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I was driving yesterday and admiring the huge masonry barrier walls between the Freeway and the houses, built solely for the purpose of noise retention.
I think they should have a system of outdoor speakers, perhaps on the lightpoles at the sides. Each speaker of course also has a microphone and
is capable of Active noise cancellation.
The speakers would also double as a public alert system in times of emergency.
Airports could also do something similar (speakers at the ends of runways.)
mostly where the noise is highest. Wouldn't be able to make it perfectly quiet, but it would help.
It's easiest where the noise always comes from one direction. Tunnel entrances, exits, etc.
Halfbakery: Active Noise Cancellation
Advanced_20Noise_20Cancellation
Previous discussion of this, which tries to get it to work only for one (moving) target at a time. [jutta, Apr 26 2010]
Active Noise Cancellation
http://en.wikipedia...ctive_noise_control
[ShawnBob, Apr 26 2010]
[link]
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This is an idea that people come up with periodically, here - a lot of people are bothered by noise in lots of different situations, and the near-magic promise that "noise-cancelling headphones" are sold with is hard to resist. |
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But I don't think this would work in practice. Being on a pole somewhere near close to where the sound appears to be coming from isn't anywhere near close enough. |
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Is it the distance/speed of sound thing?
Perhaps the microphone would need to be closer?
One would think that as distance increases, it would give more time to identify the sound and invert it. |
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As long as the speakers are somewhere between the freeway and the houses, then they should work? I dunno. I know they wouldn't work down on the actual Freeway because you would need to be behind the noise-canceling speakers probably. |
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I just found out that they actually do use a system like this inside tunnels(?)...and aircraft cabins. Is it that they don't work well outside for some reason? |
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Compared to low (long-periodic, maybe 100 ft or so) waves of a low rumble, an aircraft cabin or perhaps cockpit is a much smaller target than the world around an airport. (I still think that's large - there are other reactive technologies that change an engine to counteract vibration build up that are sometimes lumped up with active noise cancellation, so I wonder about the details.) |
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The trick with noise cancellation is to know the distance between your observer, the noise source and the location of the cancelling force. Headphones have a head start because the observer and the cancelling force is in essentially the same place. |
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The problem this idea faces is that you dont know exactly where the noise source is, and you dont know where your observer is. You have general idea, but you need to be accurate to within 1/4 of a wavelength or you just make things worse. |
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From an engineering standpoint it is much easier to make silent cars and planes :) |
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The problem is phase error between the noise and the cancellation signal. While it's possible to crate a "null" at a specific point, as soon as the observer moves away from that point (usually by a half-wavelength) then the effect disappears. |
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[jutta] is correct - better and easier to suppress the noise as close as possible to the source, before it proapgates and the phase starts to "smear" (different frequencies have slightly different propagation velocities, and any discontiuities in air density/temperature increase the effect). |
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I agree that the engineering of noise cancellation would be hard if not impossible in the public realm, but what about at the source? On an internal combustion engine would it be possible to put a noise sensor in the muffler , and similar to an expansion chamber create an inverted phase of each exhaust pressure wave thereby cancelling most of the noise coming out of the muffler? I feel like some sort of high speed bellows |
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// an inverted phase of each exhaust pressure wave // |
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For your homework, calculate just how much energy this would require, and the bulk, complexity and mass of the equipment involved. |
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Ahhhh....pubLic! Now it makes sense. |
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