What is the difference between active and passive sonar?

• A. Passive sonar listens for noises; active sonar emits pings and analyzes echoes
• B. Active sonar listens for noises; passive sonar emits pings
• C. Both listen for noises and emit pings
• D. Neither uses pings in detection

Answer: (B)

Active and passive sonar differ on whether sound is transmitted. In active sonar, a short sound pulse is sent into the water (a ping), and the system listens for echoes off objects. By measuring the time delay of the echo, you can determine distance, and by analyzing the Doppler shift and the arrival direction of the echoes, you can infer bearing and even relative speed.

In contrast, passive sonar does not transmit anything. It simply receives sounds produced by ships, submarines, or other sources in the water and tries to identify and locate them from those sounds alone. This mode is stealthier because there is no emitted signal that could reveal your presence, but it generally provides less precise range information.

So the correct understanding is that active sonar emits pings and analyzes echoes, while passive sonar only listens for noises. The idea that active listens for noises or that passive emits pings, or that neither uses pings, contradicts these fundamental definitions.