a plane interface, the three bottom parameters that determine the reflection loss are density, sound velocity, and attenuation coefficient, the quantity related to porosity of the sediment. But the ocean floor is not perfectly planar; both scattering and reflection take place (5). Although not as protean as the surface of the sea, quantitatively analyzing seabed reflection is complicated to of the seabed’s multi-layered composition.
All bottom sediments are to some extent absorptive. Many empirical measurements of sound attenuation—the combined effect of scattering and absorption in decay rate of the wave as it propagates through material —in sediments have been made (9). They show that the attenuation coefficient of compression waves in marine sediments is related to frequency by
α= kf n
where α is in decibels per meter, f is the frequency in kHz, and k and n are empirical constants. According to Urick, “In the equation, ‘n’ seems to be essentially in unity for many measurements on sands, silts, clays and the like; k depends upon porosity, and approximately equal to 0.5 over the range of porosities between 35 to 60 percent” (5).
Application of Underwater Acoustics in Nature
Animals have made use of underwater sound propagation for millennia. Years of research have bolstered the supposition that dolphins rely on sound production and reception to navigate, communicate, and hunt in dark or murky waters, where sight is of little use (10).
Bottlenose dolphins, for instance, are known to produce various sounds through the movement of air in the trachea and nasal sacs. Dolphins produce a wide range of sounds—clicks, moans, trills, grunts, squeaks, creaks, and whistles—at a considerable range of depths, frequencies, and intensities to communicate and navigate (10).
In communication, individual bottle-nosed dolphins have signature whistles to identify themselves. In navigation and echolocation—an ability that certain mammals possess that enables them to “see” with their ears by listening for echoes—dolphins produce a series of directional clicks, a sound carrying numerous frequencies at once, lasting from 50 to 128 microseconds, and a theoretical time resolution of around 0.015 microseconds (10, 11). The sound travels away from the source and bounces off objects. The sound that is returned to the dolphin is conducted through “the lower jaw to the middle ear, inner ear, to auditory nerve of the dolphins to hearing centers in the brain,” where the meaning of the sound is interpreted, such as the “size, shape, speed, distance, direction, and even some of the internal structure of objects in the water” (10).
Human Application of Underwater Acoustics
As a product of 20th century wartime ingenuity, underwater acoustics is employed in a variety of military, civilian, and research uses.
Sonar
Active sonar systems utilize an acoustic projector to generate a sound pulse, often referred to as a “ping,” and then listen for reflections (echoes) of the pulse. The pulses reflect off a target, and return to the ship. By accounting for the speed of the sound in water, and the time for the sound to travel off the target and back, computers can calculate distances between the submarine and the target.
Passive sonar systems consist of a receiver that pick up the noise radiated by an object such as a ship or a submarine. The sound waves are analyzed to identify the type of the object in question and to determine its direction, speed, and distance.
Passive sonar systems are usually mounted on a ship’s hull, deployed from a sonobuoy, towed behind a ship, or laid on the ocean floor to monitor sound continuously.
Underwater Communication and Data Transfer
According to Milica Stojanovic, electrical engineering associate professor at Northeastern University, “Oceans cover about 70 percent of the Earth’s surface, and much of this vast resource remains to be explored” (11). One challenge to underwater exploration lies in communication technology. While methods of wireless communication, such as cell phone and wireless internet connections, have been established above ground, underwater communication has lagged behind because many frequencies, such as ultraviolet rays and radio signals, do not propagate well in water (13).
Sound waves, on the other hand, can propagate well underwater and have been used for underwater communication. U.S. Navy submarines, for instance, use a specialized telephone system that transmits and receives human vocal sound waves through hydrophones and audio amplifiers (13).
Higher quality digital data, such as words and pictures, can be communicated via special acoustic modems, which convert digital data into special underwater sound signals (13). While underwater modems are slow in comparison to their surface counterparts, they are used in a variety of settings that require transmission of data from below sea, such as submarine maneuvering and the remote acquisition of data in oceanographic research (14).
Advancements in underwater data transfer may allow ocean acoustics researchers to study the topography and makeup of sea floors, which, despite their physical and geographic immensity, are among the least-known regions of the planet (15).
Marine Biology
Researchers also utilize underwater acoustics in the study of Marine Biology. Digital recording of sea-mammal sounds have been made since the early 1980’s beginning with Giovanni Pavan, a marine biologist from the University of Pavia in Northern Italy (15).
From 2005 to 2006, in a collaborative effort with Italian particle physicist Giorgio Riccobene, Pavan embarked on a long-term monitoring project of the Sicilian seabed soundscape——the sounds that form from an immersive environment—through an Ocean Noise Detection Experiment (ONDE) and observed a train of clicks in the recording made by sperm whales. Because Pavan lacked the funding to develop a specialized algorithm to account for acoustics analysis, he and his colleagues numbered the whales by observing the echoes directly, as “the acoustic properties of a click can give an idea of the animal’s size and sex” (15). Pavan and Riccobene’s group derived a more accurate number of the area’s sperm whale population and detected “seasonal patterns and hints of social behavior in the recordings,” including patterns of communication and dialects (15).
Conclusion
Underwater acoustics have made significant strides over the last century. From the demands of wartime necessity, researchers have advanced the field to the extent that underwater acoustics is now used in civilian life (fish finders, for instance) and in various fields of science. Further progress in underwater acoustics will allow for advancements in underwater biology, seismography, topography, tomography, and even weather patterns, allowing researchers to gain deeper insights about our planet than ever before.
References
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2. D. Ulin, How Sound Waves Work Underwater (2011). Available at http://indianapublicmedia.org/amomentofscience/sound-waves-work-underwater/ (02 December 2011).
3. J. A. Maurer, Research in Underwater Sound with Focus on Musical Applications and Computer Synthesis (1998). Available at https://ccrma.stanford.edu/~blackrse/h2o.html (02 December 2011).
4. J. D. Colladen, C. F. Sturm, Ann. Chim. Phys. 36, 236 (1827).
5. R. Urick, Principles of Underwater Sound (McGraw-Hill, New York, ed. 3, 1983).
6. D. Zimmerman, Top secret exchange: the Tizard mission and the scientific war (Mcgill Queens Univ Pr, Quebec, ed. 1, 1996).
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9. Attenuation of Sound Waves (2011). Available at http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/attenuation.htm (02 December 2011).
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11. G.L. Zaslavskiy. Ocean 2, 620-628 (2003).
12. Engineer explores underwater wireless communications (2009). Available at www.physorg.com/news157915862.html (02 December 2011).
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15. N. Nosengo. Nature 462, 560-561 (2009).