Cutting noise

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luc
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Re: Cutting noise

Post by luc » Tue May 08, 2012 5:40 pm

Mike Everman wrote:I think I can help illustrate what luc is talking about in a general sense with the attached illustration of basic methods.
What got me deep into pulse-jets in the first place was the seeming impossibility of removing the noise from the pulsejet. Any way you care to look at it, you can't separate "sound" from pressure oscillations. they are synonymous, and the one you need to be so energetic is the fundamental. In most of my small motors, it's at 200 Hz plus.
Since working on single engine approaches, I had diverted to cluster engines, so spent all my time on smaller motors. The not-so-original thought being driving the frequency up, and virtually driving that frequency up by them adopting random phasing. Enforcing a specific (semi cancelling, like in pairs or 3-phase) phasing hasn't been easily achievable with a DIY'er skill set.
Exactly ... That's what I trying to illustrate ... :wink:

And although in agreement with you about the pressure and noise close relation, I never tried to dissociate either one, but just using and "Flowing" sound so it comes and kill that sound (And pressure) once I was done with it (i.e. : Got the thrust or heat or velocity I wanted).

Always keep in mind that as gas turbine and also a majority of jet engine, gases exit the tail pipe at near ambient pressure. So once pressure is gone, you can do all you want with "Noise".

Watch my next posting and image ... I think this will raise few memories ... :wink:
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Re: Cutting noise

Post by luc » Tue May 08, 2012 5:44 pm

PyroJoe wrote:Mike,
Yes, sound and pressure synonymous. To this I ask, what drives mass acceleration? Why would we see peak +pressures near 30 psi in the CC and only +5 psi at the tail openings, dumping out as an overpressure? what happened to that other +25 psi?

luc,
Ok, got it i think. What would we like to see next?
Yeap .... PERFECT ....

Now ... Draw a big BANG at the end of the tail pipe (Were pressure go to ambient), with waves traveling all over 360° until it reaches the closed ends ... Then, think about "Reflection" and "Timing" when these waves hits the closed ends and come back toward that tail pipe, and I think you will get my point.

And it works ..... BUT ... To a certain degree ... Like I said above ...
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Re: Cutting noise

Post by luc » Tue May 08, 2012 5:45 pm

Remember this????

Look closer if you don't ... He he he ... :wink:

That's what I'am talking about ... :wink:

Some will say ... "Heyyyy ... You're PERFECTLY identical" ... And I will answer "Not geometrically ... But accoustically Yes ... :wink: "
Attachments
Decoupling.JPG
Acoustic reflecting & decoupling ... ;-)
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PyroJoe
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Re: Cutting noise

Post by PyroJoe » Tue May 08, 2012 6:05 pm

Ok luc, I see where your coming from, not necessarily to agree, but i see it.

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Re: Cutting noise

Post by luc » Tue May 08, 2012 6:22 pm

PyroJoe wrote:Ok luc, I see where your coming from, not necessarily to agree, but i see it.
:lol: ... I don't need you to agree with me ... IT WORKS ... :wink: :lol:

This concept was at the base of Conceotion GLC PHS system, and trust me, we would not have sold any units if the concept would not have worked with a noise level too high.

But again ... The level of noise reduction is equivalent to the level of perfection you're putting into the reflector, which in turn, is equivalent to the level of $$$ you want to put in your system. For GLC, her aim was anywhere close to 100 DbA and we pulled few sales with a 108 DbA, some at 115 (Monday & Friday systems I guess), which was fair enough at the time and seemed to satisfy our customers.

But as far as "Noise cancellation with Noise" is concern, I beleive this is a "Proven" science and a FACT ... Now, how we do it in real life, that's another ball game.
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PyroJoe
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Re: Cutting noise

Post by PyroJoe » Tue May 08, 2012 6:32 pm

Well I stand by that using pressure to cancel pressure requires a energy loss somewhere. As far as being able to cancel some noise with what you have illustrated and constructed, I have no problem in believing the noise can be reduced that way. Hell, if you were able to suppress 1 psi of the overpressure you could reduce the noise by near 20% in the tail opening.

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Re: Cutting noise

Post by Bruno Ogorelec » Tue May 08, 2012 6:45 pm

PyroJoe wrote:Well I stand by that using pressure to cancel pressure requires a energy loss somewhere.
Sure. But, not all the energy we perceive as noise goes into the engine cycle, does it? The trick is surely to leave the 'functional' or 'inside' noise be, while cutting the 'harmful' or 'outside' noise.

It's the same thing as with heat. You want heat to remain inside, because it does the work, not to get out, where it is harmful and wasteful. You can do it. I've seen a so-called adiabatic diesel engine that keeps the heat so well it doesn't even need a cooling system.

Doing the same with noise is a bit more difficult, but obviously doable up to a point.

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Re: Cutting noise

Post by PyroJoe » Tue May 08, 2012 7:09 pm

The trick for me is to use up as much inside noise as possible, so that when it reaches the outside there is less to suppress. Engine size, versus thrust production, versus noise production have typically been opposing characteristics.

