What distinguishes passive acoustic systems from active ones?

The key distinction between passive acoustic systems and active ones lies in how they interact with sound. Passive acoustic systems are designed to listen for sounds generated by external sources without emitting any sound themselves. They function by detecting and analyzing ambient noise, such as marine animal vocalizations or other environmental sounds, which enables them to collect data without interfering with the natural habitat.

In contrast, active acoustic systems emit sound waves, usually in the form of pulses or continuous signals, and then listen for the echoes that bounce back from various objects, such as fish or underwater structures. This process involves both sound generation and analysis of the resulting echoes to determine the presence and distance of objects in the water.

The other choices highlight aspects of active and passive systems but do not accurately describe their defining characteristics. While it's true that passive systems may require electricity for operation and that active systems measure ambient noise in a certain context, these points do not capture the fundamental operational differences between the two types of systems.