Noise Cancellation
Noise Cancellation
(OP)
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...
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...





RE: Noise Cancellation
Good luck.
Cheers
Greg Locock
RE: Noise Cancellation
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
RE: Noise Cancellation
http://directory.google.com/Top/Science/Technology/Acoustics,_Ultrasound_and_Vibration/Active_Noise_Control/
RE: Noise Cancellation
Cheers
Greg Locock
RE: Noise Cancellation
I recommend following the advice of okdb and find a passive solution.
RE: Noise Cancellation
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
RE: Noise Cancellation
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
RE: Noise Cancellation
rubber calk under it to muffle the vibration
and deaden the sound. I assume the joists
are amplifying the sound.
RE: Noise Cancellation
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