Help with noise cancellation please

Discussion in 'Home Theater Projects' started by Jeff Braddock, Aug 28, 2003.

  1. Jeff Braddock

    Jeff Braddock Second Unit

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    Hello all. I recently completed my ht(at least for now):) and I was wondering about noise cancellation. I live in a duplex and there is only one bedroom of ours that is connected to our neighbor's house. Since I like to crank my movies, I am afraid that the sound might travel through the wall and disturb them. So my questions:

    1. Could waves be played out of phase through big speakers in the bedroom that connects to the neighbor's, therby reducing or elminating noise from the ht?

    2. I know that the "out of phase waves" need to be the same as the "in phase waves" of the offending noise to effectivly cancel them, but would playing in and out of phase waves at least reduce noise in the adjoining bedroom?

    3. If that (reducing or eliminating the sound from the ht) is possible, how would I go about doing it?


    Thanks for the insightful responses.
     
  2. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Hi Jeff,

    This is an interesting idea. As I interpret your message, your bedroom is between your HT and your neighbor's wall? If so, you may be able to cancel part of the subwoofer output (which is otherwise the hardest to block and the most disturbing to hear) by simply placing a 180 degree out of phase subwoofer at the symmetrical position in your bedroom to the HT subwoofer. That is, the mirror image position on the other side of the wall joining your bedroom and your HT. Differing room sizes and geometries will make this less efficient, but you should achieve some cancellation by simply adjusting the volume of the bedroom subwoofer so that the bass volume in this room is at a minimum.

    It's late and I'm madly scrambling for CEDIA next week, so I hope that this idea makes some sense. [​IMG]

    Regards,
    Terry
     
  3. Jeff Braddock

    Jeff Braddock Second Unit

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    Terry,
    That is a good idea about the sub. I think it would be easily enough done by splitting the output from the receiver or something like that, but I was actually referring to the mid and upper frequencies. I know they carry into the bedroom, and I'm sure the bass does as well but I am not sure to what extent.

    I realize that my post was kinda hard to decipher but what I was asking about was having 2 speakers playing in/out of phase waves while the ht was going. My main question is would this decrease the noise transmited through the bedroom or not.

    The way I understand noise cancellation technology (such as headphones) there is a sensor of some sort that takes an outside wave and plays said wave 180 degrees out of phase therby cancelling or at least reducing the unwanted noise.

    So then my question is will playing a set of in/out of phase waves affect ht noise since the set of waves are not corresponding to any wave other than themselves.

    I hope I cleared up my question, though I think I may have done the opposite.
     
  4. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Hi Jeff,

    You're right about how noise cancellation works. You have an advantage, in that you don't have to use a microphone to measure what needs to be cancelled -- it is exactly the signal you are sending to your speakers. I suggested low frequencies because they are the hardest to block through walls and also the most practical to phase cancel.

    The same principle of "mirror image" speakers could in principle be used for all frequencies. The problem with higher frequencies is that with smaller wavelengths, the position of the walls, room configuration, and room furnishings will have a major effect. For example, a 10 kHz sound has a wavelength of around an inch, and misalignment of a fraction of that wavelength will cause the system not fail.

    So if your bedroom was a complete mirror image (down to every detail) of your home theater room, you could in theory cancel everything using an out-of-phase speaker for every speaker in your home theater! Since this is not very practical, the phase cancellation approach has a shot only for the lowest frequencies. However, higher frequencies can be well handled by standard soundproofing construction, such as adding extra sheetrock layers with an insulating space.

    Oh, and even the subwoofer cancellation approach may not work unless the two rooms are the same dimensions. Otherwise, differing resonant modes in the two rooms will cause problems. [​IMG]

    Regards,
    Terry
     
  5. Jeff Braddock

    Jeff Braddock Second Unit

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    I figured as much. I just wanted to cut down on some of the noise bleeding into the neighbor's place if I could.
     

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