Noise cancelling device, making one

[adlfurball]

This is an instant idea that I got right after i read an article about noise. Do you think it is possible to make an noise cancelling headphones by using the LM741. Except, i won't be amplifying the signal from the inputs (i can if choose to by just adjusting the varistor) but by using this op amp, I am able to invert the signal and somehow broadcast it back to the listener. The concept of destructive waveform is what i have in my mind.

Havent done much research yet but suspect there might be some trouble in between such as the delay that might have occur during the inversion and amplification.

legit?

 

[axemaster]

I built a similar system a few months ago so I might be able to help you. I'm a little unclear on what you're trying to do though. Are you trying to cancel external noise so you can listen to music in a loud environment?

 

[adlfurball]

Are you trying to cancel external noise so you can listen to music in a loud environment?

Yes, exactly! You've built one? would you mind sharing some information about the system you built?

 

[adlfurball]

I'm not trying to steal other people's work, I just wanted to learn more and it is easier to ask a person who have built it himself than spending hours reading articles that i won't understand and unclear if it even works.. just an opinion :)

 

[sophiecentaur]

This is an instant idea that I got right after i read an article about noise. Do you think it is possible to make an noise cancelling headphones by using the LM741. Except, i won't be amplifying the signal from the inputs (i can if choose to by just adjusting the varistor) but by using this op amp, I am able to invert the signal and somehow broadcast it back to the listener. The concept of destructive waveform is what i have in my mind.

Havent done much research yet but suspect there might be some trouble in between such as the delay that might have occur during the inversion and amplification.

legit?

This sounds a very feasible project. For audio frequencies, the electronics would be fairly straightforward, it would not be hard to provide your inversion and amplification with negligible phase error. The only problem would be that the microphone etc. which senses the external noise would need to provide the same spectral distribution as that of the mechanical noise that gets through the headphone shell (that's what you need to cancel). The microphone must also be insulated from the wanted programme signal or it will cancel that too.
You could save yourself a lot of development time by looking at existing noise-cancelling phone design and to see where they put the microphones. It seems to me that you could almost benefit by housing each microphone in an exact duplicate shell, right next to the one on your ear - but that could look a bit bizarre. For the best cancellation at high frequencies (short wavelengths), you need the sensing microphone to be as near as practical to your ear.

Looking at what I have written made me reaslise that there is an alternative approach and that is to have a microphone right inside the can and use feedback to make the sound in the can track the electrical signal. There would be no phase problems with that method.

 

[axemaster]

I will get back to you with a detailed post on how to build this in a little while; right now I'm at work.

 

[axemaster]

OK! I threw together a relatively simple circuit design for you. Keep in mind that this is intended as an example - the resistor values are pretty much random. Also, I designed this circuit for single supply operation, meaning it can operate from a battery, which I assume you would like.

A few other comments:

The 741 op amp isn't good for audio because it has a lot of noise and will sound pretty bad. Something like the OPA2134 should work much better. Here's the datasheet.

http://www.ti.com/lit/ds/symlink/opa134.pdf

A few notes:

- when it says +/- 2.5V for the supply, that means that a single supply must provide at least 5 volts
- eventually you may need to find an op amp with less quiescent current, depending on the current needs of your headset. Quiescent current is the constant "on-current" of the op amp even when it isn't doing anything, and it will slowly drain your battery.

As for the physical design of the headset, you want to set up the microphone as close as possible to the speaker, facing outwards from your head. You can find a suitable mic here:

http://www.digikey.com/product-search/en/audio-products/microphones/720968?k=microphone

Be aware that most microphones will need power to operate, and some sort of special subcircuit. The details should be in the datasheets - they usually do a pretty good job of telling you how to set them up.

In general you will only be able to cancel low frequencies, where the distance between the mic and the speaker is much smaller than the wavelength of the sound. That's why even Bose sound canceling headsets have the foam insulators - to get rid of remaining high frequencies.

Well, I think I managed to cover many of the most important issues. If you have any further questions, feel free to ask.

 

[adlfurball]

Thanks for the information that you gave, I've certainly learned something, a new IC for audio amplification OPA134 ( can't help but.. OPA GANGNAM STYLE :D ). However, I won't be building this system unfortunately since in Malaysia, lack of electronics component shop gives me a headache. But these infos are really handy when i needed to do some other projects. THANKS!

 

[axemaster]

I don't know how shipping works in Malaysia, but you are aware that the most convenient way to get parts is to order online through Digikey, right?