Sound wave is an important means of observation and measurement. Interestingly, the word "sound" in English means "sound" as a noun and "detection" as a verb, which shows the close relationship between sound and detection.
When observing and measuring in water, only sound waves are unique. This is because the action distance of other detection means is very short, and the penetration ability of light in water is very limited. Even in the clearest seawater, people can only see objects in the range of ten meters to dozens of meters. Electromagnetic waves decay too fast in water, and the shorter the wavelength, the greater the loss. Even if high-power low-frequency electromagnetic waves are used, they can only propagate for tens of meters. But the attenuation of sound wave propagation in water is much smaller. A few kilograms of bombs exploded in the deep-sea passage, and signals could be received 20 thousand kilometers away. Low-frequency sound waves can also penetrate the strata thousands of meters under the sea and obtain information in the strata. So far, there is no more effective means to measure and observe in water than sound waves.
Sonar is a device that uses sound waves to detect and locate underwater targets, and it is the most widely used and important device in hydroacoustics. It is the translation of the word SONAR, which is the abbreviation of sound navigation and ranging.
Sonar can be divided into active sonar and passive sonar. Active sonar is developed from a simple echo detection instrument. It actively emits ultrasonic waves, and then collects and calculates the echoes. It is suitable for detecting icebergs, reefs, sunken ships, ocean depths, fish schools, mines and hidden submarines with engines turned off. On the other hand, passive sonar is developed from simple hydrophone. It monitors the noise emitted by the target and judges the position and some characteristics of the target. It is especially suitable for submarines that cannot expose themselves but want to detect the activities of enemy ships.
Transducer is an important device in sonar, which converts acoustic energy into mechanical energy, electrical energy, magnetic energy and other forms of energy. It has two purposes: one is to emit sound waves underwater, which is called "transmitting transducer", which is equivalent to a loudspeaker in the air; The second is to receive sound waves underwater, which is called "receiving transducer", which is equivalent to a microphone in the air (commonly known as "microphone" or "microphone"). Transducers are often used to transmit and receive sound waves at the same time in practical use, and the transducers specially used for receiving are also called "hydrophones". The working principle of transducer is that some materials expand and contract under the action of electric field or magnetic field, resulting in piezoelectric effect or magnetostrictive effect.
Like many scientific and technological developments, social needs and scientific and technological progress have promoted the development of sonar technology.