Active Noise Reduction for Projector Fan Noise? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 14 Old 11-27-2000, 11:26 AM - Thread Starter
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I was thinking about posting this topic on the Home Theater PC Forum, but I think the people here might be as technically able to solve this problem.

Several months ago, Digital Connection’s Cliff Watson stated at the Home Theater Computer Forum, that he was thinking of building an Active Noise Reduction (ANR) system to quiet our noisy HTPC’s. I believe he never got around to that challenge though.

But my projector (Runco 940) is so relatively noisy, that I cannot hear ANY HTPC noise. I guess I was really spoiled by my previous projector, a Vidikron 40SEL. That FPTV was totally silent. I tend to crank up the volume from every source to drown out the Runco. But there is the fan noise back again on quiet passages.

For those of you unfamiliar with ANR, it electronically generates a tone, 180 degrees out of phase with the offending noise, to vastly reduce that noise. ANR is used quite extensively in aircraft headphones. Companies like David Clark and Bose make some of the best and most expensive units for this application. Some passenger aircraft even use ANR throughout the entire cabin to reduce ambient aircraft noise. The Twin Commander business turbo-prop for example, uses 13 speakers and 24 microphones strategically placed around the cabin to find and reduce aircraft noise using ANR.

So my question is, has anyone ever played with ANR?

Is there any way we can use, say a very small amp, a microphone and speaker at the projector, and our HTPC’s as a controller to vastly reduce fan noise?

Thanks,


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Bob Sliwa
"Like a Midget at a Urinal, I was gonna haf'ta stay on my toes...."



[This message has been edited by Dallas777 (edited 11-28-2000).]

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post #2 of 14 Old 11-27-2000, 12:06 PM
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You might want to try http://www.nct-active.com
I visited them years ago as a small start-up and wrote about them. They had a patent on a microcomputer-based system that relied on sampling the source frequency of noise (such as a spinning fan rotor), then generating the cancellation anti-noise within that narrow band. Not sure if they're still marketing the basic computer unit; they've branched out into ultra-thin picture-type loudspeakers and other hardware.

If you'd care to make your entire listening room 'disappear' through cancellation technology, contact Cambridge Signals Technology in Cambridge, Mass., and buy a Sigtech 2000 for $6k plus.

--John



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post #3 of 14 Old 11-27-2000, 02:19 PM
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After deep investigation into the theory and the practical realization fo active noise control systems, i have come to the conclusion that: As far as beeing a viable solution for noise control, it doesn't work at all... unless the noise is very low in frequency.

There are many reasons for such a conclusion, but trust me, it's a valid conclusion.

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post #4 of 14 Old 11-27-2000, 05:15 PM
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You might consider very seriously building a hush box for your projector. They are very effective at reducing projector noise. With imagination, you can build one to blend in with the room. They work well. At the conclusion of an after dinner double blind trial that required my hush box to be open for cable access, I closed the hush box. One of the participants asked, "Is it still on?" Closing the box dropped the noise so far that they had to actively figure out if the projector was still on or off. They were amazed how quiet it was directly under the projector.



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post #5 of 14 Old 11-27-2000, 07:43 PM
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Why not just replace the fans with PC power & cooling's 20db silencer fans?
At $12 each, it sure seems cheaper and alot less hassles.
If your interested www.pcpowercooling.com/products/alarmandaccesories/silencerfan
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post #6 of 14 Old 11-28-2000, 06:41 AM
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Dallas777,
You might want to browse some of the FAQs about the field, such as: http://www.landfield.com/faqs/active-noise-control-faq/ http://www.signalsystemscorp.com/ancFAQpart_ii.htm
A google.com search with "noise cancellation fans" shows some FAQs relating to industrial fan noise cancellation.

How does your projector sound compared to a car muffler? Lots of work in electronically active muffler noise cancellation, including a few awards some years ago: http://www.discover.com/awards/arc96/90win.html
Key question, of course, is any manufacturer currently using such technology (mufflers, not interiors) for cars?

