Towed Arrays are instruments that can emit and receive sound for acoustic research. They are towed behind a research vessel and are one to several units long. They provide an advantage over devices attached to a ship in that they are farther away from interfering engine noises and can work in concert with each other to provide an overlapping series of emitted sounds and of reflected sounds. This creates a higher resolution acoustic image.
This diagram shows an array of hydrophones towed behind a research vessel. Sound can be emitted by the hydrophones and received by them after reflecting off hard surfaces, such as a school of fish. (Discovery of Sound in the Sea)