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Special Sound Applications > Sonar - Pg. 245

Chapter | 10 Audio Electronics 245 Second , each earphone contains a built-in microphone that picks up the sur- rounding noise. It amplifies it then inverts it in phase. The original noise from the microphone is then added to the inverted noise. Being the same but exactly out of phase with one another, the two noise signals cancel one another. The cancellation is not perfect, but the noise reduction is significant. It can greatly minimize noise on a plane or in a car or other types of noisy environments. Sonar Sonar means sound navigation and ranging. It is basically the underwater equivalent of radar. It is used on ships and submarines for making depth mea- surements, detecting underwater objects, and for warfare. The two types of sonar are passive and active. Passive sonar is simply the idea of putting underwater microphones called hydrophones on a long wire or arrays of such microphones in the water to lis- ten for any sounds that occur. Amplifiers boost the signal levels and various filters help sift through all the complex signals that can be heard. In military applications, the sounds are digitized and various DSP programs help filter and identify specific sounds. Many ships create unique sounds called signa- tures that can be digitized and stored for comparison to any sounds picked up