声波原理与窃听技术（Principles of sound waves and eavesdropping techniques）.doc

声波原理与窃听技术（Principles of sound waves and eavesdropping techniques） Principles of sound waves and eavesdropping techniques In elastic medium, if the longitudinal wave frequency caused by the wave source is between 20 and 2000 hertz, it can cause the humans hearing. This kind of longitudinal wave is called sound wave, and the corresponding vibration is called sound vibration. Sound wave is a kind of wave, so it has all the characteristics of wave, which can produce reflection, refraction, interference, diffraction and so on. With the development of science and technology, the military has made many kinds of wiretap according to the characteristic of sound wave. An eavesdropper, for example, vividly called big ears in a military outpost adjacent to neighbouring countries. The bug has a particularly large disc, the disc in the front side of paraboloid, when sound waves front came across the disc, according to the principle of reflection wave, will be gathered in the focus on disc reflection, sound coming from the other direction is not focused. Put a microphone in the weak voice can receive focus on the incentive from microphone came the faint sound of positive, energy will be converted into electrical signals, amplified by an electronic circuit, and then by using the headset monitoring personnel eavesdropping. The parabolic microphone is capable of picking up large areas of sound energy and tapping distances of up to several kilometers. According to the same sound wave reflection and refraction principle, it can also be made into an external directional microphone microphone like an image expanding loudspeaker. In order to improve sensitivity and directivity, the binaural effect can also be used. Pick up with two trumpets. Binaural effect. It is the sound coming from the right side that reaches both ears simultaneously and comes from the side. Because there is a slight difference in the course of propagation, one ear is always heard first, the other ear after hearing, a