Where When How — N/D 2017 - J/F 2018
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Reprint - Dive Training Sound In The Sea
Alex Brylske

In 1956 a then-obscure former French naval officer, filmmaker and adventurer published a book and released a documentary film that won the gold medal at the Cannes Film Festival. His accomplishment became a hallmark in the evolution of diving and characterized the entire underwater experience. The author/ co-director was Jacques-Yves Cousteau and his book/film, “The Silent World.” Thus began the man and the myth.

Of course, Cousteau went on to become, well, Cousteau. But the myth that the underwater world is soundless has hardly changed — at least among those who don’t know any better. Yet, as anyone who has ever stuck his or her head underwater can attest, it’s hardly silent down there. Sound travels quite well underwater. In fact, sound travels four times faster and much, much farther than it does in the atmosphere.

Light is quickly scattered and absorbed underwater, but sound is not. So, for creatures that live beneath the waves, hearing is a far more important sense than sight. Some animals, like whales and dolphins, have evolved the ability to produce and sense sound in ways that astound even researchers who study them. But cetaceans are hardly alone in their capacity to use sound. It’s also important to many fish and even some invertebrates. And it’s sound that we sight-dependent humans have turned to as the primary means of revealing the secrets of a deep sea. Sound plays a critical role for virtually everyone and everything that lives, works or in any way uses the ocean. Therefore, it’s helpful to have an understanding of just how sound behaves underwater, and ways that we use it to understand the world below.

Sound Off — The Basics

Before delving into the nuances of sound underwater, the more basic question is, just what is sound? Sound is a form of energy transmitted by rapid pressure changes in whatever it moves through. The substance that sound travels through is called a medium, which can be anything — liquid, solid or a gas — but must be something. So, contrary to what you see in “Star Wars” or “Battlestar Galactica,” sound cannot travel in space because space is a vacuum; there is no medium through which sound can travel.

So how does sound travel in this medium? As we’re interested mainly in how sound behaves in water, imagine tiny parcels of water as individual particles. When sound transits the water, the particles receive a minimal push, then a pull,causing them to vibrate back and forth. The spot where a particle was before the sound wave came through is called its equilibrium position. The movement of the water particle is called vibration.

A sound wave is called a compressional or longitudinal wave. The particles in a longitudinal wave move parallel to the direction in which the wave is traveling. This is illustrated in Figure 1. Note that there are places where the particles are squashed together (compressed) and places where the particles are pulled apart (expanded). The compressed areas represent regions of high pressure, while the expanded areas are regions of low pressure. So, a sound wave alternately compresses and expands whatever medium it travels through.

Sound moves in a wave but these wave particles move back and forth (pushed and pulled), not up and down, as is the case with a wave on the water’s surface. Water waves are called transverse waves because their particle movement is up and down, not back and forth. However, as shown in Figure 2, we can depict a sound wave like a transverse wave by showing the change in pressure as the wave moves through a medium. When particles are compressed under high pressure, the wave goes up; and when the particles expand under low pressure, the wave goes down.

We often describe sound as being loud or soft and high-pitched or low-pitched. But these are perceptions, not physical characteristics. To understand and analyze sound, it must be characterized in ways that can be measured using instruments. “Loud or soft” relates to the intensity or amplitude of the sound, whereas “pitch” is the frequency. A sound’s amplitude (loudness) is the change in pressure as the wave passes. And, like adjusting your iPod volume, the amplitude of a wave is related to the amount of energy it carries. A high-amplitude wave carries a large amount of energy, while a low-amplitude wave carries a small amount. The average amount of energy passing through a unit area per unit of time in a specified direction is termed wave intensity. As the amplitude of the sound wave increases, so does the intensity of the sound. Sounds with higher intensities are perceived to be louder. Relative sound intensities are often given in units termed decibels, abbreviated as dB.

As sound travels through seawater its intensity decreases because of spreading, scattering and absorption. Intensity loss increases with distance, while scattering occurs when sound waves bounce off bubbles, suspended particles, marine life, the surface, the seafloor and other objects. Eventually sound waves are absorbed and converted into a very small amount of heat. The absorption of sound is also a function of its wavelength, with higher frequencies being absorbed before lower frequency ones.

