Active Noise Cancellation / Soundproofing

[jonnyb]

I would be very interested and grateful for guidance on how viable it would be to contain a noise in a relatively small area.
If the aim was to contain a noise through passive and active noise cancellation and the noise source could be contained within a small area, could the technology used in some on ear headphones be used to effectively contain/cancel a noise in a headphone ear piece?
So if a small speaker were to be placed under a headphone earpiece and a sound played ... would the noise cancelling technology in a typical set of headphones be able to help cancel that sound out within the headphone in which the small speaker is so that is not audible outside the headphone?
I am not as knowledgeable on this subject as I would like to be yet and so please forgive me.
I appreciate that the magnitude of the sound being played is relevant but it would be less than 10db.
Kind regards .. Jon

 

[yungman]

Electronics wise, it is piece of cake in the sense you mic the noise of the area and invert the signal and play it back out. You will need to do some phase adjustment in the circuit to compensate the mic and speaker characteristic. BUT the difficult part should be the acoustics. Sound can be directional and it has node and anti node depends on where you listen to it. That really get complicated. I worked on some active noise cancellation, but not have to deal with acoustics. You characterize the room, electronics is not bad. That's all I can offer as I don't know anything about acoustic other than set up my hi-fi system.

Ear phone don't have to deal with acoustic because the speaker is by your ears. The electronics should be easy. Your's is much harder. I suggest you look into Acura RL where they have active noise cancellation inside the cabin. Maybe look at some patents.

 

[jonnyb]

Thanks for the quick reply. I think I need to create a different scenario. Let's say that the noise producer is not a small speaker but a small but quite loud frog that is sitting comfortably inside the speaker headphone which is pushed against a flat surface. (No frogs will be hurt or treated badly in this scenario!) I realized that I don't want to confuse things by having an example where the noise source was electrical and therefore could be part of the circuit. Can the existing active noise cancellation tech that is incorporated in a noise cancelling headphone be modified to help dampen the sound of the noisy frog to the world outside the headphone cup.

 

[Mike_In_Plano]

I think elementing the frog's sound would not be possible because it would defy causality:

If the frog were to start talking, some time would transpire before the walls of the headphone experienced what he had done. Then, if they react, they are doing so to a noise that reached the listner already.

In the case where the frog is outside, his influence can be detected, placed through a signal processor that reacts to generate a signal, with the correct phase and magnitude, to essentially null Mr Frog's message.

The difficulty comes when you realize that the sensor, container, and speaker all have unique reactions to the various frequencies the frog can produce. Thus there is no easy solution to how you sample the noise and create an appropriate response. That's when you introduce a monitor on the listener's side, and a recursive algorithm continually varies aspects of the processing and judges success by how well it reduces the noise. So, Mr Frog may start out to be a nuisance to the untrained system, but it learns how to react over time.

 

[jonnyb]

That's very interesting. So basically it's not possible to effectively cancel the noise of the frog within the headphone space before the noise reaches the listeners side.
The process would have to be applied from the outside and even then it certianly wouldn't be simple.
A shame .. In my ignorance I thought that the process could be applied within.
Thank you for your advice.