Which pair correctly describes the two main types of sonar?

• A. Narrowband and Broadband
• B. Digital and Analog
• C. Active and Passive
• D. Long-range and Short-range

Answer: (C)

The main idea here is how sonar interacts with sound in the environment: does it emit sound or not? Active sonar sends out a deliberate acoustic pulse and then listens for echoes. By analyzing those echoes, it can determine how far away a target is and where it is in bearing, and it can even generate a picture of the surrounding underwater scene. Because it produces sound, it can be detected by others, and its operational use often involves choosing ping patterns and timing to balance detection performance with stealth.

Passive sonar, on the other hand, does not transmit any sound. It simply listens to noises already present in the environment—engine sounds, hull noises, propellers, natural marine sounds—and infers the presence and movement of targets from those sounds. This approach is covert and useful for sustained surveillance, but it doesn’t provide direct range information without additional analysis.

The other options describe different concepts: narrowband versus broadband relates to frequency content, digital versus analog to signal processing methods, and long-range versus short-range to performance capabilities, none of which define the two fundamental sonar types.