In dry air at sea level with a temperature of 70 degrees Fahrenheit (21 degrees Celsius), the speed of sound is 1,130 feet (342 m) per second (770 mph [1,232 kmph]). In seawater with a temperature of 68 F (20 C) and salinity of 35 parts per thousand, sound travels, near the water’s surface, at 4,996 feet (1.514 m) per second (3,407 mph [5,451 kmph]).


With the basics out of the way, let’s look at how we’ve come to understand sound in the sea. In the spring of 1944, two scientists from the Woods Hole Institute of Oceanography, Maurice Ewing and J. Lamar Worzel, set out on the research vessel R/V Saluda to test a theory. They proposed that low-frequency sound should travel long distances in the deep ocean. On April 3 of that year, a hydrophone was hung overboard and the Saluda recorded the sound of an explosive charge detonated by a ship nearly 400 miles (640 km) away. Ewing and Worzel had heard the first sound transmission through what was eventually termed the SOund Fixing And Ranging (SOFAR) channel. Interestingly, the explosion was not heard as a single sound, but, characteristic of SOFAR transmissions, as a number of separate, gradually intensifying sounds. In fact, the final signal was so intense that, according to research, “the end of the sound channel transmission was so sharp that it was impossible for the most unskilled observer to miss it.” Later experiments would make these first test results pale in comparison. For example, during a test in the 1960s, Navy depth charges were detonated and heard 2,290 miles (3,664 km) away from the explosion.

The U.S. Navy soon realized that the ability of low-frequency sound to travel long distances in the deep ocean could be used to increase the range at which they could detect submarines. So, in the great secrecy of the Cold War, the Navy launched Project Jezebel (later to be known as SOund SUrveillance System or SOSUS). The SOSUS project placed arrays of hydrophones on the ocean bottom throughout the world, which were connected by cables to top-secret processing centers on shore. This became one of the most successful programs of the Cold War, easily detecting and tracking the noisy Soviet submarines of that era.

One aspect of the SOSUS project, which at first was quite mysterious, turned out to be a scientific treasure house. Operators monitoring the SOSUS arrays in the early days detected many sounds whose sources were at first puzzling, but definitely weren’t submarines (one particular unknown sound operator dubbed the “Jezebel Monster”). Later the low-frequency sounds were identified as whale vocalization. Unfortunately, for many years all SOSUS data was highly classified. But with the fall of the Soviet Union, the SOSUS data was declassified in 1991 and made available to marine mammalogists, opening up a whole new area of research and understanding of the Leviathans of the sea.

By the 1960s the U.S. Navy was so enamored by the prospects of sound that it even experimented with the use of these long-range sound transmissions as a lifesaving tool. The concept was that shipwrecked sailors or downed pilots could drop a small explosive charge set to explode in the SOFAR channel. The arrival times of the signal at a number of widely spaced listening stations ashore would then be used to triangulate the survivor’s position. However, the idea was ultimately rejected in favor of radio beacons.

Oceanographers were also interested in the SOFAR channel for more than military applications. They realized that the speed and direction of deep ocean currents could be measured using floats designed to drift with the current within the channel and transmit low-frequency acoustic signals (SOSUS hydrophone arrays were initially used as receiving stations). Due to the high expense of the drifting transmitters, the project turned to lower-cost receivers, which recorded transmissions from moored stations. At a predetermined time the floats surfaced and uplinked their data to satellites. With time, aside from ocean currents, oceanographers found that precise measurements of the travel times between widely spaced sources and receivers could be used to measure large-scale ocean temperature variability.

Slowing Down Sound

While it’s easy to see why the SOFAR channel became an important phenomenon of great military and scientific interest, the question remains: What causes it? Basically, it occurs because sound does not travel at a uniform speed in seawater; its speed increases with temperature, salinity and pressure. So, sound travels faster in the warmer surface layer of the ocean than it does in deeper, cooler water. This decrease in speed continues with depth, as the water gets colder, eventually reaching a minimum at about 3,300 feet (1,000 m) (the point at which the ocean reaches a constant temperature of about 36 F [2 C]). Below that depth, the effect of increasing pressure offsets the effect of decreasing temperature, so the speed of sound begins to once again increase. As a result, the speed of sound may actually be higher near the seafloor than at the surface. Salinity has the smallest effect on the speed of sound because salinity changes in the open ocean are also minimal. While these speed variations amount to only 2 per cent or 3 per cent of the average speed of sound in seawater, they are enormously important in how sound behaves in the sea. Figure 3 shows the relationship between depth and the speed of sound.

