Manual noise cancelling?

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jamoo
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Manual noise cancelling?

Post by jamoo » Fri Jun 24, 2011 12:17 am

Is there a way to simulate what noise-cancelling headphones do, and does it make sense to broadcast it, or are we better off with Beethoven?

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Post by fossiltooth » Sat Jun 25, 2011 7:20 am

What noise canceling headphones do is an acoustic phenomenon. You couldn't broadcast it.

There are little microphones on the outsides of the headphones that capture the noise around you, and then play it back into your ear out-of-phase. Essentially, they make things sound quieter by making them louder - in reverse. Weird, right?

If you ever notice the sensation of a little pressure on your ears when you put noise cancelers on and power them up, now you know why.

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Post by jamoo » Sat Jun 25, 2011 11:24 am

Interesting, though I still don't see why it can't be broadcast. What if you recorded a "general tone" from the environment, turned that out-of-phase, then re-broadcast. Wouldn't that have somewhat of a cancelling effect? IOW, why is the isolation required on the speaker-end? Even if it's not dynamic, it seems like there should be some desired result, especially if the noise is constant.

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Post by Anthony Caruso » Sat Jun 25, 2011 11:43 am

IOW, why is the isolation required on the speaker-end?
I guess it's not, they've used this in some cars http://www.gizmag.com/toyota-employs-no ... brid/9637/, but according to the article they have to optimize for "sweet spots" due to the feedback in the car of speaker feeding mic feeding speaker feeding mic etc which would probably cause some zany possibly disorienting phase effects, possibly unwanted while driving :shock: . It seems like the speakers and mics are dedicated to the cancellation, ie the speakers aren't the stereo system speakers, but I would imagine having music playing would up the weirdness ante, as your Doobie Bros. tape feeds the cancellation mics and gets pumped back into the car out of phase, feeding the cancellation mics, pumped back out etc, and I'd bet the ADC/DAC in the Toyota doesn't have a low latency monitor mode, so imagine a 10ms delay on top of that. Yow.

Yeah, think of combining feedback and anti-matter. That's probably what it's like.

What is the scenario you are imagining this in?
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Post by jamoo » Sat Jun 25, 2011 3:58 pm

Anthony Caruso wrote:
What is the scenario you are imagining this in?
domestic. actually, the initial inspiration was the sound of someone's voice.

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Post by The Scum » Sat Jun 25, 2011 5:42 pm

There are really two places you can cancel the sound with a simple polarity reversal: at the source, or at the receiver.

If you cancel at the source, you're trying to prevent it from entering the environment, and can possibly block a single source. Real sources are often more complex than that, and there's no good way to cancel them...it's hard to build something like a mouth-muff (like a reverse headphone) that captures and cancels everything someone is saying. If you ask someone to wear the mouth-muff, you may as well hand them a ball-gag.

Most receivers tend to be mechanically simple...effectively becoming a very small point in space that does the reception - like the opening of your ear canal. If you can accurately judge what's coming into the ear canal, you can generate an inverse pressure, and cancel it. But that degree of accuracy impacts the end result. If I yawn really wide while wearing active canceling headphones, I can hear through my Eustachian tubes, which isn't picked up by the mics in the phones, and thus not canceled. I've screwed up the isolation provided by the phones.

Beyond the source and receiver, it's a pretty complex problem. The "general tone" you ask about doesn't actually exist. Each receiver will be in a different location...and thus be receiving a different signal, due to source directionality, wave propagation delay, room acoustics, etc. The signal needed to cancel at point A probably isn't the signal to cancel at point B. It may actually reinforce, rather than cancel.

This can be solved for a given set of sources and a given receiver location...but it involves precise time delays and other calculations for each source->receiver path. The doppler/phase array DSP dudes use similar techniques all the time on radio signals, so it's probably not too hard to translate to audio frequencies.

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Post by Snarl 12/8 » Sat Jun 25, 2011 6:24 pm

What about masking? You could blast a sound that was in the frequency range of the offending vocal thereby rendering it less intelligible.
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Post by jamoo » Sun Jun 26, 2011 1:59 am

i suppose i was musing possibilities. i know enough to ask the question, but not enough to realize how complex it is. it would be nice, but i imagine if it were possible we'd see it in applications. i could try masking, but the one thing about the headphones is it doesn't feel like you're listening to anything (save the pressure to a sensitive ear). i think a blasted signal would stand out.

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Post by vvv » Sun Jun 26, 2011 7:32 am

jamoo wrote:
Anthony Caruso wrote:
What is the scenario you are imagining this in?
domestic. actually, the initial inspiration was the sound of someone's voice.

I found the most effective cancelling of that to be divorce. :twisted:
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Post by jhharvest » Sun Jun 26, 2011 9:40 am

The Scum wrote:There are really two places you can cancel the sound with a simple polarity reversal: at the source, or at the receiver.
I think that you can actually build an active sound cancelling barrier due to the way sound waves behave, at least in theory. If you have several (let's say a hundred) point sources which are positioned close enough (less than quarter of wavelength), they reproduce sound similar to a single large surface. Therefore you could have individual active noise cancelling elements aligned in an array and they would cancel out noise coming from the other side within their operating range.

This would be expensive due to the number of these elements needed for operating at any reasonable SPL. The functionality could probably be replicated with low cost components, such as small full-range speaker elements, a cardioid mic element and ICs for signal processing and amplification.

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Post by MisterMark » Tue Jun 28, 2011 6:26 am

I believe the guys at Izotope came up with a hardware solution to reduce broadcast noise. I don't think it uses phase canceling, but you can read more about it here:

http://www.izotope.com/products/audio/anrb/

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Post by jhharvest » Tue Jun 28, 2011 2:19 pm

That's not really pertinent to the topic but damn, I'll order one tomorrow.

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Post by Snarl 12/8 » Tue Jun 28, 2011 8:09 pm

If the OP is talking about what I think he's (?) talking about, what about installing a couple speakers in the headrest of your favorite chair, with noise canceling mics and hardware built in there. Then at least you'd have a place to go for a respite from "the voice" and it would be pretty unobtrusive. If you wanted, I bet you could feed the equipment a line from your tv/stereo and set it so that they didn't get canceled.
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Post by jhharvest » Wed Jun 29, 2011 6:49 am

jhharvest wrote:That's not really pertinent to the topic but damn, I'll order one tomorrow.
Didn't order it yet but I've agreed with Mr. Bartone from iZotope that I'll send them audio clips for test processing next week. I hope it works as well as it says it does. A hands-off noise filter would be a real boon. Now we do that stuff in post which isn't ideal.

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Post by eeldip » Fri Jul 01, 2011 12:56 pm

how about a passive sound barrier?

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see:

http://www.sciencenews.org/view/generic ... ard_it_all

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