Cancelling Only the Noise from Afterburners

Instead of the headphones which cancel all outside sound navy jets could de ploy a microphone under the jet just after takeoff and then radio the signa l back to troops on the ground who wanted to attenuate the jet sound but no t those trying to talk or hear other sounds. A dedicated receiver headphon e would be necessary.

The signal could also be sent to the long suffering civilian residents of S an Diego and Va. Beach.

Bret Cahill

Reply to
Bret Cahill
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Why not just send them a CD of the sound?

That'd work be just as badly for sound cancelling, without providing a signal for homing AA missiles.

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Reply to
Jasen Betts

One problem with that is that the radio signal travels much faster than the sound signal, and the sound arrives at different locations at different times. It will also have been distorted by reflections by the time it reaches most people, so even if you could solve the time-delay issue, you'll end up causing more noise, not less.

Reply to
Daniel Pitts

In addition, the idea of picking up the sound from a microphone under the jet just after takeoff is a non-starter. What makes it "noise" (as opposed to a "buzz" or something more tonal) is that it is highly random. The sound at one instant in time (like just after takeoff) would not correlate (eg be cancellable) with the sound at any other time.

Also, in addition to the reflections Daniel notes above, the sound at any given listener will depend upon his orientation relative to the source. It's not a point source, so a single mic will not work unless it is near the listener. Near the source you'd nead a large array to catch all the separate emitters. Just like an array of speakers, the sound at any given angle and distance is a vector sum of all the little sources cancelling and reinforcing differently according to their instantaneous frequencies.

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

Don't expose Bret to science. It confuses him.

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John Larkin

d deploy a microphone under the jet just after takeoff and then radio the s ignal back to troops on the ground who wanted to attenuate the jet sound bu t not those trying to talk or hear other sounds. A dedicated receiver head phone would be necessary.

of San Diego and Va. Beach.

Match filter the noise signal. When the noise arrives and you have a match you know you have a few nano seconds to start cancelling a m sec of signal .

The reflections are, of course, specific to the area.

These training flights aren't really interesting. Depending on the wind di rection the jets fly the same path every time.

It should be possible to develop a reflection adjustment for each path and incorporate the reflections into the noise cancellation.

Bret Cahill

Reply to
Bret Cahill

ld deploy a microphone under the jet just after takeoff and then radio the signal back to troops on the ground who wanted to attenuate the jet sound b ut not those trying to talk or hear other sounds. A dedicated receiver hea dphone would be necessary.

of San Diego and Va. Beach.

But the changes aren't random. They can be predicted.

The happy fact is we can know _everything_ that is going to happen to that noise wave over Seatack or Pt. Loma.

The navy might be able to afford a few mics/$80 million aircraft.

"They put up a stop sign. I take it down."

-- Satchmo

Reply to
Bret Cahill

The changes I'm talking about are in the source, due to the way a jet engine works. It's pretty much a pure random noise generator, due to turbulence in the engine. It's not likely this will *ever* be predictable, with any amount of computing power.

So what are you proposing? Suppose each engine of each jet drags a big ring of mics behind it, to catch sound from multiple angles. Somehow, they will not interfere with ariflow or create drag, and will be able to withstand the temperatures.

Then what? A listener at an arbitrary (and constantly changing) distance and angle would need to get a continuous feed from each mic, along with accurate instantaneous position measurements. Then it could compute the instantaneous vector sum for that angle at the source. Then it would need to compute the arriving waveform at the listening location, using its own built-in weather radar to compensate for layers of different windspeeds, temperature, humidity, pressure, etc... all of which affect propagation delay.

But let's say we knew this *exactly* for the point in space where the listener's head is. Oh, but wait a second... he has *two ears*, facing different directions and separated by a significant distance (in terms of the wavelengths we want to cancel). No problem, we just double the amount of equipment the guy has to lug. Ahh, let's see... how much does *one* IBM "Deep Blue" weigh, anyhow? (Not to mention the portable weather radar station...)

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

The noise from the engine isn't going to predicted. It's going to be recorded in short increments and radioed back to the ground.

GPS is faster than sound.

Obviously you don't want to broadcast the jet's location to a sophisticated foe but we hardly ever fight anyone with any tech other than RPGs.