The low-velocity zone of sound varies with conditions and location, but in midlatitudes it typically occurs at around 3,900 feet (1,181 m) in the North Atlantic and about 2,000 feet (606 m) in the North Pacific. Although the speed of sound at this depth is relatively slow, the transmission of sound within this zone is very efficient because refraction tends to confine sound energy to within this zone. The outer edges of sound waves escaping from this zone will enter water in which the speed of sound is higher. This causes the wave to speed up but then pivot back into the minimum- velocity layer. Unless aimed at an acute-enough angle to escape, upwardtraveling sound waves will tend to be refracted downward, and downward-traveling waves will tend to be refracted upward, as depicted in Figure 4. As the sound waves bend toward layers of lower sound velocity, they tend to stay within the SOFAR channel.

Yet there is one other anomaly regarding sound in the sea. As you now know, sound travels at a relatively slow speed in the SOFAR layer. However, it moves rapidly near the bottom of the well-mixed surface layer near the permanent thermocline in temperate oceans (a depth of about 200 feet [61 m]). Because temperature and salinity conditions are homogeneous at that depth, sound waves are not refracted. But pressure still increases with depth, causing a thin, high-velocity layer, as depicted in Figure 5.

This phenomenon can produce a problem or an advantage, depending on your perspective, when yet another very familiar sound technology is used — SONAR (Sound Navigation And Ranging). It turns out that when vessels project sonar pulses into the water to search for marine animals, submarines or hazards to navigation, the target can be obscured or “shadowed.” Depending on the angle at which they arrive at the high-velocity layer, sound waves will sometimes split and refract to the surface or bend into the depths. Therefore, any object beyond the area of divergence may be undetectable by sonar. This is a well-known technique used by submariners to hide their location; and Tom Clancy fans may remember this phenomenon featured in an episode of the novel and movie, “The Hunt for Red October.”

The Sounds of Science

Experiments with sound in the sea have progressed significantly since the discovery of the SOFAR channel; and with the end of the Cold War much of this research has turned to peaceful purposes. Research has also shown that the SOFAR channel is a truly amazing phenomenon, and may even be used by great whales to communicate with each other thousands of miles away. Today, sound is being used to study, of all things, climate change.

It all began in 1991 with the first tests exploring what’s come to be known as “acoustic thermography.” Early that year, low-frequency sounds were generated by a ship in the southern Indian Ocean near Antarctica. The sounds — booming, lowfrequency hums — were produced by powerful underwater loudspeakers emitted in all directions for an hour. This was followed by two hours of silence, and the pattern was repeated for five days. Trapped in the SOFAR channel, the sound was detected 11,160 miles (17,856 km) away off the California coast, near Point Conception. The transit time to travel nearly halfway around the world was about three hours.

But how can sound be used to measure ocean temperatures, especially — as scientists now do — to within a few thousandths of a degree? Recall that sound travels faster through warmer water. So, water temperature differences can be calculated by recording the precise time taken for sounds to reach receivers around the world. Still, what does this have to do with climate signal with 260 watts of acoustic power.

These early experiments were forerunners to a much more ambitious program sponsored by the University of California’s Scripps Institution of Oceanography. In 1995, the Acoustic Thermography of Ocean Climate (ATOC) program began transmitting acoustic signals from the Pioneer Seamount deep off the west coast of North America. The signals were recorded on U.S. Navy SOSUS receiving arrays, as well as stations near Hawaii and near the Kiritimati Islands (just north of the equator in the central Pacific). Signals were also received by a recorder off New Zealand.

After more than two years of recording data, ATOC determined three key findings. First, the technology could, in fact, record sounds with more precision than originally thought possible (to within 20-30 milliseconds at 1,860-3,100 mile [2,976-4,960 km] ranges). Secondly, the data could be clearly related to known ocean processes. And, lastly, that the ATOC data was consistent with and complementary to satellite data. As a result, by combining both acoustic and TOPEX/Poseidon satellite altimeter data, computer models of the ocean’s general circulation could be greatly refined, and the extent of ocean warming — and thus global warming — could be much better understood.

