Mid-frequency active (MFA) sonar emits pulses of sound from an underwater transmitter to help determine the size, distance, and speed of objects. The sound waves bounce off objects and reflect back to underwater acoustic receivers as an echo. MFA sonar has been used since World War II, and the Navy indicates it the only reliable way to track submarines, especially more recently designed submarines that operate more quietly, making them more difficult to detect. | Whales And Sonar by Kristina Alexander, Paperback | Indigo Chapters