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High-Tech Window Cling is Like a Volume Knob for Real Life

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Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#11

Post by chas »

oakwhiz wrote: post #4

Selective noise cancellation could be possible using a Fourier transform. Zero out the coefficients that you still want to hear and then perform an inverse transform, then a time-domain inversion, and finally apply the appropriate amount of delay to the signal.

You might know this, but I want to state it at least for the benefit of people reading this without much signal processing experience. Just zeroing like that really isn't the best idea. Square waves (that are 1 for signals that you want and 0 for signals you don't want) inverse Fourier transform to sinc functions[1], so if you are filtering just by chopping things out, it will greatly distort the time-domain system because the inverse Fourier transform of the filter and the signal get combined (convolved) in the time domain.

[1] http://en.wikipedia.org/wiki/Sinc_function

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#13

Post by chas »

fosk wrote: post #9
oakwhiz wrote: post #4

Selective noise cancellation could be possible using a Fourier transform. Zero out the coefficients that you still want to hear and then perform an inverse transform, then a time-domain inversion, and finally apply the appropriate amount of delay to the signal.

Your answer is very fascinating, can you please explain it like I'm five?

The basic idea is that it is possible to transform a signal without loss of information from being some value vs time (so called time-domain signals) to being a bunch of different frequencies with different intensities (so called frequency domain) where the frequencies are just sine and cosine waves. A pure sine wave in the time domain gets transformed into just one value with the same amplitude as that wave in the time domain.

To see why this is useful, imagine a song you wanted to listen to that was recorded with a low-pitch hum in the background because the microphone running next to a power cable and picked up the 60 Hz signal. Removing this unwanted tone by looking at the value of the song in time is really hard because the 60 Hz signal is mixed up with the rest of the song. However, when transformed into the frequency domain, the 60 Hz hum is very easy to spot and remove. When you transform the signal back into the time-domain (so you can listen to it), the 60 Hz will still be gone.

This process can be done automatically with computers using a technique called the Fast Fourier Transform. This is the basis of many techniques in the field of Digital Signal Processing (DSP) which is the theory behind most of the communication breakthroughs of the last 40 years. I should note that many naive approaches to this problem (like the approach in my example) don't work particularly well, so as in any problem domain, there are important but subtle performance trade-offs to consider.

[1] http://www.dspdimension.com/admin/dft-a-pied/

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#14

Post by radley »

chad_oliver wrote: post #10

I'm not convinced about the ability to block out certain types of sounds (e.g. birds vs traffic), but blocking out certain frequencies should be quite possible. Of course, there's one question the article didn't address: what proportion of sound is actually transmitted through window panes? One would expect that a whole lot of low-frequency sounds (traffic) would come through the walls too.

I got the impression it would play bird sounds, i.e. a "zen" noise generator, not filter actual bird sounds through.

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#15

Post by yetanotherphd »

oakwhiz wrote: post #4

Selective noise cancellation could be possible using a Fourier transform. Zero out the coefficients that you still want to hear and then perform an inverse transform, then a time-domain inversion, and finally apply the appropriate amount of delay to the signal.

That would be more like an equalizer than selecting out particular "sounds".

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#16

Post by msandford »

chas wrote: post #11
oakwhiz wrote: post #4

Selective noise cancellation could be possible using a Fourier transform. Zero out the coefficients that you still want to hear and then perform an inverse transform, then a time-domain inversion, and finally apply the appropriate amount of delay to the signal.

You might know this, but I want to state it at least for the benefit of people reading this without much signal processing experience. Just zeroing like that really isn't the best idea. Square waves (that are 1 for signals that you want and 0 for signals you don't want) inverse Fourier transform to sinc functions[1], so if you are filtering just by chopping things out, it will greatly distort the time-domain system b…

Yeah exactly right. If you don't make it smooth in the frequency domain it won't be smooth in the time domain either.

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#17

Post by darkmighty »

chm wrote: post #7
pallandt wrote: post #3

Earlier quoted context omitted.

Yes, this should be on a crowdfunded project asap if the prototype in the video was really functional and no simulated sounds were added. I'd buy it in an instant! Maybe I'll attempt to promote it on Reddit to help with gaining more 'mass-appeal' traction.

I'm pretty sure it was CGI.

Yes, and there's whether it actually works IRL. I think it's feasible to have something like the concept, but there are quite a few hiccups along the way -- I'm no audio expert, but I suspect that a single microphone/speaker location may be unable to cancel satisfactorily the incoming sound field through the window. Also, Loud noises creep in through various places, so if your window is quite good already you many not get much improvement.

But a good concept and hard work (and hope) usually lead to quite good products...

I.e. noise cancellation in building is usually approached through "get better insulation", not through the plausibly cheaper "let's spread a speaker/mic array inside/outside to cancel what's coming form outside".

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#19

Post by brianbreslin »

Does anyone remember those suction cup speakers that attached to windows and turned them into speakers?

Years ago I wanted to splice a noise canceling headphone into one of these devices and make the windows noise canceling. Would that even be feasible?

Re: High-Tech Window Cling is Like a Volume Knob for Real Life

#20

Post by msandford »

darkmighty wrote: post #17
chm wrote: post #7

Earlier quoted context omitted.

I'm pretty sure it was CGI.

Yes, and there's whether it actually works IRL. I think it's feasible to have something like the concept, but there are quite a few hiccups along the way -- I'm no audio expert, but I suspect that a single microphone/speaker location may be unable to cancel satisfactorily the incoming sound field through the window. Also, Loud noises creep in through various places, so if your window is quite good already you many no…

I've done quite a bit of signal processing in my day. Audio, image/computer vision, radio comms, etc. This product strikes me as not terribly honest for a few reasons:

Geometry

The mic/speaker assembly are in the middle of the window but the window has substantial size relative to the dimensions of the room and the distance the listener is from the device. What this means is that you have a multipath problem; the original sound can come through the whole window (even the wall!) but the canceling sound can only eminate from the speaker.

If you draw a straight line from the original noise source, through the device and from there it happens to head towards your ears, this can work. If not things will get jumbled up and it might cancel or it might actually reinforce the noise, rather than cancelling it.

http://en.wikipedia.org/wiki/Loudspeaker#Directivity

Physics

A speaker is a device that works as a derivative of the original signal. That means it needs some amount of "advance notice" of the sound in order to be able to create the proper pressure wave to cancel out said sound. The "advance notice" is usually done by means of some distance between the microphone and the speaker but the device is very thin. Worse, the amount of "advance notice" depends on the frequency of the sound which generally makes it an intractable problem to solve for the transient case.

http://www.customanalogue.com/elsinore/elsinore_19.htm

So what people do is focus on periodic noises like hum, hiss, buzz, rumble, what-have-you. In this case you don't need to make sure that any one peak has it's inverse reproduced perfectly, you only have to ensure that ONE peak is cancelled out by SOME OTHER out of phase peak. If it takes a couple of cycles for the computer to catch on, no big deal that's just a bit of start-up noise. Bose headphones don't IMMEDIATELY make the plane quieter, they just do it so fast that you don't notice the amount of time it takes for the system to warm up. It seems instant even though it's not.

But what this means is that a bunch of the sounds being cancelled out probably wouldn't be cancelled out quite so nicely as a lot of them are transients like the bottles breaking or the construction site or the park.

I'd really love to be wrong about this. There's a joke that goes "An engineer will tell you it's impossible until you show him how to do it. Then he'll say it's obvious." It's entirely possible that this guy has figured something out that the rest of us haven't. But I will remain skeptical until I can actually put my hands on one and benchmark it for myself

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