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Discovery of Sound in the Sea
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Facts and Myths

The topic of underwater sound is complex and requires a foundation in multiple sciences to fully comprehend. It becomes even more complicated when animals are exposed to anthropogenic sounds in the marine environment. Scientists are just beginning to explore the questions that need to be answered. The scientific process is at work, and it will take time to get these answers, but we are making progress with new discoveries occurring all the time.

This quiz focuses on the science of underwater sound - what is known and what is not known. Your challenge is to determine whether each statement listed below is a fact (something that has been verified to be accurate through the scientific process) or a myth (something that is believed to be false). Once you decide, click on the gold "Answer" box and you will get the accurate response along with links back to places in the DOSITS website where you can get more information on the topic.

Good luck!

1.
The greatest uncertainty in understanding the effects of underwater sound on marine animals is knowing how sound propagates.
This is a MYTH.

The fact is scientists have developed acoustic propagation models to predict where sound will go when it is transmitted in the ocean environment. There are various models that are designed to work better at lower or higher frequency, in deeper or shallower water, or to incorporate complex bathymetry. Scientists are able to measure the necessary inputs to the models, such as water temperature and ocean bottom geologic information, with oceanographic equipment. Predicting the hearing response and geographic location of animals is typically more difficult.

How do we know?
Experiments with underwater sound sources and receivers are accompanied by measurements of the ocean temperature and salinity at various depths (within the water column) with expendable bathythermographs or conductivity-temperature-depth (CTD) sensors, measurement of surface waves and wind speeds with buoys, and the geologic properties of the seafloor through downward-looking sonars. These efforts have allowed for numerous tests of acoustic propagation models. The part of the problem that relates to animal physiology and behavior is heavily species-dependent. Hearing responses are determined from a limited amount of data from captive animals. Animal dive and movement behaviors are known from placing tags on a small sample of the animals. Animal distributions are measured from aerial and shipboard surveys that are only able to detect animals that are at the surface or vocalizing and that also only occur in limited areas of the world’s ocean.


Read more on these DOSITS pages:
Science of Sound, Sound Movement
How do you determine if a sound affects a marine animal?
Animals and Sound, Hearing Sensitivity Studies (Advanced Topic)
Potential effects of sound on marine mammals
Potential effects of sound on marine fishes
How is sound used to research ocean physics?
How is sound used to study marine mammal distribution?
How is sound used to estimate marine mammal abundance?
Technology Gallery, Passive Acoustic Recording Tags

2.
Sound rapidly gets weaker as it moves away from a source.
This is a FACT.

Sound waves lose energy as they move away from a source. If we think about sound as a rock thrown into a pond and look at the pond from the side, you will notice that the wave gets smaller as it moves away from the place where the rock fell (the source of the waves). The wave gets smaller because it spreads out and because some of the sound energy is absorbed by seawater.

How do we know?
Scientists have deployed sound sources and receivers in the ocean and measured how sound waves become less intense as they travel away from a source.


Read more on these DOSITS pages:
Why does sound get weaker as it travels?
Cylindrical vs. Spherical Spreading (Advanced Topic)

3.
Sounds from World War II are still circling the globe, trapped in the sound channel.
This is a Myth.

The fact is that sounds from World War II are not still circling the globe. Any sound generated at that time was absorbed decades ago. Sounds travel farther in the SOFAR Channel than in other parts of the ocean, but even these sounds have been absorbed, just as in air.

How do we know?
Underwater nuclear explosions in the SOFAR Channel, which are far louder than any sounds made during World War II, became undetectable after two hours anywhere within the Pacific basin. Therefore, any sounds made during World War II have been absorbed. Scientists can also predict the length of time that a sound could be detected because of laboratory and field measurements of sound absorption.


