Sounds a little like Star Trek doesn't it?!
Here is the idea. It has come from another thread in which wintermute suggested a noise cancelling headphone circuit to quiet the fan noise in my amp. It got me thinking. Could this work to keep things quiet for the rest of the house?
Last night I went into the space between my sound system and where others sleep. I could hear bass (of course) and some muffled vocals, most of the energy below 500 Hz.
I know active noise cancellation works. But does it work in 3D space with speakers, rather than just headphones. With headphones, the mic is right at your ears. Also it is easy to get the cancelling sound much louder than background noise. In my case the speaker is at least 20 db ahead at 20 Hz.
Thoughts on the concept anyone?
If it works, it's a great way to crank the music at night and not disturb anyone. It has potential for people in the house, as well as neighbours getting disturbed by subwoofers! Much cheaper than turning your room into an acoustic treated bomb shelter!
For those interested, here's the link to the schematic of the device:
http://www.headwize.com/projects/noise_prj.htm
Here is the idea. It has come from another thread in which wintermute suggested a noise cancelling headphone circuit to quiet the fan noise in my amp. It got me thinking. Could this work to keep things quiet for the rest of the house?
Last night I went into the space between my sound system and where others sleep. I could hear bass (of course) and some muffled vocals, most of the energy below 500 Hz.
I know active noise cancellation works. But does it work in 3D space with speakers, rather than just headphones. With headphones, the mic is right at your ears. Also it is easy to get the cancelling sound much louder than background noise. In my case the speaker is at least 20 db ahead at 20 Hz.
Thoughts on the concept anyone?
If it works, it's a great way to crank the music at night and not disturb anyone. It has potential for people in the house, as well as neighbours getting disturbed by subwoofers! Much cheaper than turning your room into an acoustic treated bomb shelter!
For those interested, here's the link to the schematic of the device:
http://www.headwize.com/projects/noise_prj.htm
Well, as a thought experiment, let's suppose you have a system that is successfully canceling an ambient sound field at some point in space by producing an antiphase sound. Let's further suppose that some particular noise source that is being canceled is from a specific direction and has energy concentrated around 1Khz. If you move only three inches from the null point toward that sound source, you'll get an addition of the two sources, not a cancellation, at 1Khz.
There will be other, partial nulls at that frequency as well as areas where the two sounds reinforce each other as you continue to move in that direction, with diminishing amplitude from SPL attenuation due to increasing distance from the ambient cancellation sound source.
There will be other, partial nulls at that frequency as well as areas where the two sounds reinforce each other as you continue to move in that direction, with diminishing amplitude from SPL attenuation due to increasing distance from the ambient cancellation sound source.
You might get some useful results at lower frequencies. Maybe more than one cancellation point would help here also.
I once built some resonant bass traps (adjustable helmholtz resonators) for a listening room (not quite the same thing, I know) They worked, but I was a little surprised how restricted the area of effectiveness was if they weren't right near the source of the sound they were modifying.
I once built some resonant bass traps (adjustable helmholtz resonators) for a listening room (not quite the same thing, I know) They worked, but I was a little surprised how restricted the area of effectiveness was if they weren't right near the source of the sound they were modifying.
Well, I have a few applications for this in mind. One is for a noisy fan on an amp - in this case the cancelling speaker will be right near the source of noise, so I think it should be quite effective.
So I can build this thing and try it for different uses.
I'm wondering what sort of output I'll need to make it effective. My subs have very high output down to 20 Hz. As I understand, the sound used to cancel out sound needs to be a lot louder to really work, since if you only match the level, you will only get 6db attenuation.
So I can build this thing and try it for different uses.
I'm wondering what sort of output I'll need to make it effective. My subs have very high output down to 20 Hz. As I understand, the sound used to cancel out sound needs to be a lot louder to really work, since if you only match the level, you will only get 6db attenuation.
Hi Paul,
I've read about the use of active noise cancelling in industrial environments to cancel out the noise from machinery. I also have a friend who worked for Hazletons (regional airline in Australia), and he told me that the were using noise cancelation technology in the turbo prop planes to make the interior quieter for the passengers. So I think that it's possible to do on a larger scale.
I think for the fan it should be "relatively" straight forward (as yu said it's close to the source and you want to cancel all of the noise), but for the other applications I'm thinking you have a whole lot of other problems to deal with. biggest one I see is that you only want to cancel the noise in particular directions, but not where you want to listen. with the sub I'd say it is your entire wall(s) that is radiating the noise, could be a bit tricky.