In February 2005, Scripps researchers Drs. Tim Barnett and David Pierce used ATOC data and other sources in a report that many believe removed much of the uncertainty in the debate over global warming and — according to the authors — provided “the first clear evidence of human-produced warming in the world’s oceans.” With assistance from researchers at Lawrence Livermore National Laboratory, the study combined computer models and hard, observed evidence collected over the last 40 years in the world’s oceans. The conclusion: The data closely matched the results predicted in computer models for warming caused by human activity. With a statistical confidence exceeding 95 per cent, they found that warming in the upper 2,300 feet (697 m) predicted by the model corresponded to the measurements obtained at sea. This high degree of correlation between the model and real data confirmed that the ocean warming — an increase of 0.5 C since the 1940s — is the product of human influence. Although efforts were also made to explain the warming through naturally occurring variations in the climate or factors such as solar and volcanic eruptions, this did not replicate the observed ocean warming data. As one of Barnett’s colleagues at Scripps, Dr. Jeffrey Severinghaus, said, “There is no doubt that humans are warming the planet. That’s very clear now; the data is beautiful and very strong. Humans are changing the climate, and we’re expected to change it a lot more in the future.”

Sounding Off for Whales

Though the objectives are laudable, deepsea acoustical experiments are not necessarily harmless. For example, observing the aforementioned 1991 experiment in the Indian Ocean, a researcher at Hubbs-Sea- World Research Institute, Ann Bowles, reported “unequivocal evidence of behavioral effects” on sperm, pilot, beaked, and minke whales.” The most dramatic effects, she said, were seen in sperm whales, whose clicks were frequently detected before the experiment, but fell completely silent for 36 hours afterward.

ATOC, too, was not without its detractors. Concern stems from the fact that no one really knows how the cacophony of human-generated subsea noise affects any of the ocean’s creatures. As a 1994 specialists panel convened by the National Research Council concluded, “At this time, essentially nothing is known about the auditory after effects of exposure to intense sound in marine mammals, fish, or invertebrates.”

As a result of public protests against ATOC, the project was delayed and significantly revised. The original location of the broadcast transmitter — within a marine mammal sanctuary — was changed. In addition, the broadcasts were restricted to six times a day for up to four days, which was less frequently than planned. The sound pressure was reduced a hundredfold from that of the early Indian Ocean experiment. The loudspeakers used to generate sound were moved into deeper water (3,200 feet [970 m]), where it was thought few marine mammals range; and a different frequency of 75 hertz was chosen because it was thought to be used by fewer animals. Most importantly, before climate research could proceed, the Marine Mammal Research Program was formed to measure the effects of ATOC’s low-frequency sounds on whales, dolphins, elephant seals, sea lions and sea turtles.

Though as ambitious as it was, the ATOC project was dwarfed by a clandestine program begun by the U.S. Navy involving a technology known as Low-Frequency Active (LFA) Sonar. If successful, LFA sonar will be able to detect submarines by broadcasting low-frequency sounds that are 10,000 times more powerful than the ATOC transmissions. The problem — for whales, that is — involves the power of the sound signals. Each loudspeaker in the LFA system’s array can generate 215 dBs. That’s as intense as the sound produced by a twin-engine jet fighter at takeoff. And some midfrequency sonar systems can put out even more — more than 235 dbs. That’s equivalent to the launch of a Saturn V rocket. Even 100 miles (160 km) from the LFA system, sound levels can approach 160 dbs, which is well beyond the Navy’s own safety limits for humans. Opposition to the project has been led largely by the non-profit Natural Resources Defense Council (NRDC), which revealed that the system had already been field-tested and plans were to deploy it in 80 per cent of the world’s oceans.

Sadly, the concern over LFA sonar is not speculative. In March 2000, whales of four different species stranded on beaches in the Bahamas after a Navy battle group used active sonar in the area. At necropsy the whales were found to have internal bleeding around their brains and ears. The NRDC said, “Although the Navy initially denied responsibility, the government’s investigation established with virtual certainty that the strandings were caused by its use of active sonar.” Furthermore, since the incident the local population of Cuvier’s beaked whales has disappeared because they’ve either abandoned the area or died at sea.

