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Screen talk

作者:何肟纫    发布时间:2019-03-07 07:06:10    

By Barry Fox WHEN graduate researcher Young Jin Kim sent a pulsed voltage to an LCD screen he was testing, he was sure he heard a clicking noise. His supervisor, Jay Patel of the physics department at Penn State University, couldn’t hear a thing. But when a group of students gathered round they all heard the sound. “I realised I was getting old and my hearing is not so good as it was,” says Patel. “But the panel was making sound as well as showing an image.” Over the past year, Patel’s department has used grants from the US Defense Advanced Research Projects Agency and the American Chemical Society to investigate this curious phenomenon. “We now think we know why it is happening,” says Patel. “We may eventually be able to use an LCD screen as a digital loudspeaker or a phased array that beams sound in a chosen direction.” An LCD screen is made from two closely spaced transparent plates, usually made of glass, with liquid crystal material in between. Each plate is covered with fine strip electrodes laid parallel to each other. The electrodes on opposite plates are oriented at right angles to each other. The zone where two of these electrodes intersect acts as a pixel, or picture point. When a voltage is applied to a pair of electrodes defining one of these pixels, molecules of the liquid crystal reorient themselves, causing a visible change. Targeting a pattern of pixels builds up an image. Kim was using an experimental LCD which worked with a 40-volt supply. When he analysed the sounds he found that even a 5-volt screen produces audible emissions when fed with pulses at certain frequencies. A train of 10-millisecond pulses at 7460 hertz makes the screen emit sound at a frequency of 3730 hertz, and four multiple frequencies. Patel and his colleagues believe that the switching current makes the liquid swirl round the cell as its molecules reorient themselves. When the frequency of the switching signal matches the resonant frequency of the cell, the faint sounds this motion generates are amplified. So Patel expects to be able to generate a range of sounds around 2 or 3 kilohertz by using different cell sizes, and switching the frequencies. The sound level and pitch are also affected by the thickness of the transparent plates. In theory it should be possible to make an LCD screen with a wide range of cell sizes, which are individually switched to create sound which matches the image displayed. Patel has worked with LCDs for 25 years, but says he has never come across research indicating they emit sound. He searched the literature and found nothing. He tried other fluids, but only liquid crystal worked. He has now published his team’s findings in Applied Physics Letters (vol 75, p 1985). The Defence Evaluation and Research Agency in Britain has special reason to be interested in the work. DERA’s basic patent on a flat-panel loudspeaker—arising from sound-cancelling technology for aircraft cockpits—has been successfully exploited by the British company NXT. Guy Bryan-Brown of DERA, who specialises in display technologies, says he is unaware of any prior work on the resonant effect discovered at Penn State. “It seems to be a completely new approach which is well worth investigating,

 

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