I think the technology works best if you are canceling the noise as close to the source as possible, stopping it from getting a chance to radiate in the first place (or in the case of the headphone as close to the listening position as possible). It's a long time since I read about it, and they weren't giving away all of their secrets, but I suspect that the were multiple speakers involved surrounding the noise source.
But go for it with the fan, and if you succeed there's no harm in experimenting further
Tony.
I've read about the use of active noise cancelling in industrial environments to cancel out the noise from machinery. I also have a friend who worked for Hazletons (regional airline in Australia), and he told me that the were using noise cancelation technology in the turbo prop planes to make the interior quieter for the passengers. So I think that it's possible to do on a larger scale.
I think for the fan it should be "relatively" straight forward (as yu said it's close to the source and you want to cancel all of the noise), but for the other applications I'm thinking you have a whole lot of other problems to deal with. biggest one I see is that you only want to cancel the noise in particular directions, but not where you want to listen. with the sub I'd say it is your entire wall(s) that is radiating the noise, could be a bit tricky.
I think the technology works best if you are canceling the noise as close to the source as possible, stopping it from getting a chance to radiate in the first place (or in the case of the headphone as close to the listening position as possible). It's a long time since I read about it, and they weren't giving away all of their secrets, but I suspect that the were multiple speakers involved surrounding the noise source.
But go for it with the fan, and if you succeed there's no harm in experimenting further
Tony.
That thought crossed my mind once as well, and I came to the conclusion that the most beneficial application would be some sort of "bass shakers" on the walls that in part cancelled the bass transmitted through them from inside the room. This would only work for closed rooms... wouldn't be much help in keeping the livingroom sound from spreading loudly into the kitchen, for example.
I worked for many years in the field of active noise control, and my brother is still in this field, primarilly designing noise cancelling headsets.
Unless you can get close to the source of the noise, then noise cancelling is effective only over a very small area.
The amplitude of the two noises must match within better than 1dB, and the phase better than a few degrees to get decent cancellation. Clearly in a room where the source of the sound is unknown, then the best you will get is maybe 10dB of cancellation in a region of only a foot or two around the cancelling microphone, and only up to a hundred or so Hz.
If you can get to the source, then you can achieve much better results. I have fitted speakers to a ships funnel, which was radiating over 130dB at 35Hz, and achieved better than 20dB of cancellation.
Of couse with headphones the situation is easier. The enclosed volume of the region where cancellation is required is much much smaller, so cancellation is feasable up to around 1000Hz.
As for the fans, the noise spectrum from these is likely to be in the few hundred Hz range, so not a good candidate for free space noise cancelling.
Andrew
Unless you can get close to the source of the noise, then noise cancelling is effective only over a very small area.
The amplitude of the two noises must match within better than 1dB, and the phase better than a few degrees to get decent cancellation. Clearly in a room where the source of the sound is unknown, then the best you will get is maybe 10dB of cancellation in a region of only a foot or two around the cancelling microphone, and only up to a hundred or so Hz.
If you can get to the source, then you can achieve much better results. I have fitted speakers to a ships funnel, which was radiating over 130dB at 35Hz, and achieved better than 20dB of cancellation.
Of couse with headphones the situation is easier. The enclosed volume of the region where cancellation is required is much much smaller, so cancellation is feasable up to around 1000Hz.
As for the fans, the noise spectrum from these is likely to be in the few hundred Hz range, so not a good candidate for free space noise cancelling.
Andrew
With the fan:
So if I understand correctly, I will have to place the mic in the listening position, and place the speaker as close as possible to the source of the noise? If I do this, do I have a reasonable chance of it working? I think the range is 200 Hz and above for the fan. I suppose I'd want to be able to get 6db of reduction to make it worthwhile.
So if I understand correctly, I will have to place the mic in the listening position, and place the speaker as close as possible to the source of the noise? If I do this, do I have a reasonable chance of it working? I think the range is 200 Hz and above for the fan. I suppose I'd want to be able to get 6db of reduction to make it worthwhile.
The speaker and the mic have to be located close to the source of the noise.
It is precisely the feedback from the mic to the speaker that causes an "acoustic virtual earth" and provides the noise cancellation. You have to be very careful in establishing the loop conditions for the feedback, otherwise you will get instability.