Its typically been a struggle of: here are 3 positive characteristics, pick only 2. I think the future looks good though.

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Re: Cutting noise

Post by luc » Tue May 08, 2012 9:26 pm

Bruno Ogorelec wrote:
PyroJoe wrote:Well I stand by that using pressure to cancel pressure requires a energy loss somewhere.
Sure. But, not all the energy we perceive as noise goes into the engine cycle, does it? The trick is surely to leave the 'functional' or 'inside' noise be, while cutting the 'harmful' or 'outside' noise.

It's the same thing as with heat. You want heat to remain inside, because it does the work, not to get out, where it is harmful and wasteful. You can do it. I've seen a so-called adiabatic diesel engine that keeps the heat so well it doesn't even need a cooling system.

Doing the same with noise is a bit more difficult, but obviously doable up to a point.
Very well said Bruno. In fact, I could not have said it better (i.e : Keep the good and usefull and try to eliminate or reduce the less usefull) ... :wink:
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Re: Cutting noise

Post by luc » Tue May 08, 2012 9:28 pm

PyroJoe wrote:The trick for me is to use up as much inside noise as possible, so that when it reaches the outside there is less to suppress. Engine size, versus thrust production, versus noise production have typically been opposing characteristics.

Its typically been a struggle of: here are 3 positive characteristics, pick only 2. I think the future looks good though.
Very well said too ... And on the right track. But like you said, the forever question is "How or what is the best way of doing it ...

Question of our life ... Damn it ... :lol: :lol: :lol:
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Re: Cutting noise

Post by Mike Everman » Tue May 08, 2012 10:00 pm

The motor needs to be designed for the conditions, though. If it did not have the noise cancelling shroud, then the motor is designed for the pop to hit ambient air pressure at the exit. If the shroud is there, then there is now a negative pressure wave arriving at the exit at the same time as the pop, so the exit conditions are less than ambient. That sounds like a good thing to me.
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Re: Cutting noise

Post by Bruno Ogorelec » Wed May 09, 2012 4:34 am

Mike Everman wrote:The motor needs to be designed for the conditions, though. If it did not have the noise cancelling shroud, then the motor is designed for the pop to hit ambient air pressure at the exit. If the shroud is there, then there is now a negative pressure wave arriving at the exit at the same time as the pop, so the exit conditions are less than ambient. That sounds like a good thing to me.
Right. If one could get the shroud to function as a Meredith-style passive ramjet, too, so that it contributes to thrust, rather than just adding mass to the assembly...

Remember how the fanjet cuts the jet noise. It produces a 'tube' of cool air that encloses the hot gas jet and eases its expansion over a longer period and greater length. A ramjet exhaust, being hotter and less dense would be less effective, and it would have to work on a greater sound problem -- amplified single low frequency beat, rather than white noise -- but it would add to the sound dampening.

Finally, a turbine would dampen the noise dramatically. It produces some noise of its own, but much weaker and much less unpleasant, while absorbing a great deal of the combustion drumming and almost all of the explosive expansion at the tailpipe exit.

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Re: Cutting noise

Post by PyroJoe » Wed May 09, 2012 1:36 pm

I think the problem is in the pop itself. There is a very big difference between pop that has an overpressure near 5 psi and a overpressure that is near 0.10 psi.

If you could place a augmented fraction engine (near double length) producing a very low overpressure into a Meredith shroud, things would become very quiet, compared to a typical engine.

I often wonder what a 0 psi overpressure at the outlet would sound like? Maybe white noise of the mass flow? There would probably be some ringing of the duct material itself. Strange to think about.

Mike, how quiet were you looking to make these?

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Re: Cutting noise

Post by PyroJoe » Wed May 09, 2012 4:19 pm

In looking at overpressure there is probably some useful information in the "Pressure drop in a pipe" math.

The math basically describes:
pressure drop = (pipe friction coefficient)x(Length/Diameter)x(Density/2)x(Flow Velocity squared)

as we look at this, pipe friction can probably be set, and also density.

That results in two components:
first is a Length over Diameter ratio
second is Flow Velocity squared

What I propose that when we observe overpressure, that the L/D ratio doesn't allow the complete transfer of pressure to a flow velocity change. Therefore ideal flow velocity is not achieved and the excess pressure is released as overpressure.

The overpressure release from the intake is a considerable problem as its L/D is painfully close to the pressure source. A augmentor will catch some, but what isn't caught isn't transfered to velocity. Velocity squared is a hell of a thing.

In theory there should be a way to find L/D pressure utilization. Maybe for 50%, 75% 95%, admittedly I wouldn't know where to start.

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Re: Cutting noise

Post by Bruno Ogorelec » Wed May 09, 2012 6:43 pm

PyroJoe wrote:The overpressure release from the intake is a considerable problem
This is probably where the double-walled intake with perforations I have mentioned comes in. It absorbs some of the overpressure. Here's a picture.
Attachments
Surge chamber inlet.jpg

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