[andrzejpw]

Ok, now that I've got your attention. . .

Don't worry, this is a question about bose. Actually, a technology that they use.

We've all heard about the $300 bose noise cancelling headphones. Laughed at them, etc. But, the technology got me thinking. I'm guessing this is totally wrong, but here goes.

Supposedly, the noise canceling part works something like this: it "hears" the noise around you, and it produces a wave that is "negative" to it. right? So, that got me thinking. Say there's a steady noise that you want to block out. Something, like, say, an airplane's engines. What if you took a mic, recorded the cabin sound, put it in cooledit, and took the negative?

If you were on an airplane, and played that sound, would the engine sounds be blocked out? I'm sure it wouldn't be as good at canceling as the bose, but would it work? And, on top of that, could you play, say music, or a movie?

Now, I'm SURE this is all wrong. . . anyone?

 

[skippy]

to keep things simple, in some spots the certain frequencies of the signal would be out of phase with the noise, and you'd get some cancellation. at other spots certain frequencies of the signal would be in phase with the noise and the noise could be amplified.

 

[stallion11msu]

I saw something similar on a television program a while back. Some airline techs mic'd the engine, reversed the wave, then played the result back in real-time through three speakers mounted in front of the passenger cabin. They didn't say how well it worked, though it hasn't showed up in any planes as of yet which makes me skeptical.

 

[andrzejpw]

ahh, so my idea isn't great and awe-inspiring?

I'd like to try it sometime though. . .

 

[eric343]

There's actually an article in the HeadWize library about building a noise-cancelling amplifier...

http://headwize.com/projects/showpro...=noise_prj.htm

 

[Dusty Chalk]

The problem is, "noise", by definition, is variable. One way to look at it is that it is the sum of all frequencies. Another is that it is the sum of all frequencies of constantly varying phase.

Short answer: it needs to be real-time.

The speaker problem doesn't work because...well, speakers work differently in different places (I.E., if they are nearfield speakers, then they are most accurate in the near-field; if they are "normal" speakers -- "far field" -- then they won't work in the near field, etc.). If you put the microphones exactly where the speakers are, facing the opposite direction, you might be able to cancel the sound out that is right there, but that wouldn't help the people in the back of the plane.

 

[Clubkill]

Yea, real-time only.

Cancelling noise in the controlled environment of headphones is one thing, doing it in the cabin of an airplane and making it work for each person in the cabin is another

I did read an article that along with physical modificaations to the wings etc. one company did plan on using cancellation to reduce noise. It gets complacated when you have multiple speakers etc. and a cabin that large though. It is possible that if you are getting crossfeed from another speaker not close to you that you could actually amp the noise, so it is not as simple as it seems. I imagine that te first real efforts might be in rich-people planes (i.e. the concorde, private jets, air force one, etc.). If there is a significant improvement in the technology and it is fiscally feasable, maybe it will show up in more planes for the general public.

 

[Satori]

Everything i have read about noise cancelling is that it will amplify the signal except for the targeted location. It does need a mic to be able to provide the cancellation wave due to variations in the frequencies.

But the concept of Nulling out a certain frequency is in use today for many applications other than audio equipment.

 

[andrzejpw]

well, I mean, I don't want the effect for the entire plane. I just wanted it for headphones, etc. IS there any difference?

 

[eric343]

Heh, you just earned yourself a Custom Title!

 

[andrzejpw]

lol, I noticed. Finally! I feel like a real boy!

 

[RMSzero]

This wouldn't work. As they said, you'd have to have all the sounds in phase. To give you an idea about how complicated this would be, airplane noise contains components in many many different frequencies. The thing about "making the wave negative" is that for a single consistent frequency, making the wave negative is the same as a 180 degree phase shift. So by making the wave negative, and then playing it back with an incorrect phase shift, you will amplify the sound.

The problem is that you have to get all the phases of all the frequency components correct in order to cause an overall cancellation. Not only that, but the guys above me said that noise cancellation only works in a specific place: that goes for the microphone too -- for instance, noise cancellation headphones would not work if the microphones were not at your ears.

Anyway, the short answer is no, that wouldn't work.

 

[Audio Redneck]

I remember back in the early 80's, a TV show ("That's Incredible" maybe?)was showing a car with a noise cancelling muffler - muffles the sound and no restriction to air-flow like a regular muffler. Haven't seen anything about it since. I can imaging how long the drivers would last with exaust constantly passing near them.

 

[gerG]

Man oh man, is there ever a market for Bose cancelling headphones. Just to be able to walk through Fry's electronics and never hear a word that their planted sales rep says...nice. Come to think of it, ear plugs would not be a bad idea in that place in general. It would also filter out all of the guys trying to impress their girlfriends by turning up the subwoofers really loud back in the car-fi section. What is that all about?

As above, noise cancellation needs a real time input of what you are trying to cancel. That means a mic very near the location where you want to cancel, which is your ear (one mic each). You also need a processor to reverse the phase. Easy at low frequencies, but it gets tougher as the wavelength gets shorter. Finally you need a speaker to emit the inverted signal at the same amplitude as the noise. One final trick, the microphone needs to be isolated from the speaker, otherwise it will feed back and oscillate or go unstable. When you look at the requirements, cans are a natural for this. The speaker is at the ear, minimizing phase inaccuracies. The microphone can go on the outside so that it only hears the noise, and not the sum of the noise plus cancellation signal.

I don't understand why nobody has built a high end NC headphone. Most of the ones out there don't even do a good job on the infrasonic frequencies, which are a problem even if you can't directly hear them.

I gave up my quest for these things when I discovered Etymotics. Now I am on a quest to find a portable mixer that will let me selectively control the external sounds that I want to let in. Unfortunately I am not sufficiently skilled (or patient) to build such a device.


gerG

 

[Joe Bloggs]

Hmm... on a related note (?), Fortress, which I guess would be the local equivalent of Fry's here in HK, is actually quite nice. In the big shop in Times Square they have a HiFi section, and they actually have some decent stuff there. (while cheap crap is sold outside) I looked at a NAD C520, asked the sales rep there if they have some more up-to-date and higher model stuff from NAD, and he pointed me to the S500 selling for ~$800!