Read more on these DOSITS pages:
Science of Sound, How does sound travel long distances? The SOFAR Channel
Sound Travel in the SOFAR Channel
History of the SOFAR Channel
Science of Sound, Why does sound get weaker as it travels?
Science of Sound, How does sound move?
Science of Sound, Cylindrical vs. Spherical Spreading (Advanced Topic)

4.
Background sounds heard by porpoises are dominated by human noise.
This is a MYTH.

The fact is that harbor porpoises hear best at high frequencies (60-120 kHz), where background, or ambient, noise is mostly due to spray and bubbles associated with breaking waves. Ambient noise primarily due to humans is noise generated by distant shipping in the frequency range of 20-500 Hz.

How do we know?
Scientists have made measurements of the frequency ranges over which animals respond by playing sounds to captive animals, including harbor porpoises. These measurements provide behavioral audiograms of the frequency ranges over which these species can hear. Underwater recordings on hydrophones have been analyzed to determine the sources of man-made and natural noise at these frequencies.


Read more on these DOSITS pages:
Science of Sound, What are common underwater sounds?
Science of Sound, Ocean Noise Variability and Noise Budgets (Advanced Topic)
Science of Sound, How is sound measured?
Science of Sound, What sounds can animals hear?

5.
Background sounds heard by the large whales are dominated by human noise.
This is a FACT

The largest whales include all species of baleen whales, such as blue, fin, and humpback whales. There are no direct measurements of the hearing abilities of baleen whales, but these animals appear to be adapted to hear best at low frequencies (20 Hz to 2 kHz). In the frequency range of 20-500 Hz, background or ambient noise is primarily due to humans (noise generated by distant shipping).


How do we know?
Studies have been conducted of the whale auditory system using necropsies and computerized imaging. Other studies have analyzed recordings of the animals’ vocalizations. These combined studies suggest that baleen whales hear best at low frequencies, within the range of their vocalizations. Underwater recordings on hydrophones have been analyzed to determine the sources of man-made and natural noise at these frequencies.


Read more on these DOSITS pages:
Science of Sound, What are common underwater sounds?
Science of Sound, Ocean Noise Variability and Noise Budgets (Advanced Topic)
Science of Sound, How is sound measured?
Science of Sound, What sounds can animals hear?

6.
Scientists understand how important the effects of anthropogenic sounds are on marine mammals.
This is a MYTH

The fact is that increased background noise and specific sound sources might impact marine mammals in several ways. However, much more research is needed to understand the potential impacts. It is also not clear how important these impacts are to the well being of the animals and their populations.

Certain sounds might:

  • cause marine mammals to change their behavior
  • prevent marine mammals from hearing important sounds (masking)
  • cause hearing loss (temporary or permanent) or tissue damage in marine animals.

How do we know?
Although the effects of noise on humans have been studied extensively, only recently has a substantial amount of effort been devoted to studying the effects of sound on marine mammals. A number of factors affect the impact of sound on these animals including: the sound level, its frequency, and other characteristics; the hearing sensitivity, age, sex, and behavior of the animals; and the environmental conditions under which the animals experience the sound. To understand how anthropogenic sounds may affect marine mammals, the animal's reaction to known sounds must first be measured. Observations of normal behavior, "control" or "baseline" data, provide the reference points for measuring any changes occurring during or after sound exposure. Scientists also use data on the way in which the animals respond to similar sounds and sound levels to estimate potential impacts. Collecting these data is difficult and time-consuming work. There is a lot more to do before scientists have a comprehensive understanding of the potential effects of sounds on marine mammals.


Read more on these DOSITS pages:
Animals and Sound, What are the potential effects of sound on marine mammals?
Animals and Sound, How do you determine if a sound affects a marine animal?

7.
Most marine mammal strandings are due to underwater sound.
This is a MYTH

The fact is that observations as far back as Aristotle and illustrations from the Middle Ages show us that marine mammals have been stranding for millennia. There are many causes of strandings, such as disease, ship-strike, injuries, storms, and entanglement. Only very few strandings have been attributed to sound. One controversial and unresolved issue is how the use of military sonar relates to strandings, particularly strandings of some species of beaked whales. In several cases worldwide, there is sufficient information about military sonar operations and the times and locations of strandings to connect the strandings with sonar use. In the last fifty years, in all possible sonar related strandings, fewer than 50 animals are known to have stranded. In comparison, about 1,000 cetaceans and 2,500 pinnipeds strand annually in the U.S. alone.

