Questions

[Drizzt]

What's the problem with my suggestion? It's a well known way of reducing noises...

[Dogbert]

Drizzt,

I understand it's some sort of noise cancelation thing.
Why two mics ? Does it really work ???

Nuno,

I don't have much choice here for heatsink/fans so I took the best they had in the store.
In the worst case I'll pad my case's door with a noise absorbant.

[Marshmallowman]

aviation headset use noice cancelling microphones..

and there is an electronic device called a woofer stoper, its a noise cancel mike/speaker on a dog collar to stop dogs from barking!!.

I have often thought about making one for my car..

[Drizzt]

It's a well known system to reduce noise:

In an ideal world, the mic receive a sound and pass it to the op, the op invert the phase of the signal and pass it to the speaker.
The output sound from the speaker, being in (shit, is "counterphase" correct?), nullify the initial sound.

It's really simple to build, the complexity lies in making it work well; there is not a general rule about it.

For example, if you have your PC a bit far from you, you may want to use a directional mic located half way from the PC and you.
But this would be efficent just for you while other people in the room would not beneficiate too much, unless the PC is surrounded in three way by a wall, in wich case the directional mic would receive everything and really nullify all the noises...

Instead, if you have the PC in the open, the best way to use a system like this is having a mic on each side of the PC (well, should be sufficient on the left, front and back side cause on the right there is the mobo absorbing noises and on the top, if you have a decent case, there's so much metal that just do a decent shield), each mic with is own circuit and speaker. Obviously the mic is turned to the case and the speaker is turned from the case.

And we can stay here all the day speacking of this, but my english is beginning to suffer...

[VJ]

Drizzt:

I too like the idea (was aware of the concept; Sony makes such noise-canceling headphones).

But did you actually manage to get such a setup to work ?

Perhaps a strange question: where did you get such a sound-inverter (or did you use your soundcard) ? (my 4x80 mm fans and 2 10K drives are making a lot of noise...) Do you have links about constructing this ?


Jörg

[Drizzt]

Originally posted by VJ
Drizzt:

I too like the idea (was aware of the concept; Sony makes such noise-canceling headphones).

And there are car manufacturers that are experimenting this to reduce engine noises...

But did you actually manage to get such a setup to work ?

Perhaps a strange question: where did you get such a sound-inverter (or did you use your soundcard) ? (my 4x80 mm fans and 2 10K drives are making a lot of noise...) Do you have links about constructing this ?

This is a painful argument...for the first and last question, the main problem is that I have made up something like this a few years ago, and now I'm a bit too rusty about electronic projecting to have the ability to write down some circuitry.
Perhaps some other Murcer is enough able with electronics to produce something.

About the middle question, "where did you get such a sound-inverter":
There is no need to use the sound card. In elec. exists a simple component called operational amplifier capable of huge number of operation on one or more signals. For example, it can make + or - between two signal, it can produce the logarithm of a signal or it can be used to amplify it. So, if you amp a sound for a factor of -1 you simply reverse it phase.

The nice thing about op-amp is that their cost can be very low - one good op-amp can cost 1 €

[VJ]

Thanks !
(this might be something to try when I have a little more time )

Jörg

[Greebe]

Drizzt et al, it does NOT work. The phase shifting that is induced, the affect of transient responce because of mass of the microphone/speakers to name a few reasons it won't.

Bose's noise cancelling headphones reduce alittle (3-5dB max) but this is all designed with specific parameters considered. Special filters reduce noise in a specific range, special feedback amp is employeed, as is a specially designed microphone and it's placement. Also remember this is within the closed space within the ear cup (sealed back).

Low frequency periodic common mode noise can be reduced in ductwork/factories, but this is well below the range users are speaking of here.

[VJ]

Well, I have been to technology demos where similar setups was actually working. One was the construction of a "silent corridor" : it was just like a corridor, but when you stepped inside, it became quiet (not completely, but the noise was significantly less).

And yes, I doubt it is possible to get this working at home (most likely it won't), but it might just be nice to experiment with.


Jörg

[Marshmallowman]

I have seen small ones working on dogs!!!, It doesn't complety stop the sound but does a worthwile job..