A bit lost with Active Noise Cancellation - Page 1
Forum section: electronics / projects

author: bvanderlaan
author_rank: Newbie
timestamp: September 27, 2015, 07:37:01 pm
content: Heya,I'm new to the forum, so I'm hoping I chose the right place to put this. I want to build an active noise cancellation solution, I understand that there are a lot of variables at play with this type of project. From what I understand the main points I should be worried about are the frequency response of the microphones, the headphone drivers, and the amplifiers used to preform the actual cancellation.After a bit of digging I stumbled upon this circuit diagram:It's a little over 10 years old, but it got the point across. That was my jumping off point while starting this project. Since then I have found the AS3410, and I am intimidated to say the least. But the lower cost has high appeal for me, as well as the low current draw. My goal is to build a ANC solution for my headphones while I'm at work. I do understand that my mileage may vary, and it would be easier to go buy a prefab solution, but I'd really love to just built it. Here's the spec on the AS34XX:http://media.digikey.com/pdf/Data%20Sheets/Austriamicrosystems%20PDFs/AS3400,10,30.pdfAnd here's the example I am working off of:Where I got a bit lost is looking at Figure 30 (page 43) specifically around the values for the microphone filters on pins 1, 23, 24 and 11, 12, 13.So the list of questions I have built up so far:Where should I be starting on this?I like the idea of using the IC due to lower cost (and I've been meaning to try SMD soldering), smaller footprint, and I can control it using I2CHow do I select my filter values?I feel like I should be doing something based on my microphone to decide what to filter (is that based on the noise floor? or?)How should I select the microphones for this?When the example says there are other possible topologies, what else can I do? Does it just mean my filter design is up to me - go nuts?Looking forward to your responses!

author: Kilrah
author_rank: Unknown
timestamp: September 27, 2015, 08:01:02 pm
content: QuoteSounds a lot like "try a bunch of values until you're happy"...

author: microbug
author_rank: Frequent Contributor
timestamp: September 27, 2015, 08:06:02 pm
content: I suspect that most noise cancellation these days is done with a DSP. One of those circuits might work but a DSP would no doubt be more reliable / accurate. I haven't dabbled in DSP myself, but it seems like a fairly simple project.

author: bvanderlaan
author_rank: Newbie
timestamp: September 27, 2015, 08:16:26 pm
content: So for trying a lot of values I have a couple of studio monitors that I can use to generate pretty accurate sounds, my thought was to use them as my baseline, take whatever mic I have and then look at its response using audio chirps and a scope.That helps with the mic side at least in terms of knowing what frequencies I should be worried about? (That's my understanding)For the headphones, the only thing I've thought of so far is to have a parametric eq on the mic input and set that eq based on the frequency response of the headphones to maximize the actual cancellation.What I'm worried about is the phase offset from the mic to the headphones - I don't know how an eq would change the phase (or if it would change it all)---From what I've read, most noise cancellation today uses DSP using hybrid feedback and feed-forward techniques simultaneously to achieve better (>30db) cancellation at frequencies up to 3Khz(http://www.edn.com/Pdf/ViewPdf?contentItemId=4422370)I don't know where to start with DSP, so I thought this would be a good jumping off point

author: IanB
author_rank: Super Contributor
timestamp: September 27, 2015, 08:32:41 pm
content: I once bought a cheap pair of noise canceling headphones from a convenience store, which for only a few bucks probably used an off-the-shelf one-chip solution as you are considering. They worked, sort of, but the noise reduction was minimal.I have since splurged on a Bose QC15, and I can tell you the difference is night and day. When you turn on the QC15 it is like turning off the outside world.I think getting noise cancellation to work well requires a huge amount of research, testing and fine tuning. Unless you have a well instrumented laboratory and are able to spend months measuring, adjusting, tuning and refining, I doubt you will get anywhere close to a commercial product.This is just my opinion, but I think sometimes good results come from a lot of research, and unless you can duplicate the research you will not easily duplicate the results.

author: Kilrah
author_rank: Unknown
timestamp: September 27, 2015, 08:42:09 pm
content: That's one thing, the next is the "controlled environment" aka with that chip you're supposed to adjust the caps and filters for one particular headphone you'd then mass-produce. Even if you could get good results after a lot of testing on one, I doubt stretching that to other headphones with just an EQ like you want to do would get anything decent. Maybe it can work, but you won't know without trying and spending dozens of hours on it as it simply isn't meant for it and thus isn't specced for it.Love my Bose QC15 as well, was one of my best purchases ever 5 years ago. Of course they use closed-loop feedback, with mics not only for ambient sound but with a 2nd pair that also samples the sound that is actually emitted in the earcups.

author: eneuro
author_rank: Super Contributor
timestamp: September 28, 2015, 04:03:07 pm
content: When we were working in a big company in open space I tried different noise cancelation devices and it didn't worked for us.I've made another aproach to do not cancel noise (mainly humans voices from everywhere ) but... I've created in PC software 9 OpenAL sound sources around and played randomly ocean waves in each this point in space around listener, so quite uniformly distributed additional "noise" (sound of falling ocean waves) quite succesfully helped remove humans voices perceptionWe use this "ocean waves" music these days too, since it helps concentrate on task and eliminate any voices from background....