Alarmingly, the Bahamas incident is not isolated. Additional strandings and deaths associated with military activities and LFA sonar have occurred in Madeira (2000), Greece (1996), the U.S. Virgin Islands (1998, 1999), the Canary Islands (1985, 1988, 1989, 2002, 2004), the northwest coast of the United States (2003) and coastal waters off North Carolina (2005).

The creatures that live underwater are easy to see and appreciate, so it takes little imagination to understand how studying them can give us valuable insight into their world. But it’s just as important for us to understand the less obvious physical science of the sea. How light, sound and other forms of energy behave in the ocean can lead to insights never imagined, and provide a global perspective that we can never achieve by looking at individual organisms or even entire ecosystems. Clearly, the “silent world” is a misnomer. Sound is as much a part of the sea as any of the creatures that live within it.


For more information on the effect of LFA sonar on whales and other marine animals, and what’s being done about it, check out the Natural Resource Defense Council’s website: www.nrdc.org/issues/ocean-noise “Discovery of Sound in the Sea” www.dosits.org/

Protect Dolphins from Captivity In the Turks and Caicos Islands

A proposal to build a captive dolphin facility on Grand Turk for the entertainment of tourists is being considered. The keeping of captive dolphins should be considered counter to the Turks & Caicos Islands “Beautiful by Nature” slogan.

Dolphins are complex and highly intelligent animals. Some characteristics that make dolphins more like humans than other animals are:

• They exhibit complex behavioural, cognitive and social traits.

• The brains of dolphins are highly developed and larger than humans.

• They have clearly demonstrated the ability to understand how different things function and how to understand combinations of complex instructions.

• They have highly developed communication skills.

• Dolphins experience emotions much like humans.

What happens to these highly intelligent, social animals when they are kept in captivity? Many of their natural skills and attributes begin to change.

Dolphins during the capture process suffer tremendous trauma and stress. There is a six-fold increase in the mortality rate of dolphins captured from the wild in the first five days after capture. In fact, this increase in stress mortality happens each time a dolphin is transported.

Given the confined space of all captive habitats in which dolphins are held, physical activity is greatly reduced. Sea pens (fenced off portions of open seawater or lagoons) are thought to be better, but even the largest sea pens greatly reduce the space available for the dolphins. They also generally don’t provide protection for the animals from hazards such as hurricanes, potential pollutants from the land, and potential exposure from human waste.

Wild dolphins catch their food, which consists of live fish. Captive dolphins must be taught to eat dead fish which is lower in nutritional value.

Social structures within dolphin communities are quite dynamic, but in captivity dominance determines the hierarchy resulting in a substantial increase in aggressive behavior between the dolphins.

Dolphin communication skills change, or don’t develop as they would in the wild. The vocalisations decrease in diversity and new vocalisations are learned, often imitating noises found in their new environment.

Dolphinaria operators cite the fact that lifespans in captive facilities is comparable to that found in the wild. There is no increase in lifespan for captive dolphins and there is continuing debate about whether lifespans in captivity are actually worse.

The health of dolphins in captivity is also a challenge to monitor. The lack of mobile facial expressions (the “smile” on their face is a fixed, unchanging expression) makes it difficult to identify animals in physical distress. Most often the first sign of a problem is a lack of eating with dolphins often dying within a day or two of this observation.

Captive dolphin programs require dolphins. Captive breeding programs do not generate enough dolphins to fulfill the demand from new and existing dolphinaria. This means more dolphins must be captured from the wild and the methods used to capture dolphins are traumatic and lead to many dolphin deaths.

Swim with dolphin programs provide no educational benefits, they merely exploit the animals while exposing them to additional risks, such as increased stress related to too much exposure to humans, increased exposure to health hazards and health risks from inadvertent or intentional touching of sensitive areas such as the blowhole and eyes. Humans also are at risk during these programs from aggressive behaviour from the animals.

Bottom line, captive dolphins are much like captive humans or slaves.

This is not an attraction that is compatible with the “Beautiful by Nature” Turks and Caicos Islands, and only harms the eco-friendly image of this country.