--John



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post #7 of 14 Old 11-28-2000, 07:19 AM - Thread Starter
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Gentlemen, thanks for all the replies.

Specifically:

John - thanks for all the links. I visited the nct-active yesterday. I'll check out the others today. Being an old gear head, I am aware of some of the development done with auto muffler applications. And being a private pilot, I am up to speed on the headset applications.

Dr. Kuo - I was considering a hush box, but in my application my projector is hanging from an overhead horizontal beam in my cathedral ceiling HT room. I've seen photos of many hushboxes, but they were all around the 4 or 5 sides on a conventional flat ceiling. It still may be an option, but I think it would look weird in my room. ANR would have no effect on the appearance of the projector, which I think is kinda cool.

Ergin - Agreed, cancellation at the source sounds impossible. But, as the Twin Commander aircraft that I referred to with the 13 speakers in the cabin, "anti noise" throughout an environment is possible. How about just a microphone at the projector, and the 180 degree out of phase "anti noise" originating from our 5.1 or 6.1 speakers? Could the computer input the "anti noise" simultaneously while playing a DVD soundtrack? That would be cool.



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"Like a Midget at a Urinal, I was gonna haf'ta stay on my toes...."

[This message has been edited by Dallas777 (edited 11-28-2000).]

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Click here to see my Home Theater "Corvette Cinema"
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post #8 of 14 Old 11-28-2000, 07:21 AM
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Noise cancellation systems within the context of mufflers, has a fighting chance of working, due to the restictive, controlled orientation and 'volumetric' (pressurised cavity) considertions of the noise. It is a pressure-volumetric situation, with a controlled release apature, which aids greatly in any attempt to 'remove' any noise output. The big problem lies in the quality and RELIABILITY of the noise cancelling hardware. Failure rates are high in this regard. Any and all high frequency harmonics are still subject to release, this includes the lag times involved in dealing with time sensitve measurements and feedback systems. Varying conditions (and thus the noise cancelling system's response) are still high enough to render such solutions as being impractical.

Passive systems still work best. They cannot kill all noise, but their inherent reliability speaks volumes for such as a solution.

All of the above is why all negative feedback systems in audio amplifiers are generally invalid. what this means is that they are an imperfect, incorrect solution, and -therefore-, are not a solution.

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post #9 of 14 Old 11-28-2000, 07:38 AM - Thread Starter
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But KBK remember, lag times and varying conditions are not a consideration with projector fan noise.

The fan drones along for hours at a constant rate and frequency the entire time the projector is running. We are talking about no constant changes, as in a muffler application, here.

It seems to me that one could just record the noise for a certain model projector and then add the "anti noise" to the 5.1 output and eliminate the microphones completely?????

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Bob Sliwa
"Like a Midget at a Urinal, I was gonna haf'ta stay on my toes...."

Bob Sliwa
"Like a Midget at a Urinal, I was going to have to stay on my toes...."

Front Projection Home Theater since 1979.

Click here to see my Home Theater "Corvette Cinema"
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post #10 of 14 Old 11-28-2000, 02:25 PM
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Quote:
Originally posted by Dallas777:
It seems to me that one could just record the noise for a certain model projector and then add the "anti noise" to the 5.1 output and eliminate the microphones completely?????
[/b]
Great!!!!!!!! Finally I found someone else that is following the same path I was walking in order to reduce the noise.
My approach is the same. At the moment I'm focussing whether any kind of software is able to apply a 180 degree phase filter on the sound coming from the mic. Obviously there should be such kind of filter (it is not definitely anything difficult to implement on certain kind of software). I think that we should try to post this kind of question in some forums that deal with software like: cubase, wavelab and similars.

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post #11 of 14 Old 11-28-2000, 03:02 PM
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If you are going to go through with this, then here are the things that you have to do.

1) You have to find and use a fan system that has a method of realizing the exact rotational characteristics of the given unit, ie., one that has a rpm guage of some sort. Similar to that of a stepper motor for robotic systems. (postional guage of some sort)

2)You would have to analyse the noise spectra on a digital storage system, and simply create an anti-noise signal. The reason for this approach is that the noise spectra is complex in structure, and high in overall frequency - meaning it is full of harmonic structure.