If the noise is repetitive, then other schemes can be used such as predictive optimisation. This is the scheme we pioneered back in the early eighties. However fan noise is too random for this to work.
The idea of noise cancellation is attractive in concept, but as is usually the case, far from trivial to implement.
Andrew
It is precisely the feedback from the mic to the speaker that causes an "acoustic virtual earth" and provides the noise cancellation. You have to be very careful in establishing the loop conditions for the feedback, otherwise you will get instability.
If the noise is repetitive, then other schemes can be used such as predictive optimisation. This is the scheme we pioneered back in the early eighties. However fan noise is too random for this to work.
The idea of noise cancellation is attractive in concept, but as is usually the case, far from trivial to implement.
Andrew
Hmmm.... the whole noise cancelation thing, its not quite my cup of tea. I think for the fan, you should just replace it. Intsead of worrying about an entire system to elimate some fan noise, just get one of those ultra quiet brushless DC motors.
Bass cancelation seems quite possible. What I think you'd need to do is put subwoofers in the sleeping quarters (under the beds) and the microphone/feedback circuit would be right there in the area to be canceled.
You probably woudnt need anything close in terms of output to what you have in the listening area because the bass would be attenuated by the walls and such before it even enters the bedrooms.
all in all the whole project is impractical
Bass cancelation seems quite possible. What I think you'd need to do is put subwoofers in the sleeping quarters (under the beds) and the microphone/feedback circuit would be right there in the area to be canceled.
You probably woudnt need anything close in terms of output to what you have in the listening area because the bass would be attenuated by the walls and such before it even enters the bedrooms.
all in all the whole project is impractical
I'm not going to mess with anything inside the amp, it's the first amp I've bought new, and I've had enough hassles! This thing is still under warranty. Outside of warranty I may consider this, however improvement may be limited by the aerodynamics inside the amp, since there is more to having a quiet fan than the fan itself.
Placing the sub cancelling device under the bed sounds promising. Andrew, any comments on this? Since I can't cancel at the source, since the source is the walls of the entire room, it seems my only choice is cancel at the sleeping location! Do you think it's feasible?
Placing the sub cancelling device under the bed sounds promising. Andrew, any comments on this? Since I can't cancel at the source, since the source is the walls of the entire room, it seems my only choice is cancel at the sleeping location! Do you think it's feasible?
Placing the mic at the listening position and the speaker at a remote location will not work for a feedback based virtual earth noise canceller. This would require feedforward, a whole different kettle of fish.
Placing the sub and mic under the bed should work fine for low frequency noise.
You will have to do a lot of experimentaion to stabilize the feedback loop however. I found in my early work that the natural response of a bandpass loudspeaker works quite well if you have no experience of compensating a feedback system.
Whatever you do, be very carefull. If the system starts oscillating due to poor loop response you are liable to blow your ears, driver or amp!
Andrew
Placing the sub and mic under the bed should work fine for low frequency noise.
You will have to do a lot of experimentaion to stabilize the feedback loop however. I found in my early work that the natural response of a bandpass loudspeaker works quite well if you have no experience of compensating a feedback system.
Whatever you do, be very carefull. If the system starts oscillating due to poor loop response you are liable to blow your ears, driver or amp!
Andrew
Nelson Pass once developed a product, I think it was called the Shadow. It was a bass cancelling device, basically a long tube, with a woofer mounted at either end, and mics right near the woofers. The thing stood up in a corner, so that each woofer was near the floor/walls or ceiling/walls corner. It basically inverted the phase of whatever the mic picked up and amplified that throught the woofer. The inverted phase of the woofer cancelled whatever bass was building up in the corners. I spoke with him about it, he felt that something simple like a chip amp, a panasonic mic capsule, and a simple woofer in a sonotube would work well. Maybe do a patent search for the product. It was well reviewed in the hifi magazines of the time.
Ron
Ron
Just a further question on doing sound cancellation, would this work if i made a small box around a generator and put a speaker inside with a mic to cancel the sound from the inside? or if i had one on the outside as well to give feedback? What kind if circuit would i have to make to do this plus how big would the amp have to be to equal the noise produced by the generator. the box i have now is 30x30 and 28 high with hard blue foam and sound absorbing material inside as well.
Thanks in advance
Thanks in advance
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