How do we know?
The term stranding refers to an aquatic animal, particularly a marine mammal, landing on a beach or in shallow water, dead or alive, and probably in distress. Animals may strand singly or in groups. Determining the exact cause of a stranding or death of a stranded animal is often difficult. Scientists must reconstruct what happened, but most often have little or no information about the animal’s history or the circumstances that preceded the stranding. On average, a cause of death can be determined in only about half of all stranding cases.

Much more scientific research is needed to understand why a relationship in time and location may exist between cetacean mass strandings and the use of military sonar. At present, no definitive answer has been provided by the available research.

It is also important to put the potential of impacts to marine mammals from military sonar in perspective. In addition to anthropogenic sound, marine mammals face threats from many different human activities, including fishing, habitat destruction, ship strikes, and whaling. Of these threats, the most significant is fisheries by catch, which causes more marine mammal deaths than any other human activity. Globally, it is estimated that more than 650,000 marine mammals are killed each year by being accidentally caught in fishing gear. This can be compared with the less than two dozen strandings considered to be related to military sonar over the last two decades.


Read more on these DOSITS pages:
What are the potential effects of sound on marine mammals? Strandings
DOSITS Fact Sheet, Strandings
Hot Topic, Mass Strandings

8.
Marine mammals must be able to hear a sound in order for it to affect their behavior.
This is a FACT.

Not all animals are able to hear all sounds equally well. What is a loud sound to one animal may not be as loud or even audible to another.

How do we know?
Direct measurements of hearing have been obtained from a variety of marine mammals, including dolphins, beluga whales, harbor seals, and elephant seals. These measurements provide behavioral audiograms of the frequency ranges that each individual tested can hear. Observations of animals have shown that behavioral responses to sounds depend first on whether the animal can hear the sound, but also on other factors, such as, tolerance to noise, exposure to the same noise in the past, age, sex, and group composition.

Read more on these DOSITS pages:
How do you determine if a sound affects a marine animal?
What are the potential effects of sound on a marine mammal? Behavioral Changes
What sounds can animals hear?

9.
Marine mammals use sound to communicate, navigate, and locate prey.
This is a FACT.

Marine animals use sound to accomplish many tasks. Since light does not penetrate very far underwater, sounds can provide more information over greater distances to help find food or a mate, navigate, and communicate.

How do we know?
Scientists record underwater sounds while observing the behaviors of marine mammals to determine how the animals use sound. For example, experiments found that blindfolded Weddell and ringed seals could accurately locate breathing holes in the ice based on sounds such as tapping, water splashing, and/or scratching in the vicinity of the hole. Without these acoustic cues, the blindfolded seals could not locate the breathing holes. Studies with captive animals have also demonstrated how dolphins can use biosonar to detect prey and different objects.

Read more on these DOSITS pages:
Why is sound important to marine animals?
Marine Mammal Communication
Marine Mammal Navigation
Marine Mammal Feeding/Echolocation

10.
Like marine animals, people use sound underwater to accomplish many of the tasks for which we use light in air.
This is a FACT.

People use sound in the sea for activities such as research, exploration, navigation, fishing, and communication.

How do we know?
Measurements show that sound travels much farther than light underwater. People have therefore developed a number of technologies that use sound underwater to perform a wide variety of tasks. For example, ships use sonar to avoid obstacles that cannot be seen.

