Why the Ocean Is a World of Sound

For most humans, the ocean appears silent — an impression shattered the moment you put on a hydrophone and listen. The ocean is, in fact, one of the noisiest environments on Earth, filled with the calls of whales and dolphins, the snapping of shrimp, the rumble of distant earthquakes, and the low drone of ship engines. To understand why sound plays such a central role in marine life, we first need to understand the physics of how sound behaves underwater.

Speed: Sound Travels Much Faster in Water

In air at sea level, sound travels at roughly 343 metres per second. In seawater, that speed jumps to approximately 1,500 metres per second — about 4.4 times faster. This is because water molecules are much more tightly packed than air molecules, allowing pressure waves to propagate more efficiently.

But the speed of sound in the ocean is not constant. It varies based on three key factors:

  • Temperature — warmer water increases sound speed
  • Salinity — saltier water increases sound speed
  • Pressure (depth) — greater pressure increases sound speed

These three variables interact to create a complex, layered acoustic environment that varies by location, season, and depth.

The SOFAR Channel: Nature's Acoustic Highway

Perhaps the most remarkable feature of ocean acoustics is a phenomenon called the SOFAR channel (Sound Fixing and Ranging channel). At roughly 600–1,200 metres depth in most of the world's oceans, temperature and pressure effects combine to create a zone of minimum sound speed. Sound waves naturally bend toward areas of lower speed, so sounds produced near this depth layer tend to become trapped and guided horizontally for enormous distances.

This is why fin whale calls — low-frequency moans around 20 Hz — have been detected by hydrophones thousands of kilometres away. Many researchers believe whales instinctively use the SOFAR channel to communicate across entire ocean basins.

Frequency and Range

Not all sounds travel equally well in water. Low-frequency sounds lose energy much more slowly over distance than high-frequency sounds — a principle called acoustic absorption. This explains a key pattern in whale communication:

  • Baleen whales (blue, fin, humpback) typically use low frequencies (10–1,000 Hz) suited for long-range communication across hundreds or thousands of kilometres.
  • Toothed whales (dolphins, sperm whales, orcas) use higher frequencies (1,000–200,000 Hz) suited for short-range echolocation in their immediate environment.

Acoustic Shadows and Refraction

Because sound speed varies with depth, sound waves in the ocean don't travel in straight lines — they refract, bending continuously as they pass through layers of different acoustic speed. This creates shadow zones where sound cannot easily penetrate, and convergence zones where sound is naturally focused and amplified. Oceanographers and navies have long studied these effects; cetaceans appear to have evolved to exploit them naturally.

Why This Matters for Whales

An understanding of underwater acoustics reveals just how finely tuned whale communication is to the physics of their environment. The frequency ranges, timing, and repetition patterns of whale calls are not arbitrary — they reflect millions of years of evolutionary pressure to communicate effectively through a medium that bends, focuses, and absorbs sound in complex ways.

It also reveals why human-generated noise — from shipping, sonar, and seismic surveys — poses such a serious threat. When the acoustic landscape is disrupted, the communication channels whales depend on for feeding, navigation, and reproduction are compromised in ways that are difficult to measure but impossible to ignore.