Active Noise Filter

I understand that its been in use for some time one of its uses is in reducing the sound level in car interiors...

There are many other cases where it has been used in industrial processes etc...

I wish someone would develop a personal system that can be carried around and used in a disco, A sort of anti-walkman thingie!

John.

There are noise cancelling headphones out there already:

http://koss.com/koss/kossweb.nsf/p?openform&pc^nc^QZ77

http://koss.com/koss/kossweb.nsf/p?openform&pc^nc^QZPRO

Elderbrock is currently selling a vehicle muffler that uses a noise cancellation design to reduce noise and resonance. http://store.summitracing.com/egnsearch.asp?N=700+400065+4294923429+4294922710+115+311045&autoview=sku

This design is mechanical not electronic. There are a few ways to skin a cat.

The most basic form of electronic noise cancellation senses the incoming acoustic level and generates a corresponding antiphase signal. You could regard this as simulating a wall of infinite mass between the signal source and the shielded region. Essentially, the incoming wave is reflected over a wide range of frequencies. This is not practical using passive (purely acoustic) systems that are both compact and low mass.
(N.B. related systems have been in use to concert halls for a much longer time, to correct for faults in the intrinsic acoustic design - sometimes it is desirable to simulate a different delay, sometimes simply to increase or reduce the reflection at different frequencies. High frequencies, however, are generally dealt with using acoustic methods)

In order for noise cancellation to work for all source locations and throughout a space, you would need this to cover the entire region surrounding the listener, and divide the transducers into sufficiently small segments to cover all directions of interest. The alternative is to provide cancellation that is only fully effective for specific sources and/or for specific regions within the protected area. (Low frequency sources are easier to deal with in this respect, of course)

In order to widen the effective frequency range, noise cancellation systems for cars can use prior knowledge of the locations of the noise sources and the listeners to optimise the overall effect. They can also have some measure of prediction built in, so they don't need to monitor every source separately.

Fyz

Hi FYZ,

Thanks for the information but I wonder why this method of sound suppression is not used in buildings specially those located in heavy traffic streets. It would be extremely popular specially in summer time when people like to leave windows open but due to the noise from the street, its almost impossible. The traffic noise is normally of low frequency type and it shouldn't be hard to kill by active noise technique.

baser5nature (#7) is, unfortunately, correct. Just considering sound coming through an open window, and trying to screen the whole room:Using transducers that surround the opening, you could do a reasonable job for longer wavelength sound - down to about the half-wavelength across the shorter dimension of the window. If the constraints on input direction were known, you might manage a slightly broader wavelength range with the same performance. If you are reasonably high above street level, you could absorb the shorter wavelengths where they reflect from the ceiling. But even then there would normally be a region that was largely unattenuated, that you could only reduce with an array of transducers spread over the window, which would rather spoil the view - and possibly also the ventilation.

(What makes it really difficult, unlike a car, is that neither the location of the listener nor the location or characteristics of the source are well known)

Fyz

Then the partial solution might be insulating the bed room by placing the sensors arround the bed.

JFB

I don't know why, but I was thinking about offices. Bedrooms should be easier, because you know roughly where your head will be. Needs an adaptive system*, of course, because not all rooms are the same. Transducers around the window should be capable of doing the job, or even on a stand near the window's centre (it's always easier to cover a reasonable area if the speakers are near the path of the sound). SFIK, no-one yet makes such a thing for domestic environments - though I would be surprised if the spooks weren't already on to it (in reverse, to stop sound going out). Not a trivial project...
*with a microphone in the bed to get it set up.

Fyz

Actually I am thinking of a quiet bed with open window to the street. Do you have any idea where can I get technical information, if I want to try this project?

JFB

Sorry, outside my scope, I'm afraid - I know the theory, but am not involved; plus, by non-trivial, I mean exceeding the level of published stuff. Also, would need new software development. Quite a bit of hardware too. If you have the skills, you could go for venture capital and set up a team - it shouldn't take more than two or three years for a suitably qualified group (full time). Unless someone knows different...

Fyz

These guys's business is occupational health and safety for mining, factories and industry at large. They address noise pollution, noise control and specifically communication in a noisy enviornment. Check it out! Proudly South-African company
www.noiseclipper.co.za

Physicist, mentioned one key thing the "shielded region". For headphones this is your ear canal... a relatively small area, very easy to "cancel" the noise here. For something like a building this would be a mammoth task. I think that Physicist's reference to the "wall" could be misleading... Active Noise cancellation can only be focused on a general "spherical" region. That means there are regions outside the "shielded region" that you will be able to hear the original sound, and there will be regions that you will actually hear the antiphase sound.

Now if I can find a device that cancel my snoring, my wife would love to buy one.

MidniteFighter

Try nose tape