Read more on these DOSITS pages:
People and Sound
How do people and animals use sound in the sea?
History of Underwater Acoustics
DOSITS Career Gallery

11.
Decibel levels in air are the same as decibel levels in water.
This is a MYTH.

The decibel is a relative unit of measure of intensity, not an absolute one. Confusion arises because relative intensities in water are referenced to a sound wave with a pressure of 1 microPascal (µPa), whereas sound waves in air are referenced to a sound wave with a pressure of 20 microPascals (µPa). The intensity of a sound wave depends not only on the pressure of the wave, but also on the density and sound speed of the medium through which the sound is traveling. Therefore, relative sound intensities given in dB in water are not the same as relative sound intensities given in dB in air.

Read more on these DOSITS pages:
How does sound in air differ from sound in water?
Introduction to Decibels (Advanced Topic)
What units are used to measure sound?

12.
Sound travels faster in water than in air.
This is a FACT.

Sound moves about 1500 meters per second in seawater. That is approximately 15 football fields end-to-end in one second. Sound moves much more slowly in air, at about 340 meters per second, only 3 football fields a second.

How do we know?
Scientists have measured how long it takes sound to travel known distances in water and in air. The speed is calculated by dividing the distance traveled by the time it took to travel that distance. The exact speed depends on environmental conditions which, in water, include, pressure, temperature, and salinity.

Read more on these DOSITS pages:
How fast does sound travel?
The First Studies of Underwater Acoustics
History of the SOFAR Channel

13.
The impact of sound on marine life is magnified because sound travels faster through water than it does through air.
This is a MYTH.

The speed at which sound travels is not directly related to its potential impact on marine life. Impact is, however, related to energy received for a given sound. Potential behavioral effects also depend upon whether the animal can hear the sound. The sound intensity received by the animal, the frequencies the sound contains, and the sensitivity of the species in the area are all important for understanding if a sound may impact an animal.

How do we know?
Scientists have measured the impact of sound on animals in air and in water. For example, temporary threshold shift (TTS) has been found to be related to the energy of a signal and not the speed at which the signal travels.

Read more on these DOSITS pages:
What is sound?
How fast does sound travel?
How do you determine if a sound affects a marine animal?
Advanced Topic: Temporary Threshold Shift (TTS) Studies

14.
Sound can be used to measure ocean temperature, currents and waves.
This is a FACT.

People routinely use sound in the sea for many applications. Because sound travels or propagates differently with different temperatures, currents, and waves, the precise measurements of sound propagation can be used to collect information about the ocean's characteristics.

How do we know?
Measurements of the speed at which sound travels in the ocean have shown that the speed depends upon temperature, salinity and pressure (which is directly related to depth). For example, sound travels faster in warmer water than in colder water. The temperature of the water can be measured by sending a sound pulse from an underwater sound source to a hydrophone some distance away (up to thousands of kilometers). By combining the time the sound takes to travel from the source to the receiver and the distance between the source and the hydrophone, the speed of sound can be calculated. If the salinity and depth where the sound traveled are known, the temperature of the water can be calculated.

Similarly, sound will change frequency with water movement due to the Doppler effect, allowing for the measurement of currents and waves. Scientists have compared measurements made with sound to measurements made with other instruments to demonstrate the accuracy and precision of the measurements made with sound.

Read more on these DOSITS pages:
How is sound used to research ocean physics?
How is sound used to measure temperature in the ocean?
How is sound used to measure currents in the ocean?
How is sound used to measure waves in the surf zone?
History of Underwater Acoustics

15.
Fish species produce many sounds, including grunts, croaks, clicks, and snaps.
This is a FACT.

Fishes produce a variety of sounds using different mechanisms and for different reasons. The three main ways fishes produce sounds are by striking or rubbing together skeletal components, using sonic muscles that are located on or near their swim bladder, and by quickly changing speed and direction while swimming. The majority of sounds produced by fishes are typically below 1,000 Hz.

How do we know?
Scientists use a variety of passive acoustic systems to listen to underwater sounds produced by marine fishes and attempt to correlate them with fish behaviors. Hydrophone surveys have been used to record fish sounds and locate spawning areas.

