Most of us are aware that marine mammals make sounds or noise for communication, navigation, searching for food or self-defense. In fact, the oceans are so filled with sounds and noises, it can be difficult or impossible to determine what the various noises are. Man-made devices, such as boat motors and ship engines add to the noise of the marine environment. In truth the silent deep is far from silent. A safe assumption is that any marine life that is much larger that a microscopic organism will make noise.
A swimmer or diver in the ocean will not hear much in the way of sound, except perhaps during a close encounter with a medium or large marine mammal such as a whale or dolphin. The sound is there, but we can's hear much of it because our ears are designed to work in air rather than water.
Most of us have heard of the marine-sonar and passive-listening devices that were developed before World War II. Their use was, and is, essential in both submarine and anti-submarine warfare. It is known that schools of fish and marine mammals would often foul up those sonar operations. We can use that fact to listen to the denizens of the deep.
The hydrophone plans for this project is an electromechanical listening device that works on the same principal as the passive sonar devices in submarines. It has the same basic components: an acoustic transducer, a high gain amplifier and connections for headphones, speaker or tape recorder.
