1. Why Is It Hard to Hear Someone Underwater When You Call from Above?
You may have experienced trying to call out to a friend at the bottom of a pool from above the water. It is difficult for your voice to reach them because sound waves in the air are almost entirely reflected by the water surface [1]. The acoustic impedance of water, which represents how difficult it is for sound to pass through a material, is about 3600 times that of air [2].
Because of this huge difference, about 99.9% of the energy of underwater sound is reflected back into the water when it hits the surface [2]. Only about 0.1% of the energy passes through. It is important to note that this figure refers to the ratio of energy, not the loudness measured in decibels. This shows that the water surface acts almost like a wall for sound. Voices from outside have trouble entering the water, and sounds from inside have trouble getting out. Once sound is trapped inside this "wall," how does it travel?
2. Why Sound Travels Faster in Water Than in Air
The speed of sound is approximately 343 m/s in air and about 1480 m/s in water [1]. According to NOAA (the National Oceanic and Atmospheric Administration in the United States), the speed is roughly 1500 m/s in water and 340 m/s in air [3]. Dividing these numbers shows that sound travels more than four times faster in water. Why is this, given that water is heavier than air? While higher density tends to slow sound down, water is much "stiffer" than air, meaning it is harder to compress. This stiffness outweighs the delay caused by density, resulting in faster sound travel in water [1]. The common explanation that "sound is faster because water particles are packed closely" is not entirely accurate on its own.
These figures are approximations. The speed of sound in water changes with temperature, salinity, and water pressure, so 1480 m/s is just a typical value [1]. To visualize the difference, consider a distance of 1000 km. At 1500 m/s, sound would take about 670 seconds (about 11 minutes) in water. At 343 m/s, it would take about 2900 seconds (about 49 minutes) in air. In reality, sound in air does not travel this far, but this calculation helps illustrate the speed difference. Beyond speed, does the ocean have a mechanism to carry sound over long distances?
3. How Sound Can Travel Thousands of Kilometers in the Ocean
NOAA explains that under the right conditions, sound signals in the ocean can travel thousands of miles (several thousand kilometers) with little loss of strength [4]. The distance sound travels depends mainly on water temperature and pressure [4]. As you go deeper, water temperature drops until a certain depth, after which it remains nearly constant. However, water pressure continues to increase with depth [4]. This combination creates a layer where sound travels most slowly. Sound waves bend toward this layer and become concentrated there, forming a "channel" for sound in the ocean.
This channel is called the SOFAR channel. In low and mid-latitude oceans, it is located about 600 to 1200 m below the sea surface [3]. The depth of this channel varies by location; it is shallower in oceans closer to the poles. As sound bends toward the slower layer, it becomes difficult for the sound to escape the channel. Unlike the water surface, which acts as a wall, the ocean interior traps sound at a specific depth, allowing it to travel far. How do living creatures use this "world of sound"?
4. How Whales and Fish Use Sound Underwater
Marine animals rely on sound for communicating with one another, determining direction, finding food, defending territory, and escaping predators [5]. Low-frequency sounds can travel across entire ocean basins [5]. The calls of large whales, such as blue whales and fin whales, are low-pitched and have been reported to reach microphones on the seafloor hundreds of kilometers away [6]. However, in actual research, sounds used for calculating positions were typically those within 50 km. Fish detect vibrations in the water using otoliths, which are calcium carbonate masses in their inner ears [7]. Fish essentially receive underwater sound through these stone-like structures in their inner ears. Since underwater sound is a vital source of information for these creatures, how do humans perceive it?
5. Can Humans Hear Underwater and Tell Where a Sound Comes From?
In a 2022 study by Sørensen, Christensen-Dalsgaard, and Wahlberg, seven participants aged 23 to 49 tested how faint a sound they could hear underwater [8]. Previously, it was thought that humans heard underwater mainly through bone conduction. However, this study reported that hearing was 26 dB better than expected. For a 500 Hz sound, the average threshold for hearing began at 71 dB. Researchers suggest that air trapped in the middle ear may resonate with sound, amplifying it and making the ear more sensitive [8]. This is still a hypothesis, and the exact mechanism of human underwater hearing is not fully settled.
On the other hand, determining the direction of sound is difficult. While humans can locate sound within a few degrees in air, underwater, the perceived direction can be off by up to 90 degrees [8]. The researchers state that one should not assume they can determine their orientation solely by hearing when jumping into the sea. Even though the human ear can detect underwater sound, which is trapped in the ocean but blocked at the surface, it is hard to pinpoint where it comes from.
6. A Bathtub Experiment: Comparing Sounds Heard Underwater and in Air
You can try this experiment in a bathtub, always with an adult present. Do not try this in a pool or the sea. With an adult beside you, sit securely and gently lower just your ears into the water while keeping your nose and mouth above the surface. Have another person tap two spoons together underwater so you can hear the sound. Then, have them tap the same spoons outside the water to compare how the sound differs. You can also test which direction the underwater sound seems to come from. Close your eyes and point to where you think the sound is. Whether you are correct may vary by person and bathtub. If water gets into your ear, tell an adult immediately [8].
Try another test with an adult. With your ears underwater and your mouth above the surface, make a small “ah” sound. How does it differ from hearing your own voice in the air?