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RE: Noise cancellation-opposing frequencies...non stereo topic....

64.13.14.96

In Reply to: RE: Noise cancellation-opposing frequencies...non stereo topic.... posted by Carl G on February 27, 2010 at 00:30:05

I think it would be pretty tough. Especially if you don't want to just sit in one place. The problems is spatial, there are many paths for the noises and positions for the listener.

I don't know about acura but I read an article about electronic mufflers years ago and as I recall the key was that the noise was predictable. With just a few variables like throttle position and RPM the appropriate cancellation signal was known. The other thing that made it possible was that they could apply the cancellation at the single point where the noise was being generated, the exhaust pipe outlet.

In your case the noise is unpredictable and can come from any position and the listener can wander. The general solution is a noise wall which can be made large enough in some instances to reduce the position dependencies or noise canceling headphones or earplugs.

I grew up in a quiet world but I sure don't live in one now so I wish there was an easy answer. Since noise walls can be made of glass you might consider something like a few courses of building block with glass panels on top relatively near where you want to sit.

Good luck, Rick

Follow Ups:

RE: Noise cancellation-opposing frequencies...non stereo topic.... - Carl G 08:57:17 02/27/10 (4)
Rick
Thanks for the response, the outside noise "seems" to be of similar frequencies, I am assuming now that it would have to be real time analysis of the sounds and deal with them as they occur ????
I already put up a wall to squelch some of the sound.

So , based on your comments, I would need to direct the "anti-noise" frequencies at the origin of the sound ?
Noise canceling headphones seem to direct their frequencies directly at the ear.... I am assuming I would point speakers directly at me from somewhere out in the yard.... ?

Thoughts/comments ?

Thanks
- RE: Noise cancellation-opposing frequencies...non stereo topic.... - rick_m 14:22:05 02/27/10 (0)
  I think the other posters have covered anything I might add. I'm glad that David brought up trees. I didn't think of them but they really do help. We have some deciduous trees along our fence and there are quite a few between us and the freeways on other lots, I can definitely hear the difference come Fall. However since I usually huddle inside out of the rain in the Winter the sound doesn't matter and I like the extra light.
  
  You might investigate your wall and try to determine where it works the best and worst. Lay down on your back with your head near it in the center and see how that sounds, then try standing up moving, around etc. (Tell your neighbors that you've taken up yoga if they ask.) The idea is to figure out if what you are hearing is mostly coming over, around, or through it. Maybe it can be made more effective.
  
  If you try noise-canceling headphones let me know how they work out.
  
  Regards, Rick
- RE: Noise cancellation-opposing frequencies...non stereo topic.... - Ivan Beaver 11:24:14 02/27/10 (2)
  You cannot use frequency to cancel out other freq. Despite what the add says. But that is what the general public can understand.
  
  What will work is the opposite polarity at the same freq. So basically what happens is you generate the same signal-but out of polarity.
  
  Of course you first have to determine the proper polarity of each of the recieved sounds/freq. They will not all be the same.
  - To add to this� - David Aiken 12:47:02 02/27/10 (1)
    As Ivan said, what you need is a signal that's out of phase with the noise.
    
    The noise signal is complex. It has some variation in frequency from different vehicles plus also the Doppler effect which produces a change in frequency as a moving sound gets closer or further away from you. The vehicles are moving which means that there will be some Doppler effect. In addition you're hearing noise from a length of freeway and noise from different parts of that freeway will be reaching you at different stages of the phase cycle because of that variation in distance to the different parts of the freeway. Plus, as a final complication, the level of noise is constantly varying as the amount of traffic, and also the vehicles involved, vary.
    
    To cancel that noise you need real time analysis of the sound, then synthesis of an out of phase signal but the signal has to reach you over a distance because you're using speakers, just as the noise reaches you over a distance. Your distance from the speakers is varying as you move and that adds another complication to the calculation of the signal you need.
    
    The cancellation signal also has to reach you at the same level as the sound it has to cancel. That means that you won't hear it, or the cancelled noise, but it's still a noise and it is being radiated in other directions including towards neighbours. Since it won't match the noise they hear from the freeway in level and phase, it can actually add to the level of noise they hear in their homes and yards which may mean that they take action against you for creating a noise nuisance.
    
    The reason it works with headphones is that the mic for analysing the sound to be cancelled is in basically the same location as the speakers, ie at the headphones, and the headphones are where your ears are. No matter how the sound varies or where you move, what is being analysed is always the sound at your ear position and what is being generated is always out of phase with that so cancellation can occur. You can never truly get that situation over a wide area using speakers.
    
    Tall walls create a "noise shadow" in which the sound level is much reduced. That's basically what you need. Noise walls along the side of freeways do that but the sound diffracts over the top of the wall so the "shadow" only covers a limited area close to the wall. The trick is to make the wall tall enough so that the "shadow" extends far enough to reduce the noise to the level you want over a large enough area, and there are limits to how high such walls can go, just as there are limits to how high a wall you can build in your yard. Failing all else, a tall closely planted row of trees might help since you can usually let trees grow taller than any wall you're allowed to build but then you have to deal with problems from the spread of the trees as well.
    
    Probably the simplest and cheapest approach is to look for some reasonably comfortable noise reducing ear plugs. These don't totally eliminate noise but reduce it's level by a set amount. You should be able to reduce the noise of the freeway to a more bearable level using them, but they will also reduce the level of other sounds like the voices of anyone else in the yard with you who is talking to you.
    
    Sorry but there are no perfect solutions and your noise cancellation approach will not work effectively over an area like a yard.
    
    David Aiken
    - Thanks - Carl G 13:33:58 02/27/10 (0)
      David,
      Thanks for the detailed info, it appeared to be easier than it actually is to cancel the noise. Looks like I will need to live with the extraneous sounds.
      
      Much appreciated

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