The only biometric technology which does not measure the visual features of the human body. In voice recognition the sound vibrations of a person is measured and compared to an existing sample. The person to be identified is usually required to pronounce a designated password or phrase which facilitates the verification process. Voice recognition can be used on the phone, but has the weakness of technology is its susceptibility to interference by background noise.
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The water-clocks are an early long-distance-communication-system. Every communicating party had exactly the same jar, with a same-size-hole that was closed and the same amount of water in it. In the jar was a stick with different messages written on. When one party wanted to tell something to the other it made a fire-sign. When the other answered, both of them opened the hole at the same time. And with the help of another fire-sign closed it again at the same time, too. In the end the water covered the stick until the point of the wanted message.