Can cetacean sounds be heard hundreds of kilometres away?

Yes, and well beyond “hundreds of kilometres”. The lowest sounds produced by certain great whales can cross entire ocean basins, up to several thousand kilometres in favourable conditions. This capacity, long suspected by marine biologists, was confirmed during the Cold War thanks to a network of military hydrophones that became, once declassified, the most powerful tool in modern bioacoustics.

Three broad acoustic categories

To make sense of this acoustic diversity, three broad categories need to be distinguished among cetaceans. The first is that of the baleen whales (mysticetes): blue whale, fin whale, humpback whale, grey whale. They use low frequencies, very deep sounds, sometimes below the threshold of human hearing (20 Hz). These infrasounds have the physical property of travelling extraordinarily far in water.

The second category covers most of the toothed whales (odontocetes): dolphins, sperm whales, killer whales and others. They use high frequencies, mainly for echolocation. These frequencies offer good spatial resolution (useful for locating prey) but travel much shorter distances.

The third category is that of the specialised small odontocetes: porpoises and freshwater dolphins, which use ultra-high frequencies, up to more than 150 kHz. The harbour porpoise, for example, produces its clicks at 130 kHz, wholly inaudible to the human ear unless the recording is slowed down to an extreme degree.

The physics of underwater sound

The propagation of sound in water depends mainly on frequency. At high frequency, absorption by water molecules is strong: the sound loses intensity quickly, carrying only a few kilometres at best. At low frequency, absorption is almost negligible: the sound can in theory travel thousands of kilometres before dying away.

But absorption is only part of the story. The speed of sound also varies with depth, as a function of temperature and pressure. This variation creates layers in which sound is trapped by refraction, like light in an optical fibre. That is what is called an acoustic waveguide.

The SOFAR channel

The most important of these waveguides is the SOFAR channel (SOund Fixing And Ranging). It is an ocean layer lying between 600 and 1,200 metres deep, where the speed of sound reaches its local minimum. Sounds emitted in this layer are bent by refraction back towards its centre, staying trapped horizontally and travelling considerable distances with minimal attenuation.

Discovered by military oceanographers during the Second World War, the SOFAR channel was used to locate enemy submarines and wrecks. During the Cold War, the United States Navy installed the SOSUS network (Sound Surveillance System), a mesh of hydrophones placed in the SOFAR channel across the oceans, designed to track Soviet submarines.

Blue whales, the long-distance champions

The blue whale (Balaenoptera musculus) produces the most intense and the most sustained songs known. Its songs range around 10 to 40 Hz, within the optimal acoustic window of the SOFAR channel. With a source level that can reach 188 decibels, these songs can in theory travel thousands of kilometres.

The SOSUS recordings, opened progressively to civilian research since 1991, have confirmed this range. In favourable conditions (calm ocean, no anthropogenic noise), blue whale songs have been detected more than 2,500 kilometres from where they were produced. In a silent pre-industrial ocean, two whales could in theory have communicated from one continent to another.

The songs of humpback whales

Humpback whales (Megaptera novaeangliae) produce more complex songs, but at higher frequencies (50 Hz to 2 kHz). Their range is therefore more limited, generally of the order of 10 to 30 kilometres. What has made them famous is the structural sophistication of these songs, organised into phrases, motifs and themes.

Only the males sing. Each population has its own repertoire, which changes from year to year, a cultural phenomenon documented in detail in Australia, in Hawaii and in the Atlantic. When a new motif appears in one population, it spreads gradually to other neighbouring populations by way of the polar feeding grounds where whales from different regions meet.

The problem of anthropogenic noise

This extraordinary capacity for long-distance communication is today compromised by anthropogenic noise. The low-frequency rumble of the engines of cargo ships and tankers occupies exactly the same band as the songs of the great whales. In areas of heavy traffic, the effective range of communication has been reduced by a factor of 10 or more.

For a North Atlantic right whale, whose population is below 400 individuals and which has to find a mate hundreds of kilometres away in order to breed, this loss of range is a silent but critical threat. Songs barely reach 1 or 2 km in the Bay of Fundy today, against 10 km in the past. Without a reduction in ocean noise, several threatened populations could never recover.

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