Read more on these DOSITS pages:
How do fish produce sounds?
Audio Gallery: Fishes
How do people and animals use sound in the sea?
How is sound used to locate fish?
How is sound used to identify fish?
How is sound used to study the distribution of marine fishes?
Technology Gallery: Acoustic Fish Tags
Technology Gallery: Acoustic Datalogging Systems

16.
Stranding events involving multiple beaked whales have coincided closely in time and space with military activities using sonar.
This is a FACT.

In five well-documented stranding cases, there was sufficient information to know that military multi-ship exercises contributed to the stranding event. These occurred in Greece (1996), the Bahamas (2000), Madeira (2000), and the Canary Islands (2002 and 2004).

How do we know?
Determining the cause of a stranding or death of a stranded animal is normally investigated by necropsies, which have shown that there are many causes of strandings, including storms, disease, entrapment, and in some cases, human activity. Mass strandings of beaked whales are rare, with only 136 mass stranding events reported from 1874 to 2004. Careful investigations of circumstances surrounding five beaked whale stranding events listed above, which included necropsies, concluded that the military sonar exercises precipitated the strandings. The necropsies that were performed found multiple injuries, but none of the animals were found to have acoustic trauma.

Much more scientific research is needed to understand why a relationship in time and location exists between some beaked whale mass strandings and the use of multiple, mid-frequency sonars in critical areas. It is still not clear if it is simply the sound of the sonar, or other aspects of the military exercises, such as multiple ship maneuvers, that resulted in the strandings.

Read more on these DOSITS pages:
What are the potential effects of anthropogenic sound on marine mammals? - Strandings
Stranding Fact Sheet

17.
Anthropogenic sound is the leading cause of marine mammal deaths.
This is a MYTH.

Marine mammals face threats from many different human activities, including fishing, habitat destruction, ship strikes, whaling, and sound production. Of these threats, fisheries by catch kills the most marine mammals. Globally, it is estimated that more than 650,000 marine mammals are killed annually by being accidentally caught in fishing nets (Read et al., 2006). By contrast, on average fewer than nine marine mammals per year have stranded during the last 60 years in association with anthropogenic sound.

How do we know?
Some causes of marine mammal death are well understood. The number of marine mammals incidentally caught in fisheries gear have been well documented at the species, population, and ecosystem levels.

Read more on these DOSITS pages:
What are the potential effects of anthropogenic sound on marine animals?

18.
In almost all cases, hearing loss due to noise does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.
This is a FACT.

Hearing loss from sound exposure depends mostly on the sensitivity of the animal to a sound and the interaction of three characteristics of the sound: the frequency of the sound, the intensity of the sound, and the duration or how long the animal is exposed to that sound. Hearing loss does not usually occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear. However, one other factor, the rise time of the sound, or how long it takes the sound to reach its highest intensity level, is also important. Very sharp rise times, which can occur with very intense impulse noises, can compound an injury and, in some extreme cases, can impact ears even though the peak frequency of the sound is not in the normal hearing range of the animal.

How do we know?
Scientists use various techniques to evaluate the effect of sound on marine mammal hearing. Hearing sensitivity can be measured using auditory testing methods similar to those used on humans and other terrestrial mammals. These measurements provide audiograms of the frequency ranges over which these species can hear. Impact is related to energy received for a given sound and whether the animal can hear the sound. The softest sound that an animal can hear at a specific frequency is called the hearing threshold at that frequency. If an animal is exposed to sound below the threshold of hearing, the animal cannot hear the sound. The animal can hear sounds that are above its threshold without impairment until a certain combination of intensity and duration is reached. Above this limit, the animal's threshold of hearing may be temporarily or permanently worsened.

Read more on these DOSITS pages:
How do you determine if a sound affects a marine animal?
What are the effects of underwater sound on marine mammals?
Advanced Topic: Hearing Sensitivity Studies

19.
The only way to reduce the potential effects of anthropogenic sound on marine life is to eliminate the sound source.
This is a MYTH.

