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  1. https://www.wired.com/story/ultrasonic-signals-wild-west-of-wireless-tech/ I've been thinking about ultrasound a lot lately. Thanks to information on the forum, I've found out that some companders on analog wireless devices may choke out the signal you actually want when dealing with ultrasound due to HF boosts in the system. The noise from an induction stove or an ultrasound motion detector are known examples. Apparently ultrasound is becoming more commonly used in IoT-devices for wake-up signals and inter-device communication in household stuff, so I'm quite curious about the future as well as the present state of things. How many ultrasound related problems actually come up these days? What types of common sources are there, and how could they be dealt with?
  2. Thought I'd post this up for discussion as I just found out about it, and didn't find any reference to it on this site. Something that appears to have a longer research history, but the inventor Woody Norris has developed something he calles "hypersonic sound". These are devices that uses the higher directionality of ultrasonic frequencies to create speakers with similar qualities. A modulated ultrasound signal is transmitted, which uses the nonlinear properties of air to make the air itself act as a demodulator. A highly directive beam of ultrasonic sound therefore causes a sound signal to be created in the air it passes through. The video is the inventor giving a demonstration and talking about the usages on a Ted conference. You never hear the demonstration, but apparantly the audience he directs it towards does. I don't quite understand how this works to allow for higher directionality, since a sound, even if it's made as he says "by your ears", should still continue to spread from that point and not be directional in itself. Maybe the demodulation happens in a way that also makes the sound propagate in the same direction as the modulated signal, or that the beam is precise enough to allow the sound level created to so low that it wont pose a problem (since it's created so close to the ears). If this actually works, and the technology becomes widely used and refined (as of now, it doesn't allow low frequencies, and we don't know about the quality), it could mean a change in how sound is emitted and experienced. True stereo can be created where each ear cannot hear more than one channel (effectively headphones without the cans). Concerts where the sound level is equal everywhere, and movie theaters that doesn't have sweet spots. For movie work, actors can be played music, ques or lines on set that is not picked up by the microphones. Maybe sometime in the future the technology can be used to create extremely directional microphones, that somehow measure the nonlinearity in the air before it that is created by the sound waves, and that data is then modulated into the represented sound signal.
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