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Science

How submarines see in the dark

Below about 200 m the sea is dark, so ships do what Sonar Ruler does: make a sound and time its echo.

Sunlight doesn't get far into the sea. A faint glow can reach about 1,000 m down, but below about 200 m there's rarely any useful light at all. Down there, looking doesn't get you far.

Sound is a different story. Water carries it well, and fast: about 1,500 metres a second, some 4.4 times faster than in air. So ships and submarines find things with their ears. They make a sound and time its echo, or they just listen.

The first echo off an iceberg

In April 1912 the Titanic sank after hitting an iceberg. Inventors soon started looking for a way to find icebergs before a ship found them the hard way.

One of them, Reginald Fessenden, built an "oscillator": a device that looked like a big loudspeaker and could listen as well as send. In April 1914 he took it to the icebergs off Newfoundland, aboard a US Revenue Cutter called the Miami. Near an iceberg that stood about 40 m (130 ft) out of the water, the oscillator sent a sound through the sea. A little more than a second later, the echo came back. The next morning they pointed it down at the sea floor and heard that echo too. A report from the voyage said it could pick out an iceberg about 4 km (2.5 miles) away.

Quartz, submarines and a name

During the First World War, the danger under the water was submarines. In France, the physicist Paul Langevin found that a thin slice of quartz could both send and hear ultrasound: sound far too high for people to hear. His quartz devices picked up echoes from as far as about 1,300 m away. It was the first sonar to work with ultrasound.

His final design, a slice of quartz sandwiched between steel plates, vibrated about 40,000 times a second (40 kHz). It passed a test at finding submarines in February 1918, and Britain's ASDIC used transducers based on his design. That came too late to matter much in that war, but by the Second World War it mattered a lot. Doctors use ultrasound echoes too, to see inside the body.

In 1942 Frederick V. Hunt, director of the Harvard Underwater Sound Laboratory, came up with the name we still use: sonar. It's usually spelled out as sound navigation and ranging.

Ping or listen

There are two ways to use sound under water.

Active sonar pings. It sends out a pulse of sound and listens for the echo. The echo has made the trip twice, out and back, so:

distance = speed × time ÷ 2

If an echo takes one second to come back, the target is about 750 m away (1,500 m/s × 1 s ÷ 2). In air, the same second would mean only about 170 m.

Passive sonar only listens, sometimes with long lines of underwater microphones (hydrophones) towed behind a ship or a submarine. It hears the sounds the target makes itself. It can't time a round trip, because it doesn't know when a sound set off. But it has one big advantage: it gives nothing away. A ping is loud, and the submarine you're looking for can hear it too. If you want to stay hidden, you listen.

Which echo is the target?

The sea is full of things that echo. The sea floor, a rock, a school of fish and a whale all send a ping back, and a trained sonar operator learns to tell their echoes from a submarine's. It helps to watch how each echo moves. A rock stays put. A submarine is going somewhere.

Try it with Sonar Ruler

Sonar Ruler's Live Wall is active sonar in air. Your phone chirps from the speaker in its bottom edge and listens. Every echo it hears glows as a line under the tape on the screen.

  1. Point the bottom edge at a wall about 2 m away.
  2. A real room gives you a few lines: the wall, the ceiling, the back of a sofa. Now be the sonar operator and step slowly closer. The wall's line moves with you, and most of the others stay put.
  3. Tap the wall's line to track it, and save the reading when it says STEADY.
  4. Then point the phone up at the ceiling and take a depth sounding, like Fessenden's sea floor.

From a wall 2 m away, the echo is back in about 12 thousandths of a second. Under the sea, with sound going 4.4 times faster, it would take less than 3.

Sources

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