What are small, expendable floating hydrophone units called?

The correct answer is the term used to describe small, expendable floating hydrophone units, which is "Passive Sonar Buoys." These buoys are specifically designed to detect underwater sounds without emitting any signals themselves, making them valuable tools for listening to marine life or detecting submarines and other vessels. They operate by capturing acoustic signals from their environment and can provide critical information for naval operations and marine research.

Passive sonar technology relies on the principle that sounds in water travel considerable distances, and having a network of these buoys can enhance the capability to monitor a wide area effectively. Other types of sonar buoys, such as active sonar, send out signals and listen for their echoes, which is not how passive sonar works; this distinction is crucial in understanding why passive sonar buoys are designed the way they are.