A hydrophone array is made up of a number of hydrophones placed in known locations. These hydrophones maybe placed in a line on the seafloor, moored in a vertical line in the water column, or towed in a horizontal line behind a boat or ship, for example. Sound arriving at the array from a distant source, such as a submarine, will reach each hydrophone at slightly different times, depending on the direction from which the sound is coming. This time difference is known as the time-of-arrival-difference and can be turned into a direction. Using this information from all the hydrophones in the array, the direction from which the sound is coming can be pinpointed.
Even a simple array consisting of only two hydrophones can give the approximate direction from which a sound is coming. People do this all the time in air with a “receiving array” that consists of two ears. Sound arriving from a source, such as a person speaking, will reach each ear at slightly different times, depending on the direction from which the sound is coming, making it possible for the listener to tell the direction to the speaker.
