Underwater sound is a complex topic, requiring a foundation in multiple sciences to fully comprehend. It becomes even more complicated when animals are exposed to anthropogenic sounds. 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 progress is being made, with new discoveries occurring all the time. While many uncertainties remain about how underwater sound affects marine life, enough is known to be able to say that some concepts are true-”facts”- and some concepts are false-”myths”.  Separating fact from myth is not always obvious, especially in the internet era.  Below is an exercise to test your impressions.

This quiz focuses on 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 “See the Answer” box and you will get the accurate response along with links back to DOSITS content where you can learn more on each topic.

Good luck!


1. The greatest uncertainty in understanding the effects of anthropogenic underwater sound on marine animals is understanding how sound propagates.

This is a MYTH

There are many factors that contribute to the uncertainty of how underwater sound impacts marine animals. These include the animals’ hearing sensitivities, responses to sound, and typical geographic location. However, the fact is that scientists have a good deal of certainty about how sound propagates in water. Scientists have developed detailed acoustic propagation models to predict where sound travels when it is generated in the marine environment. There are even different models that specialize at lower or higher frequencies, deep or shallow water, and flat or complex bottom bathymetry. Scientists are able to measure the necessary inputs to the models, such as water temperature and ocean bottom geologic information, with oceanographic equipment.

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. In many simple cases, mathematical solutions are available that can be checked against the models, and different models can be checked against each other.

By contrast, we know relatively little about marine mammal hearing and potential responses to anthropogenic sound, in part because 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.

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2. Baleen whales can hear each other from thousands of miles away.

This is a MYTH

Based on current scientific understanding, this is a myth. There are no data to provide information regarding baleen whale hearing sensitivities or the distance at which baleen whales can detect underwater sound. Furthermore, the distance that whale vocalizations may travel depends on many oceanographic parameters. For example, ambient noise varies at different locations and the amount of background noise will influence whether an animal is able to detect a sound.

How do we know?

Anatomical evidence, computerized imaging, and recordings of baleen whale vocalizations imply that all mysticetes hear well at low frequencies, within the range of their vocalizations. However, there are no direct measurements of hearing thresholds for baleen whales and the exact mechanism that mysticetes use for hearing is still being researched. In addition, high quality descriptions of the structural, functional, and propagation characteristics of baleen whale sound production are also needed. Once scientists better understand sound production and reception in baleen whales, the existing knowledge of sound propagation can be applied to estimate detection distances.

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3.  Sound levels generally get less intense (weaker) as sound travels away from a source.

This is a FACT

Sound waves lose energy as they move away from a source. 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.

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4.   Sounds from World War II are still circling the globe, trapped in the sound channel.

This is a Myth

This sentence is wrong in two ways. The first is that very few sounds have the capability of even traveling and being detected halfway around the planet. The sounds that do are either very loud (nuclear explosions) or quieter signals specifically designed to take advantage of the underwater SOFAR channel. The second problem with this statement is that sound is absorbed pretty quickly in the ocean. Even a loud sound is absorbed and vanishes within a couple of hours of being generated.

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, it is understood that quieter sounds would be absorbed relatively quickly, making it a myth that sounds from WWII circled the globe. Scientists can also predict the length of time that a sound could be detected because of laboratory and field measurements of sound absorption.

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5.  Background sounds that are able to be heard by porpoises are dominated by anthropogenic sound.

This is a Myth

The fact is that harbor porpoises hear best at high frequencies (60-120 kHz), where background sound is mostly due to spray and bubbles associated with breaking waves. Background sound primarily due to humans is 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.

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6.  Background sounds heard by the large whales are generally dominated by anthropogenic sound.

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.

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7.  Scientists understand the biological impacts of anthropogenic sounds 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 significant these impacts are to the well-being of the animals and their populations. More research is needed to develop a comprehensive understanding of the potential effects of sounds on marine mammals.

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?
A number of factors affect the impact of sound on marine mammals 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 specific 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.

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8.  Most marine mammal strandings are due to anthropogenic 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 long before people created underwater sounds. There are many causes of strandings, such as diseases, ship-strikes, injuries, storms, and entanglement. One controversial and unresolved issue, however, 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 the timing and location of both military sonar operations and beaked whale strandings to connect the strandings with sonar use. In the last fifty years, fewer than 50 cetaceans are known to have stranded in vicinity to sonar operations. In comparison, about 1,000 cetaceans and 2,500 pinnipeds strand annually in the U.S. alone, with no relationship to anthropogenic sound.

How do we know?

Determining the exact cause of a stranding or death of a stranded animal is often difficult, and most often, scientists 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. Scientists do their best to determine cause and effect, but, in the case of marine mammal strandings, they often rely on correlated events to inform their conclusions. For example, mass strandings of beaked whales are rare, with only 136 mass stranding events reported from 1874 to 2004. Two of these events were associated with the use, timing, and location of sonar.

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9. Some 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, which included necropsies, 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.

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10. Marine mammals must be able to detect a sound in order for it to affect their behavior.

This is a FACT

Not all types of animals, nor individual animals within a species, 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, 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.

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11. Marine mammals use sound to communicate, navigate, and locate prey.

This is a FACT

Marine animals use sound in their normal behaviors. Since light does not penetrate very far underwater, sound provides e 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 (echolocation) to detect prey and different objects.

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12. 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.

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13. A 100 dB sound in air is the same loudness as a 100 dB sound 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).

Even if we convert decibel levels to reference the same unit, there is a second, more subtle reason, why dB levels in air are difficult to compare directly with dB levels in water. 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. Since water and air have different densities and sound travels through them at different speeds, relative sound intensities given in dB in water are not the same as relative sound intensities given in dB in air.

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14. 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 soccer fields end-to-end in one second. Sound moves much more slowly in air, at about 340 meters per second, only 3 soccer 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.

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15. The impact of anthropogenic 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.

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16. 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.

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17. Many species of fish produce underwater 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 striking the swim bladder with boney structures. 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.

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18. In almost all cases, hearing loss due to noise does not occur if the frequency of the sound to which the marine mammal 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.

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19. The only way to reduce the potential effects of anthropogenic sound on many marine species 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 on a seasonal or permanent basis.
  • Detecting animals and modifying the anthropogenic sound-producing activities so that the animals are no longer exposed.
  • Modifying the sound source.
  • Ramping-up the sound signal intensity.
  • Sound screening (such as bubble curtains).
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. Research is ongoing to increase knowledge of marine animal hearing sensitivities. This information may assist in mitigation efforts by allowing the design of devices that decrease the intensity of sounds within the hearing range of the animals.

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20. There is scientific uncertainty about how underwater sound may be affecting marine animals.

This is a FACT

Researchers suggest that increased background noise and specific sound sources might impact marine animals in several ways. The potential 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 may affect 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 and the received levels to which animals might be exposed. 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.

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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.

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22. 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.

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