Noise Cancellation

[new2hvac]

Are there any commercially available products that can be used for noise cancellation? Specifically, I'm trying to cancel the noise produced by a fan above a false ceiling.

It would seem this would be pretty simple technology...wouldn't it be the same as what is being used on noise cancellation headphones?

http://homepage.ntlworld.com/nigel.gough/noise.htm

The obvious difference is that the microphone and speakers of these headphones are in close proximity. There may be additional complexity when trying to silence an entire room.

Any and all thoughts are appreciated...

 

[GregLocock]

No there is no commercially available device that will provide effective antinoise in large volumes. You may be able to home-brew a system using a mic and loudspeaker and a variable delay circuit.

Good luck.



Cheers

Greg Locock

 

[okdb]

Hi,

My advice: try good old sound insultation. It works and isn't that expensive. I'm not sure what kind of false ceiling you have and how extensive it's the problem, but if you need 3 dB reduction or so, try to lay a 100mm tick layer of dense glass wool like AF-75 over an area about 3m x 3m. You could also replace some of the tiles by gipsum tiles. that will give you maybe 6-10 dB attenuation. Last option, build a gipsum box around the fan.

OkdB

 

[pheon]

Greg is right but I doubt you will be able to home-brew a system. There are many technical difficulties to be overcome to make this work (which is why there are no products). For more information about active noise control see

http://directory.google.com/Top/Sci...ltrasound_and_Vibration/Active_Noise_Control/

 

[GregLocock]

I have heard a home-brew system that did work, built as I described. It was the proof of concept they built at Lotus which led to our obsession with antinoise for engine noise, for several years in the late eighties. If you have a fixed frequency point source then it should be possible to put something together that will work.



Cheers

Greg Locock

 

[pheon]

I regret that I must beg to differ from Greg, or at least qualify his assertion. Yes, it might be possible to make a home-brew ANC system work if you have a fixed frequency point source at one location. However, unless you are very lucky (or very clever), you will only get reduction at one location in the room, elsewhere, it could be louder. (I write from painful experience, having implemented ANC systems for planes, trains and automobiles and many other things).

I recommend following the advice of okdb and find a passive solution.

 

[GregLocock]

I think persistence and a multiplicity of transducers is more important than cleverness, for a fixed frequency point source. Luck will win, of course. If you have a point source and place speakers between the source and the listeners, then it can be made to work. Basically you are building a spherical wave source around the original spherical wave source.

The best demo of that I did was to cancel the external exhaust noise of a car, in the car park, using mics and speakers on the car itself.

My boss (almost) fell over when he heard it.

There are some provisos - I am talking about low frequency noises with speakers placed within the near field of the source.

Since I've made systems work in both heavily modal and free field, I know /that/ isn't a big issue in itself.

Which ANC system did you use? Our development system was the ISVR Nelson/Elliott setup, limited to about 200 Hz and 4 speakers x 6 mics (or 8) I think when I used it.

Cheers

Greg Locock

 

[pheon]

Greg,

I worked with Nelson and Elliott on one of their first ANC system. Most of the systems I have used since then have been 'home-brewed' (if you can call systems like a 196 actuator by 64 sensor control system 'home-brewed').

I'm impressed that you could cancel exhaust noise using the internal speakers. It certainly solves the 'how on earth can I make speakers that survive in an exhaust pipe' problem. Isn't it a bit uncomfortable for the passengers though?

Alan

 

[diamondjim]

Could you pull the fan mounting and use
rubber calk under it to muffle the vibration
and deaden the sound. I assume the joists
are amplifying the sound.

 

[GregLocock]

They were subwoofers in the boot. Hmm, I don't remember sitting in the car while it was cancelling, I wonder what it sounded like? Pretty bad, I bet. So we'd really need some mics inside the car as well.

One of my 'research' (=keep Greg out of mischief) projects at Lotus was external exhaust noise cancellation.

We tried an air powered noise source from ISVR, but it was too flaky (and big and had too much distortion) to be used in an exhaust. I used the external speakers, as above. Then we decided to use a modulated turbine in the exhaust, using a high speed alternator with a variable load. That was a pretty good idea, until I read around and discovered that turbos are normally tuned to absorb the exhaust pulses - in other words there was no need for any fancy electrics, just a normal turbocharger would do. That is why the exhaust on a turbocharged car doesn't really need resonators, just high frequency absorption.

Then we must have had another contract come in, as I dropped external exhaust cancellation and no one else picked it up while I was there.

I did get some useful stuff out of that exercise though, including the correct way of describing the intensity in the duct, with a moving airstream, and a better understanding of how tailpipes work. All forgotten now, of course.


Cheers

Greg Locock