[davidhyde]

Technology for noise cancelling headphones is fairly straight forward. You put a mic near the entrance of the ear and then reverse the wave. The ear is basically a one dimensional sensor in that configuration. Now, take the headphones away then think about what you have to do to cancel noise. You have this two dimensional window in your home that lets through three dimensional sound waves. The best that you can manage is a two dimensional speaker array which is what you see there attempting to create some sort of sound hologram. It’s insanely complex and anyone who thinks that a company who makes good noise cancelling earphones would therefore be capable getting technology like this to work is just very naive. It’s another magic leap. A whole bunch of people who think they are not buying into snake oil when they really are.

[jacquesm]

Three dimensional speaker arrays are definitely possible, a simple one would be a cube, a bit harder would be a cylinder.

Yes, it is extremely hard. The question is though, can it be done? Phased arrays are insanely hard as well, they do work though.

In the 80's there was a free space noise cancelling demonstration on a cross roads in a major city somewhere, I can't seem to dig up any references to it. IIRC that was a completely analog setup, I have no idea how it worked but they had a volume within which background noise was substantially reduced. I can't stand it when I am 100% sure that I read something but I can't find it.

[ww520]

It's a hard problem. I wonder whether they can combine a specially made window that funnels the traveling of the sound wave to the active noise cancellation mic and speakers to make it easier.

A back of the envelope thought. A quad-pane window with the double-pane at outside to cut down the noise, a middle air gap, and then a double-pane inside. There's an opening at one end of the outside double-pane to allow air to go between the middle air gap and an opening at the other end of the double-pane inside to allow air to flow through. The ANC can be placed along the middle air gap to reduce the noise more.

       air in
    ===      ===================
    |   noise ->   <- ANC      | 
    ===================      ===
                       air out

It would be expensive but it's a first step.

[Anechoic]

First thing: unless that middle air gap is very wide (10+ inches), you've just created a resonant chamber that almost completely negates the sound reducing performance of the two double-pane layers. This is way party walls consisting of two separately framed walls (drywall-stud-drywall-gap-drywall-stud-drywall) tend to be very poor at blocking sounds between units.

There are other issues regarding the impedance mismatch between the outside and middle air gap openings.

As you said, it's a very hard problem.

[ww520]

Can the resonant chamber actually help in the sense that it tunes the sound into certain frequency range? Making the job of the ANC easier? Also can the glass material be tuned to reduce the resonance? Music instruments need to be make certain way to take advantage of the resonant chamber effect; any deviation would produce no sound.

[coldtea]

>Now, take the headphones away then think about what you have to do to cancel noise. You have this two dimensional window in your home that lets through three dimensional sound waves. The best that you can manage is a two dimensional speaker array which is what you see there attempting to create some sort of sound hologram.

First, what you be the problem of creating a three dimension speaker array? Sounds totally feasible...

Second, is that really "the best" that we can do? I've read about a mesh of special design that you can put on your window that still lets you see outside and lets air go through, but cancels certain movement/frequencies, cutting off noise.

[davidhyde]

You’re right, I should have been more explicit. Passive noise cancellation is far superior for a window. I was trying to point out that active noise cancellation is not feasible (not impossible though) because of the extra dimensions and that the engineering behind active noise cancellation headphones is not the same as what it would take to cover a whole window. By several orders of magnitude.

Honestly, people should just drill an extra hole in their walls, add a quiet fan and put a series of passive sound plates and other known geometry to cancel the sound. Or pipe fresh air from the roof.

[IshKebab]

A 3D speaker array is completely pointless. A 2D array can already create all possible sound fields.

[qqii]

In reality noise cancelling headphones don't "just reverse the wave", since that's often not fast or accurate enough (too noisy). Instead it's more like:

mic -> fft -> secret suace analysis and filtering -> unfft -> speaker

A simple filtering would be a high pass filter, to avoid random high frequency sounds from constructively interfering producing more high frequency static.

Just a random tidbit, if you listen to rtings' examples or have a pair yourself I'd suggest playing some different colours of noise to see for yourself!

[leeoniya]

Light Field Cameras (with arrays of micro-lenses) also seemed extremely hard (to me), yet Lytro managed to get the tech working at non-laughable resolutions in a small package, so...shrug.

[amelius]

Yes, but we're talking about a device here that's not sitting in front of your window, but makes it look black.