A variety of approaches have been used to reduce the potential effects of anthropogenic sound, although the extent to which these measures are effective has not been determined:

  • Avoiding marine mammal habitats.
  • Detecting animals and modifying the sound-producing activities.
  • Modifying the sound source.
  • Ramping-up the sound signal intensity.
  • Sound screening.

How do we know?
Federal laws such as the Endangered Species Act (ESA), Marine Mammal Protection Act (MMPA), and National Environmental Policy Act (NEPA) that aim to protect animals from harassment (which includes impacts from sound sources) have motivated the development of mitigation techniques and alternative technologies. There has been a great deal of research devoted to increasing our knowledge of hearing sensitivities amongst all marine animals. This new information may also assist in mitigation efforts.

Read more on these DOSITS pages:
How can we moderate or eliminate the effects of human activities?

20.
There is scientific uncertainty about how underwater sound may be affecting marine life.
This is a FACT.

Researchers suggest that increased background noise and specific sound sources might impact marine animals in several ways. The effects vary depending upon the intensity and frequency of the sound, and other variables. It is also not clear how important these impacts are to the well being of the animals and their populations.

How do we know?
There are many factors that influence if and how much a sound source affects marine animals. How loud the source is, what frequencies it transmits, where it will be used, and what species might be in the area are all factors that need to be considered. Scientists have developed sophisticated sound propagation models that predict the sound field around an acoustic source. These models consider the sound speed as it varies in the ocean and determine how the sound will spread. Knowledge of the hearing sensitivities of marine animals, and conditions under which they may be affected by a sound source, are not nearly as well known. Most hearing studies are performed on animals in captivity, so the hearing information that is available tends to be for a small number of individuals of the smaller-sized marine mammals such as pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and many odontocetes (toothed whales). Behavioral responses to sound vary greatly and depend on an individual animal's hearing sensitivity, tolerance to noise, exposure to the same noise in the past, behavior at the time of exposure, age, sex, and group composition.

Read more on these DOSITS pages:
What are the effects of anthropogenic sound on marine animals?
Scientific Uncertainty
Scientific Method

21.
Low-frequency sound levels in the ocean have increased since the introduction of steam-powered shipping.
This is a FACT.

In the frequency range of 20-500 Hz, distant shipping is now the primary source of background noise (ambient noise) in much of the ocean. Noise generated by shipping has increased as the number of steam or diesel-powered ships on the high seas has increased.

How do we know?
Comparisons of low-frequency background noise levels made in the early 1960’s using the hydrophone arrays installed as part of the U.S. Navy Sound Surveillance System (SOSUS) with more recent measurements show that the ship traffic noise in the eastern North Pacific and western North Atlantic oceans showed that noise increased by approximately 0.55 dB per year up through the early 1970’s. More recent and extensive measurements have now shown that the changes in the levels of shipping noise vary in different places in the ocean depending on the number and types of ships.

Read more on these DOSITS pages:
How does shipping affect ocean sound levels
What are common underwater sounds?
Advanced Topic: Ocean Noise Variability and Noise Budgets

22.
Science strives to be objective. Feelings and religious or political views should not influence research or conclusions.
This is a FACT.

Scientific investigations are based on observations, measurements, and reproducible experimentation. Science is a process for asking questions about the natural world and testing the answers. Personal feelings and religious or political views should not be incorporated into the development of hypotheses, theories, or conclusions.

Read more on these DOSITS pages:
Scientific Method
Scientific Uncertainty

23.
One way to arrive at "scientific truth" is to conduct an opinion poll of scientists.
This is a MYTH.

An opinion is a personal judgment or belief, not necessarily based upon fact. On the other hand, “scientific truth” is arrived at through the scientific method, which is an orderly and very well-established process for asking questions about the natural world and testing the answers. Hypotheses that have been consistently validated through observations or experimentation can eventually be advanced to the status of theory. A theory is a thoroughly substantiated explanation of some aspect of the observable world. Theories come as close to objective truth as possible.

Read more on these DOSITS pages:
Scientific Method
Scientific Uncertainty