Ocean Acoustics Explained Simply with Easy Examples

Ocean acoustics is the study of how sound travels through seawater. Sound moves through water at roughly 1500 metres per second, more than four times faster than in air, and low-frequency sound can cross entire ocean basins, which is why whales sing across hundreds of kilometres and submarines listen rather than look.

It matters because the ocean is opaque: sunlight fades within about 200 metres, so below that depth sound is the only practical way to see. Mapping the seafloor, tracking whales, detecting submarines and warning of tsunamis all depend on listening to the sea.

Key Concepts

• Sound speed in seawater: about 1500 m/s, increasing with temperature, salinity and pressure (depth).
• Refraction: sound bends toward regions of slower speed, so rays curve up or down following temperature layers.
• SOFAR channel: a natural "sound tunnel" around 1000 m deep where sound speed is at a minimum; trapped sound can travel thousands of kilometres.
• Echo sounding: measuring depth by timing a sound pulse's round trip to the seafloor.
• Active sonar: sends pings and listens for echoes, like shouting in a cave.
• Passive sonar: only listens, used by submarines and whale researchers to stay silent.
• Absorption: high frequencies fade quickly; only low rumbles survive long journeys.
• Decibel (dB): the loudness scale; underwater dB uses a different reference from in-air dB, so the numbers cannot be compared directly.

Formulas

• Depth from echo time: d = v × t / 2, where d is depth in metres (m), v is sound speed in m/s and t is the round-trip time in seconds (s). The division by 2 accounts for the down-and-back journey.
• Approximate sound speed: v ≈ 1449 + 4.6T, with T the water temperature in °C, good enough for rough estimates near the surface.

These assume a straight-line path and constant speed along it. Real rays bend with temperature and pressure layers, so survey ships correct the raw timing with measured sound-speed profiles.

Worked Example

A ship's echo sounder receives the seafloor echo 1.6 seconds after the ping. Sound speed is 1500 m/s. How deep is the water?

1. Round-trip distance = v × t = 1500 × 1.6 = 2400 m.
2. Depth = 2400 ÷ 2 = 1200 m.

As a second check of scale, a whale call crossing a 3000 km ocean basin takes 3,000,000 ÷ 1500 = 2000 seconds, about 33 minutes, which is why researchers must be patient listeners.

Common Misconceptions

• "Sound travels slower in water." It travels much faster: 1500 m/s versus 343 m/s in air, because water is far less compressible.
• "The deep ocean is silent." It is a noisy place: ship propellers, breaking waves, earthquakes, cracking ice and chorusing whales all fill the water with sound.
• "Sonar harms nothing." Very loud military sonar can injure or disorient whales and dolphins, which is why navies now follow mitigation rules during exercises.

Real-World Applications

• Seafloor mapping: multibeam echo sounders have charted the ocean floor in detail, revealing mountains and trenches.
• Submarine detection: navies use passive sonar arrays to hear vessels hundreds of kilometres away.
• Whale research: hydrophones track migration and population by identifying individual calls.
• Tsunami warning: underwater microphones detect earthquake rumbles within minutes.
• Underwater communication: acoustic modems link subsea instruments where radio waves cannot reach.
• Fishing: fish-finders are small echo sounders that spot shoals beneath boats.

Where light gives up, sound takes over: ocean acoustics turns the sea's darkness into something we can map, monitor and understand, one echo at a time.