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noise cancellation as stealth technology-what do you know?


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These techniques have been in use by the military on certain helicopters for quite a long time. The specifics were classifed and, my guess why you're not finding much detail on the internet, is that they still are classified.

 

Of course, it doesn't make them truly "silent," but it does cut way, way down on the rotor noise. The idea, of course, being that you fly below radar and avoid even on-site detection (by flying quietly) until it's too late for the enemy to do anything about it.

 

Cheers,

Mark

"I don't know anything about music. In my line, you don't have to."

-Elvis Presley (1935-1977)

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I'm with you, Mark!

 

Yeah Phil, I saw that page. It talks about noise suppression, but not noise cancellation, of the Comanche (which has been subsequently discontinued). I saw another blurb somewhere, but can't find it, that someone had tested a noise cancellation device on the Comanche. Using the technology that noise cancelling headphones use, someone used it to "cancel" the sound of the rotors and engine, particularly in front of the craft so that targets couldn't hear it coming.

 

Yeah, Hard Truth, the proliferation of noise cancellation headphones these days added to my interest. As did recent posts that talk about silent UFOs.

 

I want to create noiseless drones to become the ultimate Paparazzi!

 

:D

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Lots of Luxury car manufactors use the Bose technique. An out of phase amp that cancells outside and road noise. What the military did was go to five blades on the main rotor instead of 3 or 4. It was just on The Military Channel last week. Slowly

 "Let It Be!"

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I suppose you could use noise cancellation electronics such as those in noise cancelling headphones with several high power amps driving speakers mounted in the fuselage.

 

This would be novel in a moving object, I guess, but it's done on a large scale in some factories so as to meet OSHA guidelines without workers wearing isolating noise suppression headphones.

It's easiest to find me on Facebook. Neil Bergman

 

Soundclick

fntstcsnd

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Originally posted by DC:

I've ridden in a Blackhawk, a CH-43 and a Sh#thook, and they were all louder than hell. How can you make them quiet?

I thought these were interesting and are relevant to Neil's post. Also, the second talks about how active noise cancellation can also save energy and reduce vibration. This is neat stuff!

 

http://doctord.dyndns.org:8000/Pubs/CSXmuf.htm

http://doctord.dyndns.org:8000/Pubs/energy.htm

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Please oh please let this noise cancellation technology filter over to private and commercial aircraft as a requirement.

 

People like me who unwittingly relocated to an area that is in a low altitude small plane flight path would really appreciate the reduction of the extreme noise pollution.

Just a pinch between the geek and chum

 

 

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Originally posted by Slowly:

Lots of Luxury car manufactors use the Bose technique. An out of phase amp that cancells outside and road noise. What the military did was go to five blades on the main rotor instead of 3 or 4. It was just on The Military Channel last week. Slowly

There is a Japanese car manufacturer that very successfully uses this technology to cancel the engine noise to people outside the car.

 

It's been in use for a while now.

Jotown:)

 

"It's all good: Except when it's Great"

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Originally posted by DC:

I've ridden in a Blackhawk, a CH-43 and a Sh#thook, and they were all louder than hell. How can you make them quiet?

You're thinking of it from the wrong perspective. Doesn't matter a bit how loud it is from inside, what matters is how loud it is outside. Rotor design can lower the sound inside as well, but most noise suppression techniques focus on what happens outside. Besides, a lot of the noise is going to get transferred through the frame...not much you can do about that.

 

Still, as I noted above, I'm not aware of any techniques that make them actually quiet....just quieter.

 

Cheers,

Mark

"I don't know anything about music. In my line, you don't have to."

-Elvis Presley (1935-1977)

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Originally posted by Jotown:

Originally posted by Slowly:

[qb] There is a Japanese car manufacturer that very successfully uses this technology to cancel the engine noise to people outside the car.

 

It's been in use for a while now.

Really? I'm surprised they'd care that much about noise outside (as opposed to reducing engine noise from within the passenger compartment).

 

--Mark

"I don't know anything about music. In my line, you don't have to."

-Elvis Presley (1935-1977)

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Originally posted by Red Winger:

Originally posted by Jotown:

Originally posted by Slowly:

[qb] There is a Japanese car manufacturer that very successfully uses this technology to cancel the engine noise to people outside the car.

 

It's been in use for a while now.

Really? I'm surprised they'd care that much about noise outside (as opposed to reducing engine noise from within the passenger compartment).--Mark
Making the car quieter in general makes it quieter in the passenger compartment.

 

I am not a scientist, but most noise cancelation is done not by "soundproofing" but by actually generating "negative noise" that cancels, or masks the original sound source.

 

A friend of mine who is involved in this field of endeavor for the auto industry (and is also a musician) told me that basically when you look at picture of a sound wave you are seeing the positive sound source. If you invert that signal and play it at the same time as the positive sound source you get silence.

 

I am sure that Bruce, or one of our engineering friends could explain the tecnnical term for this phenomena.

 

It is a kind of yin-yang effect.

Jotown:)

 

"It's all good: Except when it's Great"

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My city of Long Beach just plunked down $6M or so on a pair of "stealth" helicopters (replacing a pair that cost less than a quarter mil a piece when they were new) -- supposedly since that would make them less objectionable to residents.

 

They are, of course, a shabby joke. Much louder from most angles than the old choppers.

 

Yes, if the NR technology is turned on and you're at a certain angles, they can be momentarily somewhat quiet. But as soon as they shift position or the wind shifts, they sound like a devil brigade of threshing machines.