And for civilian areas which like to be in the same real estate market as the navy, i.e., ocean front property, broadcasting GPS data would not be an issue at all.

The OP which was deliberately shortened to save time. It's not trivial. But it isn't impossible or even prohibitively difficult either.

There are many issues/corrections that can be factored in with every day weather data, flight data, terrain data like overpasses, piers, etc.

It's a fun engineering problem that may yield other completely unrelated inventions.

Conventional noise cancellation headphones do not eliminate _all_ the noise, yet Walmart has Sonys on sale for $50.

Why is the standard perfection here?

"Don't make the perfect the enemy of the possible."

Reply to
Bret Cahill

You're spot on saying that 'active' form of cancellation is a bit complex. [hi, Bob!] The microphone functions can be replaced with other ways to measure 'acoustic disturbance' without the drag, etc, but not very important unless you can figure out what to do with the information. As you point out, even with 'total' information cancellation for ALL is a bit daunting compared to cancellation for ONE. A lot to do with the dispersive nature of atmosphere and the placement of all those audial 'receivers'

I proposed a method a few years ago for mitigating landing and takeoff noises around commercial/military airports. It requires some humongous electrical power, but can drop noise for EVERYONE outside by over 16-30 dB. Doesn't 'sound' like much, but sound is a bit insidious, once made difficult to get rid of, and usually your only hope is blocking/reflecting.

So, what other ways are used? From memory I believe there are some techniques done right at, or near, the source, very imaginative solutions too, but the details are still classified. And don't know what, if any, is the overhead cost to flight performance is.

Reply to
RobertMacy

I think you still aren't "getting it" here. There isn't a single "noise" coming from the jet. It's the sum of all the little emitting regions in the turbulent flow. That means that the waveform (the thing you want to cancel) is different at every angle from the engine, due to constructive and destructive interference from all those millions of emitters . If you hang a single mic somewhere to try to record "the noise", it will only tell you about the waveform at that one position... totally worthless in predicting what listeners on the ground will get.

And what they get is continually changing with the relative position of the passing jet. Perhaps you have heard the phenomenon called "jetsounds" or "flanging" in music, or noted the phenomenon they are named after at an airport. As a jet moves relative to the listener, he hears a hollow "sweeping" effect that isn't present when the jet is stationary. That's due to his ears getting the sum of the direct sound, plus delayed versions reflected off the surroundings (runway, buildings, etc). If there is a single reflection plus the source, the resultant spectrum will have a series of dips and peaks called "comb filtering", due to the interference at wavelengths that are multiples of the path difference. (See for a more detailed explanation, and a link to using the free Daqarta generator to demonstrate it.) One curious thing about this is that you only notice this while there is motion, and the dip-and-peak pattern is changing... you don't notice the dips and peaks when they are static.

Anyway, that effect changes the waveform, and it's that waveform that you need if you want to cancel anything.

I suspect you are thinking of a tonal source, especially when you mention "recodred in short increments". Yes, if the jet engine sound was (say) a few strong tones and harmonics, and they changed only slowly with time, then there might be a glimmer of hope for your proposal. The ground listener's equipment could get a list of frequencies and amplitudes by radio, and set up a matched filter to cancel them, and only them. This would not use the same principle as noise cancellation, which requires the exact waveform and simply subtracts a gain-adjusted version from the local sound.

But a matched filter can't work with a broadband noise source like a jet engine, where there is energy at all frequencies, and everything is constantly changing. You have to use subtractive cancellation like the noise-cancelling headphones use, and that requires that you know the exact waveform of the incoming sound *at the instant it arrives*. There is no way to get this sent from the jet... it doesn't have "the waveform" to send. There is no way to compute it from one or even quite a few mics at the jet, even if you had a perfectly-known air column and no reflections. Even if you knew the exact distance, etc, etc.

So, why attack the impossible? If the need is to cancel the jet noise for troops on the ground, a standard noise-cancelling approach will do the best job, with mics on helmets to pick up the ambient noise. If the troops need to talk among themselves, they can use lip mics or even throat mics. This is a tried-and-tested approach.

If you *really* want to hear ambient sounds on the ground, the only hope would be to have an upward-pointing mic as input to the canceller, and hope that it didn't pick up too much of the ground sounds. This would only have a chance if the jet was way overhead, so that the arriving sound appeared to be coming from a distant point, and not too many ground reflections. But then you probably don't need it!