3) You must tie the anti-noise signal to the rotational speed and TIMING or rotation of the fan exactly.

You may have to create a computerized system, that analizes the signal at turn on, and then creates the anti-noise signal, and then institutes it as a solution. You may need a method of trimming it, or another system (with microphone, or the same original microphone used for signal analysis) that 'hunts' about for the lowest noise point, and stays there. At that point, it may be viable to set it so that it corrects itself by starting all over again if the output spectra is too high in level (set to do so at a given db...)


Once you do this neat thing...take a break.

If I felt that I wanted to program a bit I could probably do it, but at best it would only be partially effective.

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[This message has been edited by KBK (edited 11-28-2000).]

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post #12 of 14 Old 11-28-2000, 08:17 PM
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I failed to mention, for some reason, that the anti-noise signal generator would have to be placed as close to the fan as possible...

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post #13 of 14 Old 11-29-2000, 05:12 AM
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Hi Guys!

Just chiming in as no one seems to have mentioned the issue of active noise reduction actively reducing selective areas of the sound spectrum of a movie's audio playback. For some of us go to great pains and expense to have the best possible (hopefully better than) "theater sound". http://www.avsforum.com/ubb/wink.gif

It seems unavoidable that, even if only at specific frequencies, actively generated sonic signals would break down the soundwave structure from your home theater audio speakers - that is, unless you implement active noise generation with very directive pavillion speakers located near, and pointed directly at the projector, with any reverberation from these baffled by extensive enveloping suspended panels which would be far less esthetic than the ugliest of hush boxes.

So accompanying or instead of such active measures, I would suggest at least a horizontal panel placed between the projector and the viewers' main seating location to isolate them from any direct sound emanating from the projector or the noise reducers/generators, leaving only the weaker reverberated sound waves to disrupt the audio track of your movie. You can also take the less elegant but possibly more aesthetic and nonetheless effective path of directly reducing noise at the source by installing quieter fans - that way you still get to show off your enviable equipment and keep the visual impact of this suspended mass to a minimum. Maybe an authorized Runco dealer would be willing to operate a fan replacement for you (at a deserved high cost) and commit to still honoring your projector's warrantee? It is definitely worth investigating. Otherwise you can always take the low road and pool resources and know-how with others here in the forum where you will assuredly get guidance, advice and sourcing.

Hope this helps a tad.

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post #14 of 14 Old 11-29-2000, 06:36 AM
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Just a slight expansion on the NCT system(s) I mentioned earlier (11/27 above). The demo I saw was 12-15 years ago, and of course microprocessor chips (used by NCT then), as well as more advanced digital signal processing (DSP) chips, have advanced considerably. In the table-top demo I watched, a vibrating source of noise was sharply reduced; you could see the noise on a dual-trace scope, and the anti-noise, 180 degrees out of phase, which caused the sharp noise reduction. As I mentioned, NCT's patent(s) related to sampling the frequencies near the noise source, feeding them into their microprocessor, generating the anti-noise, then feeding the anti-noise signal back to a transducer near the source. I've always assumed industrial applications of noise cancellation, such as large fans, use techniques very similar to this. Whole-room noise cancellation, based on speakers in a room and aimed at a fairly narrow range of noise seems much more difficult to accomplish. (But with enough signal processing power, which seems to be the technique used by Cambridge Signals Technology also mentioned above, you can make the acoustics of an entire room disappear. A Nov. 21 Wall Street Journal article, "Psychoacoustic Chicanery," describes this system.) So, if I wanted to cancel the moderate noise coming from the two fans on my Philips 64-in. RPTV attached to a large, thin rear panel, I'd concentrate on sampling the frequencies, signal processing (a NCT-type computer), and careful placement of the anti-noise transducer(s) on the rear fan panel. Naturally I'd try specialized more-silent fans first.

--John

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