 

This town never met an overpriced boondoggle it wouldn't sink a small fortune in taxpayer money in... each successive one is usually supposed to be a traffic leader for the previous failed boondoggles.

 

So they had to build a wildly expensive aquarium to "draw people" to the Queen Mary (which teeters always on the edge of bankruptcy and has lost scores of millions of dollars since the city bought it and started calling itself the "Queen City" -- amusing our large percentage of gays, no doubt, but drawing no one here).

 

Then they had to help fund a new private shopping center built on what was supposed to be city property (reclaimed from our rapidly diminishing strip of Pacific ocean) just to draw people to the aquarium, which was supposed to draw people in the first place... But who wants to go to a shopping center surrounded by ugly, rusting cargo containers -- which clog what is left of our once-beautiful oceanfront.

 

Next, they'll have to build another boondoggle "attraction" to draw business to the shopping center.

 

 

As our stealth choppers circle far from noiselessly overhead, giving the nice warm, cozy feeling that only an armed paramilitary force in the sky can bring...

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Thanks, Jotown. :wave: I understand the difference between noise suppression and noise cancellation. I'm just surprised that any car manufacturer would do the latter for noise outside the passenger compartment (as doing so would not likely have any effect on persons inside the passenger compartment). Noise cancellation is generally (although, not always) more expensive to implement than noise suppression.

 

But, I'm not up on the auto industry, so it's certainly possible as the technology has been out there for quite some time.

 

Cheers,

Mark

"I don't know anything about music. In my line, you don't have to."

-Elvis Presley (1935-1977)

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Originally posted by theblue1:

Yes, if the NR technology is turned on and you're at a certain angles, they can be momentarily somewhat quiet. But as soon as they shift position or the wind shifts, they sound like a devil brigade of threshing machines.

And that, in a nutshell, is exactly the problem with noise cancellation as I understand it....absent some amazing new (and probably very expensive) designs, it's directional and, for most practical purposes, limited in range.

 

--Mark

"I don't know anything about music. In my line, you don't have to."

-Elvis Presley (1935-1977)

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Originally posted by Jotown:

Originally posted by Slowly:

Lots of Luxury car manufactors use the Bose technique. An out of phase amp that cancells outside and road noise. What the military did was go to five blades on the main rotor instead of 3 or 4. It was just on The Military Channel last week. Slowly

There is a Japanese car manufacturer that very successfully uses this technology to cancel the engine noise to people outside the car.

 

It's been in use for a while now.

My 11 year old Corolla is stil so quiet you can 'sneak up' in back of pedestrians in a typical city alley or street. When it was new it was much quieter still. (And I guarantee you it has no added noise cancellation features.)

 

 

But then there are those yuppie assholes who pull the baffles out of the tailpipes of their $35K "hogs" in some weird demonstration of their putative manhood... poseur wannabe jagoffs.

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A friend of mine who is involved in this field of endeavor for the auto industry (and is also a musician) told me that basically when you look at picture of a sound wave you are seeing the positive sound source. If you invert that signal and play it at the same time as the positive sound source you get silence.

 

Inverting phase. The wave peaks of the one track will align with the "valleys" of the second waveform and cancel out. You can experiment with that on your DAW. Take a track and record a sine wave on to it. Then copy that track to another track and flip the phase. Make sure both tracks are center panned. If the amplitude of the two tracks is identical (IOW, the faders are set to the same levels) the signal will cancel out. If you flip the phase back, the signal level will increase compared to just running the single sine wave track.

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Many plug ins (but by no means all) have phase reverse switch on them. The least CPU intensive way, if all you want is the phase reverse, is probably via the trim plug in... and you'll see a little circle icon with a diagonal line through it on that plug in. Depress that icon / switch, and phase is reversed. :)
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Originally posted by theblue1:

Originally posted by Jotown:

Originally posted by Slowly:

Lots of Luxury car manufactors use the Bose technique. An out of phase amp that cancells outside and road noise. What the military did was go to five blades on the main rotor instead of 3 or 4. It was just on The Military Channel last week. Slowly

There is a Japanese car manufacturer that very successfully uses this technology to cancel the engine noise to people outside the car.

 

It's been in use for a while now.

My 11 year old Corolla is stil so quiet you can 'sneak up' in back of pedestrians in a typical city alley or street. When it was new it was much quieter still. (And I guarantee you it has no added noise cancellation features.)

 

 

But then there are those yuppie assholes who pull the baffles out of the tailpipes of their $35K "hogs" in some weird demonstration of their putative manhood... poseur wannabe jagoffs.

Almost any car is quiet when its almost idling. How'd it sound at 65 MPH with "ALL SEASON" radials tires. Slowly

 "Let It Be!"

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Well, my Corolla's probably quieter on the outside at 65 MPH... :D The wind seal is still fine -- but the road noise definitely comes up through the little beast. I've been keeping my tire purchases low key on this car.

 

My last car was a Celica and I put a $600 set of high speed, wide radials on it. It stuck to the road like glue. But, yeah, the road noise was loud on the f'way.

 

Until it was stolen a few weeks later, the theives undoubtedly drawn to the older silly car by those still-black and nubby tires... When they found it, it had four "rollaways" (tiny trunk spares). It was totally trashed. They even stole the seats (leather). The only feature they didn't steal was the sunroof...

 

Sigh.

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