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Audio-Cancelling Headphones

Headphones that cancel out an audio stream rather than ambient noise
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Existing Noise-Cancelling Headphones have a microphone to pick up ambient noise in the room, and in addition to playing an audio stream (e.g. from a DVD player), they also predict any repetitive ambient sounds and inject the inverse of these sounds into the audio stream, so that the sounds are effectively cancelled out and you don't hear them.

My idea is to create headphones that take an audio stream as input and simply play the inverse of this sound into the headphones. This would allow all of the people in a room to experience an annoying TV show or song (via external speakers) while one person wears the audio-cancelling headphones and hears nothing but ambient noise (e.g. people speaking).

schwardo, Jul 08 2006

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       reversing most situations inevitably produces something unexpected....   

       what happens if they all fall silent and start advancing towards you, waving the pitchforks you failed to notice earlier? (waits to see quality of answer before rewarding idea) but welcome here, in any case, if you are "new"
xenzag, Jul 08 2006
  

       Well, first of all, this wouldn't work. You can't just play the inverse of an audio stream to cancel it out. You've got to perceive it first. The time delay of sound crossing even a small room is enough to eliminate this as a possibility. Also, I'm not aware of any noise cancelling headsets that "predict" repetitive sounds as a means of action. I'd like to see a link if that actually exists.
zigness, Jul 09 2006
  

       Let alone that the frequency curve of the original signal will have been altered by the reflective characteristics of the room, as well as smeared out over the time domain by different reflection times from internal walls. Sorry, this is impossible.
BunsenHoneydew, Jul 09 2006
  

       I suppose discreetely unplugging them won't work?
theircompetitor, Jul 09 2006
  

       I disagree [Bunsen] - playing the inverted signal directly into the ear (probably with earbuds) at the appropriate level with the appropriate delay will cancel out most of the direct signal. As you point out the frequency response will be altered and the room reflections will still get through and it will probably sound extremely odd indeed, but I don't see why you shouldn't be able to attenuate by 6 or 9 dB. I might try this at work next week.
wagster, Jul 09 2006
  

       What's wrong with this week [wagster]?
methinksnot, Jul 10 2006
  

       Well as of this morning, next week is this week.
wagster, Jul 10 2006
  

       [wagster] Perhaps an adjustable time delay will help you. Once you're more then say a quarter wavelength away from the sound source, I don't see how the out-of-phase pulses of the headphones are going to help you. At half wavelength to one wavelength distance the headphones will actually amplify the signal.   

       I think [BunsenHoneydew] has got it right. These headphones wouldn't work.   

       What would work is a very fast system that detects the incoming sound at the headphones (possibly using the headphone's normal transducers as the pickup element) and then produces pressure in the headphones to counteract the incoming air pressure waves (also known as sound). I think these are baked and widely available, [schwardo] referred to them.   

       What would be cool is a system that takes the original sound signal + the audio signal and learns the aucoustics of the room and then cancels it out. Maybe it could even "visualize" shapes in the room just by analyzing the way the room changes the sound :)
jmvw, Jul 10 2006
  

       <Leaning in to hear wagster's headphones> Shhh.. I can almost hear something... wait... no... there it is again. Damn! It's Rice Krispies... <Lithwh>
zigness, Jul 10 2006
  

       By the way, I've used the noise cancelling headsets made by Bose and also David Clark for aviation. [jmvw] is correct. That's how they work.
zigness, Jul 10 2006
  

       //What would be cool is a system that takes the original sound signal + the audio signal and learns the aucoustics of the room and then cancels it out. Maybe it could even "visualize" shapes in the room just by analyzing the way the room changes the sound//
Sonar?
methinksnot, Jul 10 2006
  

       If you sit 1m away from a pair of speakers (equidistant from each speaker), the sound coming from each speaker will hit your ears pretty much 1/340th of a second after it leaves the speakers. This will happen regardless of what frequency you are listening to - all frequencies travel at the same speed. Of course you will have to be accurate to less than 2cm or 1/17000th of a second (0.058ms) in order to phase cancel at 20kHz - the bottom end will be easier to deal with. The Klark Teknik delays we have are accurate to 21us so they will be quite good enough for this, then I'll just have to keep my head perfectly still. And avoid the client asking me what the hell I'm doing in the middle of their event.
wagster, Jul 10 2006
  

       //The Klark Teknik delays//..... as used to power the Taridis on its recent Dalek beating mission.
xenzag, Jul 10 2006
  
      
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