How Do We Research Sound?

When people think about the ocean, they often imagine a quiet world, but beneath the surface, oceans are very noisy places!

Sound travels through water at a speed of about 1,500 meters per second. That is three times faster than sounds move through the air, and equivalent to around 30 lengths of an Olympic swimming pool every second! Sound is created by vibrating objects. Objects in the ocean which vibrate create sound pressure waves that compress and decompress the surrounding water molecule, allowing a sound wave to travel through the water. Sounds waves traveling through the water are described by their frequency (pitch) and amplitude (loudness). High-frequency sounds have a short wavelength and do not travel very far. Low-frequency sounds have a long wavelength and can travel further, especially in water (Figure 1).

Scientists use underwater microphones called hydrophones to record sounds within the ocean. These recordings create a soundscape, which is a “picture” of all the sounds in one place at one point in time. Using hydrophones, we can detect weather events like hurricanes and earthquakes, and human activities such as fishing and shipping. By comparing soundscapes from various places, and from 1 year to another, scientists can track animals that travel long distances and look at changes in habitats and ecosystems over time. By understanding how each type of sound spreads underwater, we can predict how far sounds will travel in our oceans and what effects they might have on the wildlife that lives there [1].

Why Is Sound Important Underwater?

On land, most humans use sight to explore the world around them. Our eyes detect light patterns, and our brains work out what those light patterns mean. But light does not reach many parts of the ocean, so many animals rely on sound to “see” each other, to find their way around, and to hunt for food. Seals, fish, crabs, and even plankton all use sound to communicate with each other, but the underwater animal sounds we know the most about are those made by whales and dolphins.

Large whales, such as humpback, fin and blue whales, use low-frequency sounds that can be heard over long distances. Some are stand-alone calls and others form songs! Each whale species has a unique call, which makes it easier for scientists to detect who is who (Figure 2). Toothed whales, such as dolphins, communicate using high-frequency sounds, which we call clicks and whistles (Figure 1). Whistles are used to chat or pass on social information, although we do not yet know what the animals are chatting about! Perhaps they are warning others of danger or telling them where the best fishing spots are! The clicking sounds dolphins make are used for echolocation. The dolphin produces clicks that bounce off objects in the ocean, and the sound waves are reflected back to the dolphin (Figure 2). That “echo” tells the dolphin where the objects are and provides information on the size and shape of objects, allowing dolphins to “see” with sound.

Using sound to listen to environments is a great way to eavesdrop on other species without affecting their day-to-day lives, which helps us to work out how humans are affecting soundscapes around the world.

The Human Impact on Soundscapes

Humans depend on the oceans for transportation, fuel, and creating renewable energy (such as wind farms). The noise these activities make leaves a large “footprint” on the ocean’s soundscape (Figure 3). Much of the foods we eat and the things we buy have been transported across the oceans in ships. Every day, around 60,000 ships are moving across our oceans and seas, with 500 ships in the English Channel alone. These ships carry 226 million containers packed with things humans consume, and the ships’ engines produce very loud, low-frequency sounds—when a sound produced has a negative effect on the environment it becomes “noise.” Ship noise spreads great distances through the water and interferes with marine animals’ ability to communicate and find food. The increasing level of ship traffic is raising the natural sound level of our oceans!

Much of the oil and gas we use comes from reserves below the seabed, and searching for new reserves is a very noisy business. Survey ships fire loud, low-frequency sounds from acoustic “guns” into the seabed. The echoes produced from each blast enable people to map the seabed and find where oil and gas may be hidden. These surveys can fire up to 40 guns every few seconds, for days or weeks at a time, and at any one time there can be 20–40 surveys going on around the world. Underwater, these blasts sound louder than standing next to a jet engine as it takes off! A seismic airgun blast is estimated to reach up to 260 underwater decibels. If you were standing close to a space shuttle as it launched, you would experience about 160 decibels (dB). Humans damage their hearing if they are exposed to 90 dB continuously, but we do not yet know the decibel level that causes long-term damage to marine life. The noise these guns produce is deafening to animals nearby, and as the sound waves can travel further than 4,000 km they affect the soundscape of large oceanic areas. Ocean exploration happens every day, raising noise pollution to dangerous levels and exposing marine life to levels of noise that would cause long-term damage to humans.

How Does All This Noise Affect Marine Animals?

Research has shown that animals may stop hunting for food or may leave protected areas because of noise pollution. Scientists often observe behavioral changes, such as species diving into deep water to escape noise or changing the amount of time they spend at the surface, alterations in the frequency of diving for food, and even changes in how vocal they are—all as a result of increasing noise in the soundscape.

Busy coral reefs are naturally noisy places, and new research shows that young fish use sounds to find the best reefs to live in [3]. But noise pollution can mask natural sounds, making it difficult for marine animals like fish (who make relatively quiet sounds) to hear each other, which interferes with their navigation. This can cause animals to become disorientated and vulnerable to predators, or even to become stranded on beaches as they try to escape underwater noise.

Studying the polluting effects of noise is difficult. We cannot give a whale a hearing test or ask a dolphin what it thinks about noisy gas and oil survey ships. Instead, we must study changes in noise levels, along with animal behavior, to find out how noise affects marine species. We can then use this information to build an idea of the impact of noise on our oceans.

Armed with this information, we might then work to make ships quieter, perhaps by redesigning their propellers and engines. We might also look at restricting how fast ships can travel and stop them from entering sensitive areas. In Boston, USA, scientists use hydrophones day and night in busy shipping lanes to listen for endangered right whales [4]. Their work is helping to reduce the number whales hit by boats.

Similar technologies are being developed for other species, and scientists are exploring how we can put hydrophones on swimming robots that roam the oceans all year, listening to the ocean soundscape and letting us know when it changes. Artificial intelligence systems are being used to teach these robots to detect calls belonging to certain whale species, enabling robots to survey our oceans much more quickly and accurately and to reach places that have never been surveyed before—including under the ice shelves of Antarctica!²

Humans depend on the ocean for our way of life, but this way of life is causing harm to marine life. Understanding the effect anthropogenic noise has on marine life is increasingly important, and requires a deep understanding of the soundscapes. Reducing the amount of noise humans emit into the natural world is critical in our fight to protect wildlife all around us, and within our oceans.

Funding

This work was supported by the Natural Environmental Research Council [grant number NE/S007210/1].

Glossary

Frequency: ↑ The number of sound waves that are recorded by a hydrophone at a fixed point in time. High-frequency sounds have a high pitch; low-frequency sounds have a low pitch.

Amplitude: ↑ How loud a sound is. Scientists can measure amplitude by describing how high the sound pressure waves are, detected by a hydrophone.

Hydrophones: ↑ Underwater microphones used to listen to the ocean.

Soundscape: ↑ The combination of sounds created by plants and animals, sounds like wind, rain, storms, earthquakes, and ice-breaking, and sounds created by humans. A region’s soundscape can vary over time.

Echolocation: ↑ The ability to locate objects using reflected sounds. An animal sends out a series of sounds and listens for the signal that bounces off the object and returns to the animal.

Decibel (dB): ↑ A unit for sound, with a logarithmic scale. Every increase in 10 dB means the sound is 10 times louder. Human hearing is harmed by continuous sounds of 90 dB or above.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

1. ↑If you would like to listen to what the animals sound like, head to https://www.fisheries.noaa.gov/national/science-data/sounds-ocean where you can listen to the spectrograms.

2. ↑You can help! If you would like to help these swimming robots learn to recognize ocean sounds, visit Zooniverse (www.zooniverse.org). Here, you can learn to identify calls from various marine animals, then find them in recordings collected from all over the world.