Sound Canceling - Project

[keyboarder89]

Hey everyone,

I am doing a science fair project, or at least attempting to put together a science fair project on sound canceling (not just noise canceling) and I want to know if anyone has any ideas or feedback on this project. I would like to try to cancel sound out in a room so that all I hear is silence. I will probably buy a pair of noise canceling headphones or build my own circuit that I found online, and connect a better microphone to the circuit (like a Shure XLR or something) and run the preamp out to a PA system. Anything I should be carefull with? Any feedback what so ever would be helpfull.

Thanks

[Bill Fitzpatrick]

Originally posted by keyboarder89
so that all I hear is silence.

!

So you want to sit in an otherwise noisy room and have a device in the room which picks up the noise and cancels it so you hear nothing?

What do the headphones have to do with this project?

If you succeed you will be a millionaire many times over.

[Cal Weldon]

Bill,

Isn't kb89 asking about that inverse wave thing where you are inverting the phase and amplitude of every received signal and adding that to the mix, effectively cancel out the sound? I saw something somewhere on that. I believe they employ that on some Police helicopters no? Not complete cancellation but many dB reduction.

Cal

Edit: I don't think I mean invert the ampitude

[Bill Fitzpatrick]

What he wanted wasn't all that clear to me. Anyway, I'm sure that complete cancellation is not possible.

Perhaps a pair of earplugs with the outside being a mic and the inside a speaker.

Keyboader89, have a done a complete search on the subject?

[GRollins]

Okie dokie...here's a starter.
One, you will not be able to cancel all the noise in a room. You will only be able to reduce the noise in the area near the microphone/speaker set.
Two, the strategy works best at low frequencies. Somewhere in the hundreds of Hertz range you will lose control and the noise cancelling circuit will become less effective.
Three, the technique works fairly well in headphones (to the tune of 30, maybe 40 dB noise reduction), but mainly because the environment is small and well defined (the volume between the headphone cup and the ear).
Four, commercial products exist. Headphones exist now from several manufacturers. In the past, Nelson Pass of Pass Labs had a bass-cancelling device called The Shadow, though it is no longer in production. He received a patent on the device. See www.uspto.gov for details.
Five, Arthur C. Clarke had a story about such a device long ago. I believe it was in Tales From The White Hart, but don't have time to check that at the moment. No, the system will not blow up. Clarke was just having fun with the premise.
Six, great idea for a project. Have fun.

Grey

[audio-kraut]

There was actually a project in a montreal neighbourhood two years ago, where they used loudspeakers for noise attenuation during a road construction project that had to be done at night. Helped somewhat from what I saw at the discovery channel.

[kelticwizard]

I believe they use the system on airplanes to quiet the sound of the jets inside the plane. But there, the noise to be cancelled has a source outside the passenger compartment. The way this science fair project is described, the noise comes from inside the room, such as people talking.

I believe he has a better chance of success if,instead of cancelling the noise coming from inside the room, he has a loud sound outside the room and and he uses his idea then. The noise originating from inside the room should be soft, or silent.

[keyboarder89]

Hey everyone,

Thanks a lot for the feedback. I am still working on research for the project. If I were to try to cancel sound from outside of a room (like my younger sisters singing), wouldn't there be a delay or something? I mean, would I place a mic outside of the door to a room and place a speaker in the room I dont want sound in and cancel it using the amp and the phase inverter? I was talking to someone today about the project and he said that I would need to set up two speakers and sit in between the two speakers (at equal distance between them) in order for everything to cancel out to the sound picked up by the microphones. This person is also lending me his oscilloscope. I also was thinking, if I somehow added something like a few crossovers to the circuit, could I divide up the frequencies and cancel the sound more precisely? I am not sure if that is what I would need to use, but could it work? I guess I should try it, haha. If my project is still unclear to anyone, I would like to sit in a room and be able to study without lots of noise from the syblings running around and singing, for example. As far as variables for the project... any other ideas besides placing the mic, speaker or listener at different places?

Thanks again,
Zach

[Circlotron]

Originally posted by kelticwizard
I believe they use the system on airplanes to quiet the sound of the jets inside the plane.

Back in the days when aeroplanes where driven along by =seriously= big piston engines the pilot had a control of sorts where he could not only synchronize the left and right engine to the same speed to avoid any beating or heterodyne effect, but could also phase-lock them and twiddle the phase relationship of the two engines for minimum sound in the cockpit.

I also read recently where Lotus cars have a setup where anti-noise comes out of the radio speakers to make the passenger compartment quieter.

[Franz G]

Maybe, this is interesting:

http://www-cgi.cnn.com/TECH/9601/quiet_planes/

As far as I know, the Saab 340B and the Dornier 328 had some sound cancellation systems.

Franz