Sonar is a key tool for submarines, surface ships, and antisubmarine warfare–capable aircraft. Shorthand for “sound navigation and ranging,” sonar uses sound waves to detect objects beneath the ocean’s surface. It provides situational awareness and allows ships to engage targets underwater as well as across the surface-subsurface interface.
There are two types:
Active. Functioning like underwater radar, active sonar transducers send out sound energy—pings. Receivers listen for an echo as these waves bounce off objects such as submarines and surface ships. This can provide precise range and bearing information, but it has a downside: It loudly reveals the location of the transmitting unit, making it susceptible to counterdetection. Because the sound waves have to travel from the source to the target and back, active sonar can usually be detected about twice as far from the transmitting unit as its effective range.
Passive. Passive sonar uses hydrophones to listen for sounds in the water and to determine from what direction they come. It does not emit sound, so it can be used covertly, making it ideal for finding sounds emitted by targets—the noise of a submarine’s machinery or a ship’s propellers, for example. Unlike active sonar, it usually cannot provide range information without techniques known as target motion analysis or “TMA.”
Acoustic Propagation
Water temperature, salinity, and pressure affect how sound travels over long distances and therefore how well it can be detected.
Low-frequency sound (below 1 kHz) travels farther because it is less prone to absorption by the water. Sounds in this band can propagate over great distances, which is especially useful for long-range passive detection.
High-frequency sound (above 10 kHz) tends to travel shorter distances because water absorbs and attenuates it quickly. But high-frequency sound can provide more detailed information, which is valuable for active-sonar imaging or detecting objects nearby.
Propagation Paths
The way sound travels through the ocean depends on its interaction with the surface, bottom, and the water itself. There are many possible paths, but the most common three are:
Direct. This is the simplest form: Sound travels in a straight line from the source to the receiver. It occurs over short distances, where the effects of the ocean’s layers are minimal.
Bottom bounce. Here, sound waves reflect—“bounce”—off the ocean floor before reaching the receiver. The composition of the floor (rocky, sandy, or muddy) can affect the quality of the sound reflection, influencing detection capabilities.
Sound channel. This occurs within a unique layer of the ocean where sound can travel vast distances with minimal energy loss. The sound channel forms because of a combination of temperature and pressure that creates a natural duct in which sound refracts and propagates horizontally. This can result in energy being detected hundreds of miles from the source, a phenomenon potentially useful for long-range detections.
