# The Deep Ocean Highway

> One thousand metres below the surface, the ocean forms a natural acoustic waveguide where low-frequency sound travels tens of thousands of kilometres.

## Key Figures

- **18,200 km**: Underwater distance traversed by an acoustic pulse between Antarctica and Bermuda in January 1991.
- **3 h 24 min**: Exact travel time of the wave propagating at an average speed of 1,485 m/s.
- **0.001 dB/km**: Chemical absorption loss in seawater for a low-frequency tone at 50 Hz.

## Why does sound bend in water?

The speed of sound in seawater is governed by two opposing physical forces: temperature and hydrostatic pressure. Near the surface, solar radiation warms the upper ocean, speeding up sound propagation (around 1,530 to 1,540 m/s in warm latitudes). As you descend through the first several hundred metres, temperature plummets rapidly across the thermocline, slowing the speed of sound down to a low of approximately 1,480 m/s.

Below one thousand metres, temperature stabilizes at near-freezing levels (2–4 °C), but the enormous weight of the water column exerts crushing pressure, adding one atmosphere every ten metres. This compression increases the elasticity of water molecules, accelerating sound back up above 1,550 m/s near the abyssal plain.

Between the warmth above and the pressure below lies an acoustic trough of minimum velocity. Under Snell’s law of refraction, any acoustic wave trying to escape upward or downward is constantly curved back toward this slowest layer.

The resulting phenomenon is a natural oceanic waveguide: the SOFAR channel (*Sound Fixing and Ranging*). Instead of scattering against rough surface waves or dissipating against jagged seafloor rock, sound waves oscillate smoothly in cyclical loops spanning thousands of kilometres.

## The 1991 Heard Island Feasibility Test

In January 1991, an international research team led by oceanographer Walter Munk lowered a heavy acoustic transducer to a depth of 175 m near Heard Island in the sub-Antarctic Indian Ocean, broadcasting a continuous 57 Hz tone.

Three hours and twenty-four minutes later, hydrophone arrays operated by the US Navy in Bermuda—on the other side of the planet, 18,200 km away—recorded the transmission with remarkable clarity. Simultaneously, the signal was detected across the Pacific Rim, in South Africa, Canada, and New Zealand.

## Attenuation Geometry: From Spheres to Cylinders

In open space, sound radiates outwards equally in all directions, dispersing its energy across the surface of an expanding sphere. Intensity drops with the square of distance (losing 6 decibels with every doubling of distance).

Inside the SOFAR channel, refraction prevents vertical dissipation. The wave is confined to spread solely in two dimensions as a shallow expanding cylinder. As a consequence, intensity drops only inversely with linear distance (losing just 3 decibels per doubling). Over a distance of 16,000 km, this geometric wave-trapping preserves the signal over fifteen million times more intensely than free spherical radiation.

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Full interactive edition available at: https://mariuscomper.uk/sofar/en/