Are our oceans noise polluted? How does sound work underwater?

When we think about sound, we only ever tend to think of it in terms of how it works in open air. As human beings, we tend to only really interact with sound as it passes through the air, but that’s not the only way that sound can be transmitted. Since it is only defined by vibrations through some medium, sound can technically pass through a wide variety of materials and substances. This may seem like a strange idea, until one considers the ocean. 

Most people are familiar with how sound works in water, at least to some extent. Between the beautiful sounds of whale songs, the powerful invention of sonar technology, and a long list of movie scenes using underwater moments, there’s plenty of examples of how water impacts sound transfer. But how exactly does sound change when it passes through water? Is sound really that important to aquatic animals? And does the ocean, and other bodies of water, have similar problems with noise pollution as the above-sea world does?

How is sound different underwater?

Regardless of what it passes through, the mechanics of sound always work in the same fundamental way. It’s caused by the rhythmic vibration of a medium, which produces a wave that can then be picked up by different sensors, including our ears. These waves have two important characteristics, amplitude and wavelength. The wavelength is the distance from the crest, or peak, of one wave to the next crest. It’s associated with the frequency of the wave, and determines the pitch of the sound. The amplitude, on the other hand, is the distance from the still level of the wave to the crest, and determines how loud of a sound the wave will produce. The higher the amplitude, the louder the sound. 

When a sound wave passes through the air, it jostles particles in the air against each other, causing them to jostle into further particles, and maintaining the wave. The same thing happens underwater, where individual particles of water jostle into each other, maintaining the wave. Water is much denser than air, however, so the individual particles within water tend to be closer together, which impacts the sound wave in a few couple key ways. 

• Firstly, since the particles are closer together, the wave can propagate faster than it would in air, around five times faster according to most estimates. 
• Secondly, since there is less time for each individual particle to be impacted by other forces, sound takes much longer to dissipate underwater, meaning it can sound much more clear for a much longer time. 
• Lastly, this difference in speed also means that noises will tend to have a larger amplitude, and an associated louder sound underwater than the same noise would above water. The wavelength, however, is not generally affected, so a sound will seem to be the same pitch underwater as it does above water. 

This last statement might seem strange, since above water sounds tend to sound weird to use when they’re heard underwater. Although there are slight mechanical changes to the sound when it moves from air to water, these are mostly in respect to things like the angle the wave moves in, not in properties we normally detect. Instead, this perceived difference is caused by the way that we hear sound. 

Normally, vibrations in the air have a hard time propagating into our body. The eardrum is specially made to carry air vibrations, but unless a noise is particularly loud, or particularly low in pitch, the rest of the human body doesn’t really propagate the sound very much.

Water, however, is much more capable of making the human body vibrate, as most of the human body is also made of water. So whereas a sound in air only really makes your eardrum vibrate, sound underwater can vibrate your body, which your ears pick up as a very different sound. 

How is sound used underwater?

The fact that sound propagates differently through water might seem like a minor detail to us landlubbers, but in truth this slight difference is crucially important to most animals living in the ocean. On land, there is a wide variety of objects of different colors, sizes, and shapes to make sight-based movement easy and convenient. Underwater, however, there not only isn’t as much light, especially in lower layers of the ocean, but there’s less variance in object appearances to make vision-based movement particularly reliable.

Sound, on the other hand, propagates a long distance, retains its clarity for a long time, and is able to maintain a direction much easier than light, which makes hearing a much more reliable sense than sight underwater. Most creatures underwater have very accurate organs to pinpoint where a sound comes from, in addition to organs that can bounce sound off of nearby objects in what is known as sonar. Even in the darkest reaches of the deep ocean, creatures can rely on their hearing to keep track of what’s around them. 

Is noise pollution a problem underwater?

The eagle-eyed reader may notice an uncomfortable truth among these previous paragraphs; the ocean is an environment where most creatures are heavily dependent on sound, and where sound can propagate for a much longer distance than it could on land. This means that not only are marine animals impacted by noises created in or around the ocean, but also by others that might seem fairly distant, like cars on a bridge, or crowds on a pier, which can propagate very deep into the ocean, and disrupt marine life. 

How different animals are impacted varies with how dependent on sound they are, and how mobile they can be. Some larger animals, such as whales or dolphins, that are dependent on sound but can be highly mobile tend to develop new migration patterns that avoid high noise areas.

Less mobile animals though tend to have diminishing populations near high noise areas like ports and large cities. This type of large scale noise pollution not only impacts their ability to accurately map out their surroundings, it can also debilitate their ability to communicate with each other, causing further decreases in population.

Even the above mentioned whales have been found to have difficulties with long-distance communication due to the impacts of noise pollution. Although not every marine animal relies heavily on sound, and not every sound-reliant animal uses the low frequencies commonly associated with marine noise pollution, all marine animals are impacted by the shifting population sizes, migration patterns, and communication errors of the animals that are directly impacted by noise pollution under water. 

Conclusion

As human beings that aren’t usually too dependent on sound, it can be hard to picture how important sound can be for some animals. Although the fundamental mechanics of sound are more or less the same underwater, the meaning those sounds have takes on a completely different shape beneath the surface, in ways that can seem surreal to us. And while things like underwater sound can seem like interesting novelties to us, to marine life it defines the way they exist, so taking the responsibility to control how much noise we put out into the ocean is a crucial step for future conservation efforts.