If for elephants infrasound is the land, for whales it is the ocean. The calls of blue and fin whales lie at the lower edge of, and below, our hearing (roughly 10–40 Hz) and are among the loudest sounds any living creature makes.
Louder than a jackhammer
As early as 1971 Cummings and Thompson described the powerful low-frequency sounds of the blue whale.1 Later measurements in the Southern Ocean showed that the source level of blue and fin whales reaches about 189 decibels (on the underwater scale) — making them the record-holders of the animal kingdom for loudness.2
There is also a mystery: across the world, blue whale songs have been slowly dropping in pitch for decades — in the Californian population the frequency has fallen by 31% since the 1960s, and the same is true across all studied populations at once. There is no single explanation yet; one hypothesis is recovery in numbers after the era of whaling.4
SOFAR — a natural waveguide
Why is this voice audible so far away? Because of the ocean sound channel (SOFAR). At a depth of about 1 km the speed of sound is at its minimum, and sound that enters this layer goes neither up nor down but "glides" within the channel with very small losses.3 A whale's low-frequency call, once in SOFAR, can travel thousands of kilometres. It was this very channel that the military used during the Cold War for the long-range detection of submarines.
The ocean built itself a telephone line half a planet long.
But this channel has a flip side: human-made ocean noise increasingly drowns out the low-frequency communication of marine animals — scientists call it outright "the soundscape of the Anthropocene ocean" (Duarte et al., 2021).5
- The "loneliest whale in the world" sings at 52 Hz — higher than blue whales (~10–39 Hz) — and was tracked for 12 years on US Navy hydrophones (Watkins et al., 2004).
The mysterious "Bloop" sound, recorded in 1997, was long attributed to a giant unknown animal — until NOAA determined it was icequakes from calving icebergs, not a creature.
- The same SOSUS hydrophone network that listened for submarines later helped biologists discover the "song paths" of whales.
- The frequency of blue whale "songs" has been slowly declining over the decades — there is no single explanation yet.
- Growing shipping noise is shrinking the range over which whales can hear one another.
Whales and SOFAR are the best example of how nature uses low frequencies for long-range communication. The same principle of "weak attenuation" lies at the heart of our land-based infrasound network.
Sources for this article
- peer-reviewed Cummings W.C., Thompson P.O. (1971). Underwater sounds from the blue whale. JASA 50(4B). pubs.aip.org
- peer-reviewed Širović A., Hildebrand J.A., Wiggins S.M. (2007). Blue and fin whale call source levels in the Southern Ocean. JASA 122(2). pubs.aip.org
- organization NOAA Ocean Explorer. The SOFAR Channel. oceanexplorer.noaa.gov
- peer-reviewed McDonald M.A., Hildebrand J.A., Mesnick S. (2009). Worldwide decline in tonal frequencies of blue whale songs. Endangered Species Research 9. int-res.com
- peer-reviewedreview Duarte C.M. et al. (2021). The soundscape of the Anthropocene ocean. Science 371(6529). doi.org
- peer-reviewed Watkins W.A., Daher M.A. et al. (2004). Twelve years of tracking 52-Hz whale calls. Deep-Sea Research I 51. doi.org
- organizationrebuttal NOAA PMEL Acoustics. Icequakes ("Bloop"). pmel.noaa.gov