A towed array sonar is a long line of underwater microphones called hydrophones that is pulled behind a ship or submarine. It acts like a long underwater ear, listening for faint sounds from engines, propellers, marine animals, and the seafloor environment. Because sound travels much farther than light in seawater, sonar is one of the most important tools for sensing the ocean.
Towing the array behind the vessel also keeps it away from much of the vessel's own noise.
Each hydrophone measures tiny pressure changes caused by sound waves moving through water. By comparing when the same sound reaches different hydrophones, computers estimate the direction of the sound source using time delay and beamforming. A longer array can separate sounds coming from different directions more clearly, especially at low frequencies.
Operators use these signals to track distant objects, map acoustic conditions, and study the underwater soundscape.
Key Facts
- Sound speed in seawater is about v = 1500 m/s, but it changes with temperature, salinity, and pressure.
- Distance from travel time can be estimated with d = vt for one-way sound travel.
- For echo ranging, distance is d = vt/2 because the sound travels to the target and back.
- Time delay between two hydrophones is approximately Δt = d sin(θ) / v, where d is sensor spacing and θ is the arrival angle.
- A longer towed array improves direction finding because it gives more spacing over which to compare sound arrival times.
- Towed arrays are especially useful for low-frequency sounds, which can travel long distances through the ocean.
Vocabulary
- Towed array sonar
- A sonar system made of many hydrophones in a long cable pulled behind a ship or submarine.
- Hydrophone
- An underwater microphone that converts sound pressure waves in water into electrical signals.
- Beamforming
- A signal-processing method that combines hydrophone signals to emphasize sounds from one direction and reduce sounds from others.
- Time delay
- The difference in arrival time of the same sound at two or more hydrophones.
- Self-noise
- Noise made by the towing vessel, cable motion, or equipment that can interfere with detecting outside sounds.
Common Mistakes to Avoid
- Treating sonar like underwater radar is wrong because sonar uses sound waves, not radio waves, and sound behavior in seawater depends strongly on ocean conditions.
- Forgetting the factor of 2 in echo problems is wrong because an active sonar pulse travels to the target and then back to the receiver.
- Assuming the closest hydrophone always hears the sound first is wrong because arrival order depends on the direction the wavefront approaches the whole array.
- Ignoring the towing vessel's noise is wrong because engine, propeller, and flow noise can mask faint signals unless the array is placed far behind the vessel.
Practice Questions
- 1 A whale call is detected by a hydrophone array after traveling about 45 km through seawater. If the sound speed is 1500 m/s, how long did the sound take to reach the array?
- 2 Two hydrophones are 30 m apart along a towed array. A sound reaches one hydrophone 0.012 s before the other. Using v = 1500 m/s, what path difference does this time delay represent?
- 3 Explain why a long towed array can detect and locate faint distant sounds better than a single hydrophone mounted on the hull of a ship.