Hydrophone A hydrophone ( Greek " hydro " = " water " and " phone " = " sound " ) is a microphone designed to be used underwater for recording or listening to underwater sound . Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change . Such piezoelectric materials , or transducer s can convert a sound signal into an electrical signal since sound is a pressure wave . Some transducers can also serve as a projector , but not all have this capability , and may be destroyed if used in such a manner . A hydrophone can " listen " to sound in air , but will be less sensitive due to its design as having a good acoustic impedance match to water , which is a denser substance than air . Likewise , a microphone can be buried in the ground , or immersed in water if it is put in a waterproof container , but will give similarly poor performance due to the similarly bad acoustic impedance match . History . The earliest widely used design was the Fessenden oscillator , an electrodynamically driven clamped - edge circular plate transducer ( not actually an oscillator ) operating at 500 , 1000 , and later 3000 Hz . It was originally marketed as an underwater telegraph , rather than as sonar , but was later very successful , its Canadian inventor awarded the " Scientific American Magazine Gold Medal of Safety " in 1929 from the American Museum of Safety , an organization for ship captains ; some were still in use during World War II . Ernest Rutherford , in England , led pioneer research in hydrophones using piezoelectric devices , and his only patent was for a hydrophone device . The acoustic impedance of piezoelectric materials facilitated their use as underwater transducers . The piezoelectric hydrophone was used late in World War I , by convoy escorts detecting U - boat s , greatly impacting the effectiveness of submarines . From late in World War I until the introduction of active sonar , hydrophones were the sole method for submarines to detect targets while submerged , and remain useful today . Directional hydrophones . A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception . Directional hydrophones increase sensitivity from one direction using two basic techniques : Focused transducers . This device uses a single transducer element with a dish or conical - shaped sound reflector to focus the signals , in a similar manner to a reflecting telescope . This type of hydrophone can be produced from a low - cost omnidirectional type , but must be used while stationary , as the reflector impedes its movement through water . A new way to direct is to use a spherical body around the hydrophone . The advantage of directivity spheres is that the hydrophone can be moved within the water , ridding it of the interferences produced by a conical - shaped element Arrays . Multiple hydrophones can be arranged in an array so that it will add the signals from the desired direction while subtracting signals from other directions . The array may be steered using a beamformer . Most commonly , hydrophones are arranged in a " line array " but may be in two or three dimensional arrangements . SOSUS hydrophones , laid on the seabed and connected by underwater cables , were used , beginning in the 1950s , by the U . S . Navy to track movement of Soviet submarines during the Cold War along a line from Greenland , Iceland and the United Kingdom known as the GIUK gap .