Just as a loudspeaker is used to produce sound in air, an acoustic projector or sound source is used to generate sound underwater. Loudspeakers convert electrical signals into sound in air. Similarly, acoustic projectors convert electrical signals into sound in water.
Sounds can be made by vibrating an object. For example, when a drumstick hits a drum, the flexible skin of the drum vibrates up and down. The skin's vibration makes a sound by moving the air above it. When the skin moves up, the air above it is compressed and when the skin moves down the air moves with it and expands. The compressing and expanding of the air produces differences in air pressure. The pressure differences in the air move away from the drum surface like ripples in a pond, creating a sound wave. This is how the drum produces a sound that we can hear.
Underwater sound sources (may also be called a transducer) can be built in a number of different ways. Some underwater sound sources work the same way as loudspeakers in a home stereo system. They have a speaker that is vibrated using an electromagnet. Other sound sources create vibrations with special materials that expand and contract when electricity runs through them. Still other sound sources work much like a drum being played from the inside. A sound source or projector is used to create sound to do such tasks as find fish, navigate, measure water depth, and communicate underwater.
Small individual projectors can also be used together to create directional beams. These are called projector arrays. In a projector array, all of the individual projectors transmit at the same time to generate directional beams. The arrays are designed so that the sound waves generated by each of the array elements add together in the direction that the sound is wanted, but do not add together in other directions.