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

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corded in short increments and radioed back to the ground.

The sound from an emitter on one side of the noise generation volume will o bviously take longer than that from an emitter on the other side to reach a mic on the other side. It will also be somewhat fainter. Both these fact ors will certainly give a different wave form for each direction just as st ereo has 2 different wave forms.

One difference with stereo is the time lag from different emitters isn't ch anged. Here we adjust the lag according to direction of the person on the ground.

The navy ought to be good for a 3 - 4 mics / $60 million aircraft. Then it 's possible to back calculate numbers for everything about every emitter.

But we don't need numbers for every emitter here any more than in stereo re cordings.

The navy should know the speed of the aircraft well enough to correct for D oppler.

We know every last detail about the surface, the exact location of every hi ll, bridge, jetty, street and house.

A more difficult problem might be reflections off of fast moving clouds but the navy should have real time information on that as well.

And we know exactly how it changes every waveform.

Are a lot of mics and calculations necessary for the diva to sound like she 's in center stage in a stereo recording?

This one is good for more than noise abatement. It may lead to other probl ems to solve.

Reply to
Bret Cahill

Sorry, but if you were trying to cancel a stereo recording

*at the listener* using only information from the *source*, you *would* need the waveforms of each emitter (each speaker), *and* their angles and distances, as well as all relevant info about intervening transmission/propagation effects. That's very different from cancelling using mics at the listener.

We haven't even gotten around to Doppler effects yet. We're just talking about relative phase changes due to angle and distance. (Not even including the non-trivial effects of atmospheric phenomena on transmission.)

Huh? Dunno about you, but not even the NSA has a fraction of the "details" needed for this sort of thing. These are constantly-changing details, dependent on every surface and the relative angles of source and listener. It's way more involved than the seemingly-similar computer-generated graphics problem, where you "only" have to know millions of angles of incidence and reflection and a few intensities... those guys don't have to worry about keeping track of waveforms. And you may have heard about the huge computing power and time they use.

They would be if you were proposing to have her swinging thousands of feet overhead at hundreds of miles per hour, with an unknown atmosphere in between, and unknown reflection sources on the ground, and arbitrary listener positions. (Actually, the stereo effect in most recorded music is generated strictly by relative amplitude differences at the ears, without regard to timing/phase differences. That's how recording engineers can change perceived position so easily... they just fade one channel up and the other down. That won't work for cancellation.)

You mean problems like understanding sound propagation, wavefront interference, and random sources?

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

Reply to
Bob Masta

that's why I hate recorded music! well, sloppily recorded music!

Thanks. Didn't know what was causing the 'shallowness' and lack of dimension in a recording.

For example, I once listened to a Direct Recording [which means unadulterated and not homogenized by some sound engineer] of an old guitar player. The flash of insight into the performance was incredible. (You can't do this purposely has to ome like when you're daydreaming while listening), but you could envision the whole room, size of room, where he was sitting, even openings, doors, etc Absolutely, amazing! Only after hearing the recording and experiencing that 'visualization' did I get a chance to read about the history of the recording. The text pretty much described what one 'heard'

After that experience, it's really sadly thwarting to hear the way music is butchered, compressed, mixed, etc etc

Must explain why rap gained such a large audience. At least with speech, it was more difficult to remove the 'character' coming through.

Reply to
RobertMacy

See the post just above this one complaining about flat stereo recordings.

They haven't given out the details but the navy now has a ship based IR laser weapon.

They must have done a lot of research on the various atmospheric situations and how to monitor those conditions where it'll work.

Reply to
Bret Cahill

NASA once hired a symphony director to come up with jet noise reduction ide as. He wasn't successful.

deploy a microphone under the jet just after takeoff and then radio the sig nal back to troops on the ground who wanted to attenuate the jet sound but not those trying to talk or hear other sounds. A dedicated receiver headph one would be necessary.

San Diego and Va. Beach.

Reply to
Bret Cahill

You don't know the phase angle where I'm standing. No I'm over here now. Mikek

Reply to
amdx

I have a plant who works at NSA. Every time you blow your nose it appears on youtube in real time.

Lot's of advertising $.

Reply to
Bret Cahill

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Reply to
John Fields

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